Solution- and gas-phase study of binding of ammonium and bisammonium hydrocarbons to oxacalix[4]arene carboxylate
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This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC 3.0 https://creativecommons.org/licenses/by-nc/3.0/ Solutionand gas-phase study of binding of ammonium and bisammonium hydrocarbons to oxacalix[4]arene carboxylate © 2023 The Author(s). Published by the Royal Society of Chemistry Published version Cowart, Anna; Brük, Mari-Liis; Žoglo, Nikita; Roithmeyer, Helena; Uudsemaa, Merle; Trummal, Aleksander; Selke, Kaspar; Aav, Riina; Kalenius, Elina; Adamson, Jasper Cowart, A., Brük, M.-L., Žoglo, N., Roithmeyer, H., Uudsemaa, M., Trummal, A., Selke, K., Aav, R., Kalenius, E., & Adamson, J. (2023). Solutionand gas-phase study of binding of ammonium and bisammonium hydrocarbons to oxacalix[4]arene carboxylate. RSC Advances, 13(2), 1041-1048. https://doi.org/10.1039/d2ra07614d 2023
Solutionand gas-phase study of binding of ammonium and bisammonium hydrocarbons to oxacalix[4]arene carboxylate† Anna Cowart,‡ ab Mari-Liis Brük, ‡ ab Nikita ˇ Zoglo, a Helena Roithmeyer, a Merle Uudsemaa, a Aleksander Trummal, a Kaspar Selke, a Riina Aav, b Elina Kalenius c and Jasper Adamson * a Oxacalixarenes represent a distinctive class of macrocyclic compounds, which are closely related to the parent calixarene family, offering binding motifs characteristic of calixarenes and crown ethers. Nevertheless, they still lack extensive characterization in terms of molecular recognition properties and the subsequent practical applicability. We present here the results of binding studies of an oxacalix[4] arene carboxylate macrocycle toward a variety of organic ammonium cationic species. Our results show that the substituents attached to the guest ammonium compound largely influence the binding strengths of the host. Furthermore, we show that the characteristic binding pattern changes upon transition from the gas phase to solution in terms of the governing intermolecular interactions. We identify the key factors affecting host–guest binding efficacy and suggest rules for the important molecular structural motifs of the interacting parts of ammonium guest species and the macrocycle to facilitate sensing of ammonium cations. Introduction Ammonium ions play a signicant role in numerous biological processes and in many biochemical reactions that take place in the human body. For instance, the structures of biologically important amino acids and neurotransmitters, that include molecules such as dopamine, serotonin, or acetylcholine, all contain an amine as a functional group. These amine moieties in the structures are frequently protonated under physiological pH conditions, and therefore, the respective molecules circulate in human bodies as ammonium or indeed as zwitterionic species. 1–3 Specic neurotransmitters and their concentrations in the neural system can be linked with various neurodegenerative disorders, such as Parkinson's disease or Alzheimer's disease. 4–6 This correlation relates to the interest in designing biomimetic sensors and articial receptors, 7 that can be used to control and monitor the levels of the respective regulator molecules. It is similarly important to study the “host–guest” binding of drug molecules that contain the ammonium group. A lot of studies focus on obtaining the maximum binding strengths for these guest molecules. However, there might be advantages to regulating the binding strength and the respective rates of ingression and egression of the guest molecules. One such example is when the macrocycle–guest complexes are being delivered to the human body inside vesicles, similarly to mRNA, 8–10 for targeted delivery of the drug molecule to the cell. Targeted delivery to the cell is still in its infancy but delivering macrocycle–drug complexes to the intracellular environment could be an advantage for precise localization for intended treatment. 11–13 If the drug guest molecule is released from the macrocycle in the cell, it could have its local desired impact. We could consider the scenario where a vesicle, for example a dendrimersome nanoparticle, made of ionizable amphiphilic Janus dendrimers, 8–10,14,15 carries the macrocycle–drug complex to the relevant cell and releases it there. The complex is subject to dilution in the cell and some of the drug molecule is being released. The complex release rate and extent depend on the concentration of the complex and the association strength (K a ). Lower association strength could be benecial for releasing the maximum amount of the guest aer the complex is subject to dilution. As an example, decreasing the association constant of 1 : 1 binding between a host and a guest from 1000 to 100, would result in 91.6% of guest release compared to 61.8% guest release from a 1 mM solution, respectively. Similar considerations are applicable when the macrocycle–drug complex is a Laboratory of Chemical Physics, National Institute of Chemical Physics and Biophysics, Akadeemia Tee 23, 12618 Tallinn, Estonia. E-mail: jasper.adamson@ kb.ee b Department of Chemistry and Biotechnology, Tallinn University of Technology, Akadeemia Tee 15, 12618 Tallinn, Estonia c Department of Chemistry, NanoScience Center, University of Jyv¨ askyl¨ a, Survontie 9B, FI-40014 JY, Finland †Electronic supplementary information (ESI) available: NMR titration data, ESI-MS data, DFT calculations details. See DOI: https://doi.org/10.1039/d2ra07614d ‡These authors contribute equally to this work. Cite this: RSC Adv.,2023,13,1041 Received 30th November 2022 Accepted 19th December 2022 DOI: 10.1039/d2ra07614d rsc.li/rsc-advances © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2023,13,1041–1048 | 1041 RSC Advances PAPER Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue
delivered to any conned environment in the body, where the guest undergoes dissociation from the complex. In these instances, lower association strengths might be advantageous to obtain the maximized abundance of the guest species. Similarly, the reusability of sensors containing macrocycles could benet from easy guest release. In this paper, we show that the carboxyl-substituted oxacalixarene 1(Fig. 1) modulates the binding strength of different ammonium containing guest species compared to the noncyclic 2,6-dimethoxybenzoic acid 22 (Fig. 2). We suggest that this might arise from steric hindrance of the binding site in the macrocycle and that such modulated binding could be benecial for applications that require controlled release of the active ingredient from the complex with the macrocycle. Examples of articial amine receptors based on calixarenes with a carboxyl moiety have been investigated by a number of research groups. 16–18 For example, in the case of calix[4]arenes, it has been shown that the cavity is too small for encapsulation of organic guest molecules and that complexation can be achieved by designing a binding site external to the macrocycle's cavity. 19 In addition, chiral macrocycles with aromatic monomer repeat units have been utilized in the detection of chiral amines, such as in enantioselective complexation with amino acid derivatives. 20–23 Such “host–guest”binding could also nd use in environmental monitoring 24 or in cavitands for label-free enzyme assays. 25 These studies illustrate the potential of macrocycles with aromatic monomer repeat units in amine sensing. Oxacalixarenes have re-emerged recently as a prospective class of macrocycles, 26,27 however, their characterization is still incomplete and molecular recognition properties of this class of host molecules are not yet well understood. Due to their structural features, oxacalixarenes have the potential to combine the intrinsic binding motifs of calixarenes and crown ethers and could thereby expand the applicability of these types of macrocycles. The scarce examples of oxacalixarene complexation studies include complexation with cationic guest species. 28,29 For the purpose of ammonium sensing, Gattuso and co-workers synthesized a polycationic oxacalix[4]arene in 2015, which showed affinity towards paraquat and neutral aromatic compounds in aqueous media. 30,31 Previously, our group has investigated complexation properties of “naked”oxacalixarenes 32 and shown in 2020 that 1can bind paraquat in methanol and water. 33 To the best of our knowledge, there are only a few examples where calixarene complexation with amine molecules has been investigated in polar protic solvents. 34,35 The present study focuses on how to best make use of these characteristics for ammonium sensing with oxacalixarenes, and to our surprise we nd that the macrocycle modulates binding strengths in comparison to the noncyclic species 22. Furthermore, we study the complex stoichiometry in the gas phase via ESI-MS and determine the complexation site and structure using ion mobility mass spectrometry (IM-MS) studies paired with DFT simulations. We note that the environment of complexes, which affects the intramolecular interactions in solution and gas phase, is different. We show that in solution, electrostatic and p–pinteractions, with possible contributions from cation–p and hydrogen bonding interactions, inuence binding strengths. On the contrary, the ESI-MS spectra and DFT calculations in the gas phase show that complexes are formed between the ammonium guest and neutral host via a hydrogen bond only. This does not prevent the possibility of the formation of similar complexes with a neutral charge like seen in solution, but these complexes would not be visible in ESI-MS gas phase spectra due to the absence of net charge in the formed species. Results and discussion Oxacalix[4]arene 1contains aromatic rings, where one of the monomer repeat units is substituted with a carboxylic acid moiety, making it feasible for the macrocycle to participate in various intermolecular interactions, such as electrostatic, p–p, cation–p, anion–p, or hydrogen bonding. The effect of these interactions for complex formation with host 1was studied based on binding with 20 different ammonium and bisFig. 1 Structure of monocarboxylic oxacalix[4]arene 1with the optimized structure of the 1$12 complex in methanol illustrating the 1,3alternate conformation of the macrocycle, that is characteristic of 4membered heterocalixarenes. Fig. 2 Organic guest species 2–15 and 16–21 and 2,6-dimethoxybenzoic acid 22 screened in this work. Most of the guest compounds were used as their chloride salts (exceptions are mentioned in Experimental section). 1042 |RSC Adv.,2023,13,1041–1048 © 2023 The Author(s). 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ammonium guest species in methanol-d 4 . For the NMR studies, the macrocycle was used in the form of a tetrabutylammonium (TBA) salt to enhance its solubility in methanol and reduce the competition between the counterion and guest for the binding site. The structures of guest molecules investigated in this work are depicted in Fig. 2. The association constants were determined by NMR titration experiments and the data were tted using an open-source online tting tool Bindt 36,37 by simultaneously following four host 1proton signals (H 2 ,H 5 ,H 6 ,H 9 in Fig. 1). For the monoammonium guest species 2–7and 11–13 (Table 1), the best ts were obtained by tting to a 1 : 2 complexation model, judged by residuals of the t. In such complexes one host binds two ammonium guest species. Binding data showed very low binding constant and negative cooperativity (a=0.4) 36 for the second guest binding. Titration experiments for the bisammonium guest species 16–21 showed more complex changes, and therefore, quantitative analysis of binding was not possible based on the collected data. The NMR spectra and links to Bindt binding isotherms are given in the ESI.† In the series of NMR titration experiments, the monoammonium guest molecules with aliphatic substituents (entries 1–5 in Table 1) were rstly investigated. The results show comparable binding strengths with the K 11 values varying from 46 up to 167 M −1 for all the compounds. We notice that the binding strength is dependent on how sterically hindered the ammonium group is in the guest structure. Guest 2with the smallest alkyl chain has the largest association strength, which decreases with the increasing bulkiness of the side chain. Incorporation of longer alkyl (guest 3) or bulkier cycloalkyl groups (guests 4and 6) leads to a decrease in the guest's binding ability. The sterically most crowded guest species, i.e., the secondary ammonium ion 7has the lowest binding affinity for the macrocycle, further supporting the notion that steric availability of the ammonium group in the guest species is important for the binding event. It is also evident that complexation can take place if the ammonium group is the only interacting functional group in the guest species. The possible interactions leading to binding can be the formation of a hydrogen bond involving the N–H proton or electrostatic interactions between oppositely charged functional groups of the host and the guest. For a comparison, we undertook experiments with the TBA salt of 2,6-dimethoxybenzoic acid 22 (Fig. 2), which closely resembles the monomer repeat units that the macrocycle 1is made of but does not provide the similar cyclic architecture. The NMR titration experiment with guest species 2gave a higher binding constant (right hand column in Table 1), compared to host 1, while the second guest binding to host 1and compound 22 had a similar strength (K 12 ). We suggest that the difference for the rst binding (K 11 ) might arise from the steric hindrance of the carboxylate moiety in the host 1binding pocket, as the subsequent second ammonium guest can freely approach the second O-atom of the carboxylate group, though with weaker electrostatic forces. The next group of investigated guest molecules included the aniline derivatives 8,9and 10. The NMR spectra acquired upon addition of these guest molecules revealed that aer the addition of the rst equivalent of the guests to the host solution, signicant changes in the chemical shis in the spectra of 1 took place. Based on the 1 H NMR spectrum, we identied host 1 to be in its carboxylic acid form aer the preceding addition of the aniline derivatives (see Fig. S6–S8 in ESI†). We therefore suggest that proton transfer between anilinium 8,p-nitroanilium 9,p-anisidinium 10 and host 1has taken place. This indicates that the ammonium groups of the protonated aniline derivatives are stronger Brønsted acids than the neutral carboxylic acid site in host 1and they donate a proton attached to the nitrogen to the carboxyl group of the host compound. The structure of the complex between 1and 8, optimized in implicit methanol solvent, supports the proton transfer towards the carboxylate moiety and is shown in Fig. 4a. The rst of these experiments was conducted with aniline 8and, later, guest 9 with electron-withdrawing (EW) and guest 10 with electrondonating (ED) substituents attached to the benzene core were investigated to see whether these could suppress or enhance the proton transfer due to conjugation with the ammonium cation. It can be concluded that proton transfer takes place similarly in all three aniline derivatives. Based on these results, however, we observed that upon continuing the titration experiments aer the host has been protonated, no chemical shichanges were seen between this “neutral host”and the aniline guest species. This would indicate that the presence of the carboxylate moiety in the host is prerequisite for complex formation. Complexation through p–pinteractions or hydrogen bonding alone with the guest molecules does not occur unless the complexation is Table 1 K a values calculated based on NMR titration results for host 1 TBA salt complexes and 22 TBA salt with ammonium containing guests in methanol-d 4 at 298 K Entry Guest K aa (M −1 ) with 1K aa (M −1 ) with 22 12K 11 167.1 2K 11 250 20 K 12 15.9 0.1 K 12 17 1 23K 11 97.1 0.9 — K 12 5.67 0.05 34K 11 105 1— K 12 7.2 0.1 46K 11 103 2— K 12 5.6 0.1 57K 11 46 2— K 12 3.3 0.1 611 K 11 124 1K 11 500 100 K 12 8.73 0.08 K 12 6.0 0.4 712 K 11 818 30 K 11 2000 200 K 12 104 5K 12 100 7 813 K 11 536 8— K 12 31.4 0.5 914 K 11 113 2— K 12 3.13 0.08 a Given errors of the association constant values are obtained from Bindt and are based on single experiment's curve tting calculation errors. Further details of titration data are given in Experimental section and in ESI. Association constants are expressed as K 11 and K 12 for the rst and the second guest binding to carboxylates, respectively, since the complexation mode best t points to the 1 : 2 complex formation. © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2023,13,1041–1048 | 1043 Paper RSC Advances Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
supported by the electrostatic interaction between the carboxylate of host 1and the ammonium group in the guest molecules. This hypothesis could further be rationalized by conducting a control experiment with nitrobenzene 15 (Fig. S13 in ESI†) to see whether complexation is possible solely as a result of p–pinteractions. We did not detect complexation for this guest molecule and therefore, p–pinteractions alone are suggested to not lead to complex formation with 1. Equipped with the information that while the interactions between the carboxylate of the host and the ammonium group of the guest can lead to complexation, but p–pinteractions alone do not, we set to explore the role of the combination of covalent/electrostatic and p–pbonding for binding. The studied guest molecules 11–14 show that the binding strength increases signicantly upon the incorporation of the aromatic groups in the substituents of the ammonium guest molecules. An unsubstituted phenyl ring increased the binding ability of guest 13 5-fold relative to its aliphatic counterpart 6.We hypothesize that the interaction between the carboxylate group of host 1and the ammonium group of the guest molecules triggers the p–pinteractions between the same host and the guest. This notion is further corroborated by inspection of the structures of ion pairs formed between 1and guests 11–12, optimized in methanol, where T-shape p–pinteractions are visible in the optimized structures (Fig. 4b). The data suggest that two types of interactions appear in the complex, where p–p interactions require the concurrent presence of the covalent/ electrostatic interaction between the carboxylate and the ammonium cation. While comparing the results for guest 13 with that of guest 11, one can notice that the association strength decreases upon decreasing the size of the alkyl chain connecting the ammonium and aromatic groups. This observation suggests that the degree of freedom in the relative orientation of the two interacting sites to form bonds between the guest and 1has likely an important inuence for the formation of stable complexes. Additionally, we studied the effect of the nitro substituent in the aromatic rings on the guest molecules in solution. Comparing the association strength of guest 11 to that of guest 12 revealed that the introduction of the EW nitro group to the aromatic ring in the guest increased the binding ability of the guest. This is likely indicative of the favorable p–pinteraction between the electron-decient benzene ring of guest 12 and the electron-rich aromatic system of host 1in solution. For comparison, we conducted experiments with guest 11 and 12 and the TBA salt of 22 to see how much the macrocyclic structure inuences binding strengths (right hand column in Table 1). The binding strength was increased for both guest molecules with the noncyclic molecule 22 for the 1 : 1 complex (K 11 ), while it remained similar to host 1for the second guest binding (K 12 ). We suggest that steric hindrance of the carboxylate binding site in the macrocyclic structure could lead again to the decrease of the binding constant. This suggests that the conformation of 1 modulates binding strengths between the host and the guest. If under certain conditions, for example the aforementioned targeted delivery of drugs or reusing sensor materials, the oxacalixarene would need to release its cargo, the affected modulated binding strength could prove to be benecial for this release. The addition of chiral naphthalene derivative 14 resulted in some unusual behavior upon complexation as distinction of diastereotopic protons of host 1in the NMR spectra became evident in these experiments (Fig. 3c). These spectra indicate that upon complexation of host 1with the chiral guest 14, the symmetry of host 1is reduced. Therefore, there might be possibility for host 1to act as a chirality sensor for ammonium guest species. 38 We also tried to crystallize the formed complexes, but despite our attempts, with different crystallization techniques, the crystals obtained were too small to be analyzed by single crystal X-ray diffraction. As the following experiments, we decided to study the macrocycle in gas phase with ESI-MS and IM techniques, to understand if any other charged complexes between 1and the guest species will form. The gas-phase properties of host 1were studied using ESI-QTOF mass spectrometry and initially host 1 was analyzed in both positive and negative polarities. In the positive mode, singly-charged ions for 1and its dimer 1 2 , were detected through adduct ion formation with Na + and K + . However, in the negative mode, the expected deprotonated ions were not detected, and instead, the decarboxylated ion of 1at m/ z367 was seen (Fig. S23 in ESI†). Albeit decarboxylation being a common reaction for any compound containing a carboxylic acid group in ESI-MS, the detection of the decarboxylated compounds exclusively is relatively rare. Based on these results, MS analysis of the complexes was conducted in the positive Fig. 3 Stacked titration plots showing 1 H-NMR spectra of host 1with host to guest ratio of 1 : 1 for (a) guests 2–4,6,7and (b) 11–14,19. (c) Distinction of diastereotopic protons during the titration with guest 14. 1044 |RSC Adv.,2023,13,1041–1048 © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
mode due to the lack of stability of the macrocycle in the negative mode. It was anticipated that the affinity of 1towards ammonium cations would be driven by ion pair formation between macrocycle's carboxylate moiety and ammonium cation, similarly to the solution state studies. It should however be noted that in mass spectrometry, the ion pair complex is invisible due to its charge neutrality. During this investigation, the ammonium guests 2,3,5–7,10–12 were analyzed and the respective host– guest pairs were mainly detected as 1 : 1 singly-charged positive ions (see ESI for (+)ESI-MS spectra and Table S2†). Notably, the ion m/zvalues revealed that the macrocycle was in its carboxylic acid form when interacting with the ammonium cations in these complexes, highlighting that the likely host–guest interactions involve hydrogen bonding in the gas phase. This observation does not rule out the possibility of ion pair formation as another contributing factor of complexation in ESI-MS experiments because the potential species would simply remain undetected due to zero net charge. The detected complexes were further studied with dritube IM-MS (DT-IMMS). IM-MS is a versatile gas-phase method that has been used for evaluating host–guest complex conformation, structures and geometries. 39–43 In IM-MS, the collision cross section (CCS) value for analyte ions is measured. Additionally, in the presence of two or more binding modes, their respective structures can be separated. Identication and visualization of the structures is possible when IM-MS method is paired with theoretical modeling and the atomic coordinates from the modelled structures are used for calculation of theoretical CCS values. The DT-IMMS results of complexes of 1with guests 2,3, 5–7,10–12 revealed that the dritimes of the complexes are longer than the dritime for free host [1+ Na] + (Fig. 5). This suggests that the size of the complex is increased compared to the free host and that the guest is located outside the oxacalixarene cavity. Furthermore, as the size of the ammonium guest increased (Table S2†), the dritimes (and CCS values) increased as well, which evidence that binding takes place through a complex where the guest occupies space outside the macrocycle's cavity. It is noted here that the complexes observed with ESI-MS experiments are complexes between the neutral host and a cationic guest species, while by NMR titration experiments the complexes formed between the host with a carboxylate anion and cationic ammonium guest species. To rationalize the obtained experimental DT CCS N 2 values, DFT calculations of [1+2] + ,[1+3] + ,[1+11] + and [1+12] + host– guest pairs were performed and the theoretical TMLJ CCS N 2 values were calculated based on the coordinates obtained from the optimized geometries of the hydrogen-bond-mediated complexes (Fig. 6). The theoretical simulations also suggest that binding occurs through an external binding site above the oxacalixarene cavity. The description of the theoretical simulations of the complexes in the gas phase as well as their counterparts obtained in methanol solution is brought in the ESI.† The calculated and theoretical CCS values of the four complexes are shown in Fig. 5 and Table S2.†The theoretical and experimental CCS values are in fair agreement, which demonstrates the similarity between the geometries of the measured gasFig. 5 Arrival time distribution for (+)ESI-MS detected ions of [1+ guest] + 1 : 1 complexes. Second peak occasionally observed ∼35–39 ms originates from dimer [1 2 +G] + which fragmentates in drift tube to produce [1+G] + . Fig. 4 Optimized solution-phase structures of (a) [1+8], (b) [1+11] and [1+12] in methanol. Calculated at the B97D/6-311+G(2d,p) level of theory. Fig. 6 Optimized gas-phase structures of (a) [1+2] + , (b) [1+3] + , (c) [1 +11] + and (d) [1+12] + host–guest complexes. Calculated at the B97D/ 6-311+G(2d,p) level of theory. © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2023,13,1041–1048 | 1045 Paper RSC Advances Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
phase complexes and the calculated hydrogen-bonded structures. Therefore, complexation, albeit through different sets of intermolecular interactions, takes place both in solution and in gas phase. Experimental section NMR titration The K a values were determined in methanol-d 4 . All the spectra for 1 H NMR titration experiment were collected on a Bruker AVANCE III 800 MHz spectrometer. Host 1and compound 22 were used in its TBA salt forms and stock solutions (2–10 mM) were added to a vial containing a guest to keep the concentration of host 1xed throughout the titration experiment. All guest compounds were analyzed in their chloride salt form. All solutions were prepared using Hamilton® Gastight syringes and small aliquots of guest compounds were added to the solution using the same syringes. Compounds were weighed on a Sartorius microbalance (accuracy 15 mg), allowing preparation of stock solutions concentrations within 1% error. Increasing amounts of the guest stock solution were added to the NMR tube containing 600 mL of the host 1 stock solution. The concentrations of both the host and the guest stock solutions were based on the assumed association constants and all the details of total guest concentration are given in the ESI†below the titration spectra. All data points were measured at quantitative conditions and depending on the concentration of the host, data were collected using 8 or 14 scans with relaxation delay set to 10 s or 17 s, acquisition time set to 2.4 s and pulse width set to 3 ms(30°pulse).Thechemicalshiswerereferenced based on methanol-d 4 residual peak at 3.31 ppm. The K a values were determined using Bindt 36,37 analysis tool (available at https://supramolecular.org) that is based on nonlinear regression analysis. Mass spectrometry All the gas-phase studies were performed on an Agilent 6560 Ion Mobility Time-of-Flight mass spectrometer, equipped with a dual ESI ion source (Agilent Technologies, USA). The mass range for the ESI-MS and IM-MS experiments was set to m/z 300–1000. The 1 mM stock solution of host 1and 5 mM stock solutions of ammonium guests 2,3,10,12 as chloride salts were dissolved in methanol. Guests 5–7and 11 were used in their amine form and dissolved in either methanol or water. Sample solution of host 1was prepared in acetonitrile and diluted to the concentration of 10 mM. For the MS analysis of complexes, 5 mM sample of 1was prepared in acetonitrile and an equimolar amount of guest was added to the sample. The sample ow rate was set to 2 mL min −1 . In ESI, N 2 was used as the drying and nebulization gas. The source conditions for the IM-MS experiments were kept consistent throughout the screening of amines, with the following parameters: capillary voltage 4.5 kV, dry gas temperature 225 °C, drying gas ow 7 L min −1 , nebulizer pressure 4 psi, fragmentor voltage 400 V, and Oct 1 RF V pp 750 V. The IM-MS spectra were measured with high purity N 2 as the drigas. The dritube pressure was set to 3.95 torr and in the case of single-eld IM-MS experiments the dritube entrance and exit voltages were adjusted to 1700 V and 250 V, respectively. Trap lling time of 5000 ms and trap release time of 350 ms were used. The stepped-eld IM-MS experiments for determination of DT CCS N 2 were obtained by varying the dritube entrance voltages from 1074 to 1674 V in 100 V steps. Reproducibility of the CCS values was checked using the ES tuning mix standard solution (Agilent Technologies) as a quality control. 44,45 The data analysis was performed using Agilent MassHunter Qualitative Navigator (B.07.00, Agilent Technologies, USA) and MassHunter IM-MS Browser (B.08.00, Agilent Technologies, USA). Theoretical CCS values were calculated with the ion mobility calculator IMoSSuite version 1.06. 46,47 The trajectory method with Lennard-Jones potentials was used for calculation of theoretical TMLJ CCS N 2 values which appear to be in reasonable agreement with the measured DT CCS N 2 values. The calculation was based on IM-MS experimental parameters that include gas, temperature and pressure. The number of rotations used was set to 3, applying 300 000 gas molecules per rotation. Atomic coordinates of host 1for the theoretical TMLJ CCS N 2 calculations were obtained from the already published crystal structure of the macrocycle. 33 The respective coordinates of the ammonium cation complexes were obtained from the gas-phase DFT optimizations at the B97D/6-311+G(2d,p) level of theory. Further computational details are presented in the DFT calculations sections in ESI.† Conclusions The key structural aspects that contribute to binding between host 1and ammonium guest molecules are presented and analysed in solution and in the gas phase. The results show that different ammonium guest structures bind to host 1salt with various binding strengths in solution. We determine three important contributing factors that affect complexation. Firstly, the presence of a carboxylate moiety in the host is suggested to be prerequisite for complex formation even for guest molecules with aromatic substituents. Secondly, p–pinteractions can support binding and increase binding strengths and thirdly, we hypothesize that the accessibility of the degrees of freedom for favourable mutual orientation of the host and the guest that participate in electrostatic and p–pinteractions is important for stable complex formation. Furthermore, the binding is modulated and weaker compared to the noncyclic molecule 2,6dimethoxybenzoic acid 22, which could have its advantages in the release of the desired guest molecules from the macrocycle for biomedical uses. We further show that a chiral guest molecule can change the symmetry of the oxacalixarene host macrocycle. The studies in the gas-phase and DFT calculations demonstrate that binding in the gas phase can occur solely through hydrogen bonding through the formation of a complex with a binding site above the oxacalixarene cavity. These results allowed to establish the geometries for the complexes in the gas phase by DFT methods. 1046 |RSC Adv.,2023,13,1041–1048 © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
Author contributions AC performed the ESI-MS and ion mobility experiments and preliminary synthesis, M-LB performed the synthesis of the macrocycle, Nˇ Z and KS performed the NMR titration experiments, MU and AT performed the DFT calculations, HR helped with additional analysis for the revised manuscript, RA supervised the synthesis of the macrocycle and tted the titration data, EK supervised the ESI-MS and ion mobility experiments, JA conceived the project, coordinated the project and supervised the synthesis and NMR experiments. Nˇ Z and JA wrote the paper with contribution from all authors. Conflicts of interest There are no conicts to declare. Acknowledgements The authors gratefully acknowledge nancial support by the Ministry of Education and Research, Republic of Estonia (grants PSG400, PRG661 and PRG399) and European Regional Development Fund (project TK134 “EQUITANT”and Center of Excellence in Molecular Cell Engineering TK143), and instrumentation of University of Jyv¨ askyl¨ a. Notes and references 1 B. Gorain, P. Sengupta, S. Dutta, M. Pandey and H. Choudhury, in Frontiers in Pharmacology of Neurotransmitters, Springer, 2020, pp. 213–240. 2 G. Rudnick, in Neurotransmitter Transporters: Structure, Function, and Regulation, ed. M. E. A. Reith, Springer, New York, 2nd edn, 2002, p. 31. 3 M. Lieberman and A. Peet, Marks' Essentials of Medical Biochemistry: A Clinical Approach, Wolters Kluwer, Philadelphia, 2nd edn, 2015. 4 L. Brichta, P. Greengard and M. Flajolet, Trends Neurosci., 2013, 36, 543–554. 5 S. A. Factor, W. M. McDonald and F. C. Goldstein, Eur. J. Neurol., 2017, 24, 1244–1254. 6 P. T. Francis, CNS Spectr., 2005, 10,6–9. 7 S. Moerkerke, V. Malytskyi, L. Marc´ elis, J. Wouters and I. Jabin, Org. Biomol. Chem., 2017, 15, 8967–8974. 8 D. Zhang, E. N. Atochina-Vasserman, D. S. Maurya, N. Huang, Q. Xiao, N. Ona, M. Liu, H. Shahnawaz, H. Ni, K. Kim, M. M. Billingsley, D. J. Pochan, M. J. Mitchell, D. Weissman and V. Percec, J. Am. Chem. Soc., 2021, 143, 12315–12327. 9 D. Zhang, E. N. Atochina-Vasserman, D. S. Maurya, M. Liu, Q. Xiao, J. Lu, G. Lauri, N. Ona, E. K. Reagan, H. Ni, D. Weissman and V. Percec, J. Am. Chem. Soc., 2021, 143, 17975–17982. 10 D. Zhang, E. N. Atochina-Vasserman, J. Lu, D. S. Maurya, Q. Xiao, M. Liu, J. Adamson, N. Ona, E. K. Reagan, H. Ni, D. Weissman and V. Percec, J. Am. Chem. Soc., 2022, 144, 4746–4753. 11 P. T. Altenbuchner, P. D. L. Werz, P. Schöppner, F. Adams, A. Kronast, C. Schwarzenböck, A. Pöthig, C. Jandl, M. Haslbeck and B. Rieger, Chem.–Eur. J., 2016, 22, 14576– 14584. 12 S. Tang, F. Zhang, H. Gong, F. Wei, J. Zhuang, E. Karshalev, B. Esteban-Fern´ andez de ´ Avila, C. Huang, Z. Zhou and Z. Li, Sci. Robot., 2020, 5, eaba6137. 13 Y. Zheng, L. Tang, L. Mabardi, S. Kumari and D. J. Irvine, ACS Nano, 2017, 11, 3089–3100. 14 S. Zhang, R. Moussodia, H. Sun, P. Leowanawat, A. Muncan, C. D. Nusbaum, K. M. Chelling, P. A. Heiney, M. L. Klein and S. Andr´ e, Angew. Chem., Int. Ed., 2014, 126, 11079–11083. 15 V. Percec, D. A. Wilson, P. Leowanawat, C. J. Wilson, A. D. Hughes, M. S. Kaucher, D. A. Hammer, D. H. Levine, A. J. Kim and F. S. Bates, Science, 2010, 328, 1009–1014. 16 J.-A. Richard, M. Pamart, N. Hucher and I. Jabin, Tetrahedron Lett., 2008, 49, 3848–3852. 17 C. Capici, G. Gattuso, A. Notti, M. F. Parisi, S. Pappalardo, G. Brancatelli and S. Geremia, J. Org. Chem., 2012, 77, 9668–9675. 18 A. Inthasot, M.-D. D. Thy, M. Lejeune, L. Fusaro, O. Reinaud, M. Luhmer, B. Colasson and I. Jabin, J. Org. Chem., 2014, 79, 1913–1919. 19 T. Pierro, C. Gaeta, F. Troisi and P. Neri, Tetrahedron Lett., 2009, 50, 350–353. 20 K. Jennings and D. Diamond, Analyst, 2001, 126, 1063–1067. 21 A. Lled´ o, R. J. Hooley and J. Rebek Jr, Org. Lett., 2008, 10, 3669–3671. 22 B. Setner and S. Agnieszka, Beilstein J. Org. Chem., 2019, 15, 1913–1924. 23 T. Panahi, H. L. Anderson, K. I. Castro, J. D. Lamb and R. G. Harrison, Supramol. Chem., 2020, 32,71–80. 24 M. Chiesa, F. Rigoni, M. Paderno, P. Borghetti, G. Gagliotti, M. Bertoni, A. B. Denti, L. Schiavina, A. Goldoni and L. Sangaletti, J. Environ. Monit., 2012, 14, 1565–1575. 25 D.-S. Guo, V. D. Uzunova, X. Su, Y. Liu and W. M. Nau, Chem. Sci., 2011, 2, 1722–1734. 26 M.-X. Wang, Chem. Commun., 2008, 4541–4551. 27 M.-X. Wang, Acc. Chem. Res., 2012, 45, 182–195. 28 D. Sobransingh, M. B. Dewal, J. Hiller, M. D. Smith and L. S. Shimizu, New J. Chem., 2008, 32,24–27. 29 M. Panchal, M. Athar, P. C. Jha, A. Kongor, V. Mehta and V. Jain, J. Lumin., 2017, 192, 256–262. 30 N. Manganaro, G. Lando, C. Gargiulli, I. Pisagatti, A. Notti, S. Pappalardo, M. F. Parisi and G. Gattuso, Chem. Commun., 2015, 51, 12657–12660. 31 N. Manganaro, G. Lando, I. Pisagatti, A. Notti, S. Pappalardo, M. F. Parisi and G. Gattuso, Supramol. Chem., 2016, 28, 493– 498. 32 A. Peterson, S. Kaabel, I. Kahn, T. Pehk, R. Aav and J. Adamson, ChemistrySelect, 2018, 3, 9091–9095. 33 A. Peterson, M.-L. Ludvig, J. Mart˜ onova, S. Kaabel, P. Kerner, M. Uudsemaa, A. Trummal, M. Fomitˇ senko, T. Pehk and R. Aav, Supramol. Chem., 2020, 32, 313–319. 34 T. Oshima, K. Oishi, K. Ohto and K. Inoue, J. Inclusion Phenom. Macrocyclic Chem., 2006, 55,79–85. © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2023,13,1041–1048 | 1047 Paper RSC Advances Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
35 N. K. Beyeh, F. Pan, A. Valkonen and K. Rissanen, CrystEngComm, 2015, 17, 1182–1188. 36 P. Thordarson, Chem. Soc. Rev., 2011, 40, 1305–1323. 37 D. B. Hibbert and P. Thordarson, Chem. Commun., 2016, 52, 12792–12805. 38 L. Ustrnul, S. Kaabel, T. Burankova, J. Mart˜ onova, J. Adamson, N. Konrad, P. Burk, V. Borovkov and R. Aav, Chem. Commun., 2019, 55, 14434–14437. 39 D. V Dearden, T. A. Ferrell, M. C. Asplund, L. W. Zilch, R. R. Julian and M. F. Jarrold, J. Phys. Chem. A, 2009, 113, 989–997. 40 M. ¨ Oeren, E. Shmatova, T. Tamm and R. Aav, Phys. Chem. Chem. Phys., 2014, 16, 19198–19205. 41 A. Kiesil¨ a, L. Kivij¨ arvi, N. K. Beyeh, J. O. Moilanen, M. Groessl, T. Rothe, S. Götz, F. Topi´ c, K. Rissanen and A. Lützen, Angew. Chem., Int. Ed., 2017, 56, 10942–10946. 42 A. Kiesil¨ a, J. O. Moilanen, A. Kruve, C. A. Schalley, P. Barran and E. Kalenius, Beilstein J. Org. Chem., 2019, 15, 2486–2492. 43 E. Kalenius, M. Groessl and K. Rissanen, Nat. Rev. Chem., 2019, 3,4–14. 44 S. M. Stow, T. J. Causon, X. Zheng, R. T. Kurulugama, T. Mairinger, J. C. May, E. E. Rennie, E. S. Baker, R. D. Smith and J. A. McLean, Anal. Chem., 2017, 89, 9048– 9055. 45 V. Gabelica, A. A. Shvartsburg, C. Afonso, P. Barran, J. L. P. Benesch, C. Bleiholder, M. T. Bowers, A. Bilbao, M. F. Bush and J. L. Campbell, Mass Spectrom. Rev., 2019, 38, 291–320. 46 C. Larriba and C. J. Hogan Jr, J. Comput. Phys., 2013, 251, 344–363. 47 C. Larriba and C. J. Hogan Jr, J. Phys. Chem. A, 2013, 117, 3887–3901. 1048 |RSC Adv.,2023,13,1041–1048 © 2023 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 04 January 2023. Downloaded on 1/12/2023 9:24:51 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online