Inclusion of Pentamidine in Carboxylated Pillar[5]arene: Late Sequential Crystallization and Diversity of Host − Guest Interactions

: Here we describe the inclusion of the minor-groove binding drug pentamidine in its extended conformation into the macrocyclic cavity of water-soluble carboxylated pillar[5]arene. The arti ﬁ cial host − guest system is manifested as an ensemble of three crystalline forms di ﬀ ering in the pillar[5]arene − pentamidine interaction mode, solvation type, and even stoichiometry. The strong hydrogen bonds of various types of amidinium − carboxylate (between pentamidine and pillar[5]arene) and carboxylic − carboxylate (between adjacent pillar[5]arenes) are realized in all complexes, rigidifying the assembly. However, it seems that more subtle e ﬀ ects, such as local solvation/desolvation, a certain degree of ﬂ exibility of both the host and guest, and variable protonation/ deprotonation state of the pillar[5]arene substituents, play signi ﬁ cant roles in the peculiar crystallization behavior of the system. The snapshots of three crystal structures highlighting various binding geometries and intermolecular interaction modes between the same host and guest present exciting opportunities in terms of more holistic models of the host − guest systems, shifting from focusing our attention on “ visible interactions ” in one crystal structure to the appreciation of many subtle e ﬀ ects playing a combined role in the host − guest chemistry.


■ INTRODUCTION
The crystal structure of a protein−ligand complex and the extracted interaction geometry between a ligand and a host protein are an important starting point in structure-based drug design. 1,2However, it should be remembered that the crystal structure of a macromolecular protein−ligand complex represents a misleadingly static picture of the intermolecular interactions and geometric fit between partner molecules.In reality, the single protein can bind a ligand in a range of geometric arrangements starting from loose surface binding to the ligand being deeply buried inside the host protein. 3herefore, the protein−ligand complex is better described as a superposition or ensemble of different structures. 4,5The molecular recognition event between a protein and ligand is a delicate balance of multiple attractive and repulsive interactions affected inter alia by solvation/desolvation phenomena and conformational changes of one or both molecular partners. 6,7The same can refer to the simpler supramolecular host−guest systems composed of flexible partners with multiple binding interactions. 8Unfortunately, the experimental crystal structure data highlighting various binding geometries and intermolecular interaction modes between the same host and guest rarely mirror the whole structural host−guest landscape. 9,10In this work we showcase an artificial supramolecular system as an ensemble of three host−guest complexes where not all interactions occur at the same time.
The motivation of this work originated from the encapsulation studies of the minor-groove binding drug pentamidine (PTM) by the macrocyclic host carboxylated pillar [5]arene (CPA5) (Figure 1).Pentamidine, a dicationic bis-amidinium drug, is used as an antimicrobial agent in the treatment of pneumonias associated with AIDS and as antiprotozoal agent against trypanosomiasis and leishmaniasis.Recently, the exciting prospect of the combination of pentamidine with conventional antibiotics has led to the successful treatment of multidrug-resistant Gram-negative bacteria. 11Pentamidine helps to perturb the otherwise impenetrable outer membrane of pathogens via its interactions with lipopolysaccharides, thus sensitizing the bacteria to antibiotics.From a supramolecular point of view the host− guest chemistry of pentamidine is very limited, 12,13 which is surprising considering that the thriving interest in the multipurpose therapeutic use of the drug 14 is hampered by its rather poor pharmacokinetics and nephrotoxicity. 15Therefore, host−guest systems capable of pentamidine encapsulation with a view to controlled delivery and release would be desirable.
Fortunately, the amidinium cations are well-known building blocks in crystal engineering, particularly effective in chargeassisted hydrogen bonds (HBs) with carboxylate anions. 16,17hen the bis-amidinium nature of the pentamidine is taken into consideration, the pillar [5]arene host with a suitably sized cavity framed with multiple carboxylato functionalities seems to be an appropriate starting point for host−guest exploratory studies. 18,19Here we describe the successful inclusion of pentamidine in its extended conformation into the macrocyclic cavity of CPA5.Moreover, the host−guest system is manifested as an ensemble of three crystalline forms differing in the CPA5-PTM interaction mode, solvation type, and even stoichiometry (Figure 2).We show that the host−guest complexation is driven by the inclusion of various types of amidinium-carboxylate heterosynthons that perturb the typical CPA5 assembly by carboxylic−carboxylic and carboxylic− carboxylate HBs.These assemblies have been studied in the solid state by single-crystal X-ray diffraction, in solution by 1 H NMR spectroscopy, and in the gas phase via mass spectrometry.

■ RESULTS AND DISCUSSION
We wish to point out that the key to the successful crystallization of CPA5 and its complexes is to use the macrocycle in the carboxylic neutral form (in contrast, solution studies are mainly performed for the deca-anionic carboxylate form) with water−ethanol as the crystallization solvent.The inclusion of ethanol into the macrocyclic cavity increases the aqueous solubility of CPA5, while hydrogen bonding between carboxylic acids renders the possibility of a solid-state assembly, which is not feasible for the fully anionic form of the macrocycle.Despite the flourishing interest in the host− guest chemistry of carboxylated pillar[n]arenes starting immediately after first report on their synthesis and host− guest properties in 2010, 20 no crystal structure for this family of water-soluble macrocycles had been reported until our first structural authentication of CPA5 in 2015. 21Larger homologues such as CPA6 and CPA7 have not been obtained in the crystal form so far.We have recently reported the host−guest crystalline complexes of CPA5 with a series of viologen derivatives, in which guest inclusion into the CPA5 cavity takes place, and the overall assembly is driven by multiple carboxylic−carboxylic and carboxylic−carboxylate HBs between adjacent macrocycles. 22In order to disrupt this type of pillar [5]arene association, we hypothesized that the introduction of robust supramolecular heterosynthons could compete with homosynthons based on carboxylic acids.
Indeed, cocrystallization of pentamidine (diisethionate salt) bearing two benzamidinium functionalities with CPA5 by layering water−ethanol solutions of the two components afforded needlelike crystals suitable for single-crystal X-ray diffraction.The crystal structure of the host−guest complex of the composition [PTM@CPA5]•7.2H 2 O was solved and refined in the space group P2 1 /c.The ASU comprises one CPA5, one PTM spanning the macrocyclic cavity, and water molecules (mostly disordered) (Figure 3).The dicationic nature of PTM implies deprotonation of 2 of the 10 carboxylic substituents of CPA5 to satisfy the charge balance.The PTM   Crystal Growth & Design inclusion mode is quite symmetricalthe aliphatic pentanediol chain is retained in the central part of the CPA5 cavity, two aromatic rings are enclosed by CPA5 substituents, and two amidinium groups protrude from the prismatic cavity at both rims.
Each positively charged amidinium group bears four hydrogen atoms that can form strong charge-assisted hydrogen bonds with carboxylate anions or water molecules.Interestingly, there are no direct hydrogen bonds between amidinium groups and the parent CPA5; instead, amidinium−carboxylate HBs with adjacent CPA5 molecules are the preferred type of interaction (Figure 3c).Thus, two protruding amidinium groups interact with the surrounding HB acceptors in different modes.The first amidinium group (N1P, N2P) interacts with one carboxylate ina d2a1 arrangement (R 2 1 (6) ring in graph set notation) and two water molecules, while the second amidinium group (N3P, N4P) forms a paired HB with one carboxyl (d2a2 motif) (R 2 2 (8) ring), a single HB with another carboxyl in a d1a1 mode, and one HB with a water molecule (Figure 3c).The N−H•••O distances in the amidinium− carboxylate synthons are in the range of 2.77−3.01Å.It should be mentioned that charge-assisted HBs between amidinium cations and carboxylate anions are known and useful synthons in the self-assembly of various types of supramolecular systems.This directional hydrogen-bond interaction facilitated by an electrostatic attraction within the ion pair has been introduced into crystal engineering by the pioneering studies of Hosseini's group as part of the molecular tectonic concept. 23Later this interaction was richly explored by several groups for the preparation of capsules, 24 helical structures, 25,26 catenanes, 27 supramolecular organic frameworks, 28,29 and pharmaceutical organic salts. 30,31However, the utilization of a benzamidinium moiety in host−guest crystalline assemblies is quite limited.Comotti and Marchio reported the crystal structure and hydrogen bonding of two benzamidinium guests with psulfonatocalix [4]arene, 32 our group studied the inclusion of aminobenzamidiniums and diminazene (berenil) drug into a cucurbit [6]uril macrocyclic host. 33,34Also, a CSD 35 (version 5.42 and 3 updates) search on the pentamidine structures gives only two hits on the diisethionate salt of pentamidine. 36,37nterestingly, hexamidine diisethionate (with six methylene groups in the aliphatic chain) has recently been reported to exhibit extensive polymorphism and interconversions between 10 anhydrous phases and 2 hydrates. 38s can be expected, the strong amidinium−carboxylate HB between pentamidine and CPA5 molecules diminishes direct HBs between adjacent macrocycles.While extensive carboxylic−carboxylic/-ate HBs have been observed in the native fully carboxylic CPA5 structure and its host−guest complexes with viologen derivatives, here there is only one HB contact between adjacent CPA5 molecules in the crystal lattice.This HB with a very short O−H 39 and confirms the deprotonation of some of the carboxylic substituents of CPA5.Taking into account that we observed such carboxyl−carboxylate HBs with O−H•••O distances shorter than 2.50 Å also in the CPA5−viologen inclusion complexes and coordination complexes with strontium, 40 it can be stated that they are characteristic features of the solid-state CPA5 assembly and are realized also in the presence of competing heterosyntons (such as amidinium−carboxylate).Hydrogen bonding between pen-tamidine and CPA5 gives rise to a chainlike assembly via amidinium−carboxylate interactions in which PTM@CPA5 supramolecules are shifted "up and down" in relation to each other (Figure 4); in addition, adjacent chains are stapled by short carboxylic−carboxylate HBs between CPA5 molecules.
The needlelike crystals of CPA5-PTM complex I were kept in the mother solution in a tightly sealed test tube.The sample was analyzed under the microscope at 1 week intervals.After 2 months the appearance of platelike crystals among the initial needles of complex I was noticed.The single-crystal X-ray diffraction analysis of these crystals revealed the new form II of the host−guest complex with the composition [PTM@ CPA5]•C 2 H 5 OH•9H 2 O in the orthorhombic space group Pbca.The asymmetric unit contains one CPA5, one included PTM, one ethanol, and disordered water molecules (Figure 5).This new form, in addition to having a solvation mode different from that in complex I, is characterized by a somewhat different HB pattern and less symmetric inclusion of PTM in the macrocyclic cavity.Namely, one of the amidinium groups of PTM is closer to the rim of the parent CPA5 manifested in a direct HB contact (N2R•••O6B, 3.02 Å) between the included PTM and parent CPA5.The same amidinium (N1R, N2R) is involved in a d2a1 HB with an adjacent CPA5 and one HB with a water molecule (Figure 5c).The second amidinium (N3R, N4R) forms a paired cyclic HB (d2a2 motif) and two single HBs (d1a1 motif) with two adjacent CPA5 molecules.The number of HBs between PTM and CPA5 (seven HBs per one PTM molecule) is higher than in complex I (five HBs per one PTM molecule).This means that more carboxylate groups of the macrocycle are engaged in the amidinium−carboxylate synthons than in complex I.The difference in the interaction mode is also evident from the HB Crystal Growth & Design between adjacent CPA5 molecules; instead of the very short HB typical for a carboxylic−carboxylate synthon as in complex I, a longer bifurcated HB is formed toward carboxyl and ethoxy oxygen atoms of the adjacent CPA5 (O2A•••O5E and O2A••• O4E, 2.64 and 2.98 Å, respectively).It seems that the CPA5− PTM interactions are more efficient than in complex I, and all deprotonated carboxylate groups of CPA5 are engaged in HBs with amidinium donors.Also, two neighboring carboxylic groups of CPA5 have HBs to an ethanol molecule positioned in the proximity of one of the pillar [5]arene rims.The PTM@ CPA5 supramolecules again assemble into chains, with additional HBs of amidinium−carboxylate and carboxylic− carboxylic type between adjacent chains (Figure 6).
It is unclear what is behind the driving force for the late crystallization of this form II of the host−guest complex.As it is present simultaneously with the crystals of the initial phase I in contact with the mother solution, one of the possibilities is that the initially crystallized complex I is a kinetic product with low solubility that undergoes a very slow solution-mediated transformation into the more stable complex II.We previously observed such a two-step crystallization of the cucurbit [6]uril complexes with a solution-mediated transformation between different crystalline forms of different hydration and host− guest interaction modes. 41,42However, the transformation for cucurbit [6]uril complexes, for example between a kinetic exclusion form and a stable inclusion complex, was faster (several days) and more pronounced (initial form fully dissolved). 39e further monitored the morphology of the host−guest crystals in the sealed crystallization vial under a microscope for a period of several months.Both crystalline forms (needles of complex I and plates of complex II) were preserved.However, after about 3 months from the start of the crystallization experiment we noticed a few prismatic-shaped crystals that looked different from the needles of I and plates of II.These few crystals were manually separated from the mixture of crystals I and II and checked with the diffractometer.A singlecrystal X-ray diffraction analysis revealed another novel form for the host−guest complex III, [PTM@CPA5] 2 •PTM• 11.8H 2 O, in the P2 1 /c space group.The asymmetric unit consists of two CPA5 molecules, each including PTM guests (PTM X and PTM Z), and an additional PTM molecule outside macrocyclic cavities (PTM Y), together with some disordered water molecules (Figure 7a).The stoichiometry of the complex suggests deprotonation of three carboxylic groups per each CPA5, since PTM is in the dicationic form.The inclusion mode of PTM X and Z is similar to that in complex I; there are no intracomplex amidinium−carboxylate HBs as in complex II.Despite being complexed outside the macrocyclic cavity, PTM Y has an extended conformation similar to that of included PTM X and Z; a closer look at its neighborhood shows that it is indeed situated in the lattice cavity formed by the outer walls of four CPA5 molecules (Figure 7b,c).The HB network is quite rich, displaying all types of amidinium− carboxylate supramolecular synthons and HBs with water molecules.PTM X (in green) forms a paired cyclic d2a2 HB with one carboxyl group of the adjacent CPA5 and two water molecules (one amidinium end), one d1a2 bifurcated HB, two single d1a1 HBs and one HB with a water molecule (another amidinium group) (Figure 8a).PTM Z (in dark green) forms one paired cyclic d2a2 HB and one single d1a1 with carboxyl groups of an adjacent CPA5 (one amidinium), d2a1 with carboxyl groups of an adjacent CPA5, and two HBs with water molecules (another amidinium) (Figure 8b).Interestingly, the HB pattern of PTM Z is identical with that in host−guest complex I. PTM Y (in sea green) complexed outside the cavity forms d2a2 and d1a1 HBs with carboxyl groups of an adjacent CPA5 and an water molecule (one amidinium) and d2a1 and two d1a1 HBs with carboxyl groups of three CPA5 molecules (Figure 8c).Although multiple HB interactions between PTM and CPA5 are present, there are still several HBs between  .The crystal packing is characterized by a chainlike assembly further stapled into layers via additional HBs between adjacent chains (Figure 9).On consideration of the bizarre crystallization behavior of the CPA5-PTM system, there are more questions than answers at the present.We repeated the crystallization experiment followed up by a microscopic observation of the crystal morphology and subsequent determination of the unit cells of the selected crystals at the diffractometer.We noticed a very similar behavior of the system (several days of deviation for the appearance of platelike and prismatic crystals of complexes II and III).Such a late sequential crystallization of different forms of host−guest complexes may be related to the following factors or their combined effect.First, there are multiple interaction points between the host (10 carboxyl groups) and guest (2 amidinium groups that are potential donors of 8 HBs); moreover, the amidinium−carboxylate interactions can be intercomplex (preferable) and intracomplex (as in complex II) when the inclusion of PTM into the CPA5 cavity is less symmetrical.Second, the variable protonation/deprotonation modes of 10 carboxylic groups of the pillar [5]arene and some conformational flexibility of these substituents (carboxyl connected to arene via a CH 2 −O linker) contribute significantly to the variability of stoichiometry of the complexation (charge balance) and extra HB network between adjacent pillar [5]arenes, apart from CPA5-PTM host−guest interactions.The third is the role of the solvationethanol is a component of the crystalline assembly in complex II, forming HBs with pillar [5]arene.As was mentioned above, we have previously observed peculiar crystallization behavior and solution-mediated transformations for cucurbit [6]uril host− guest complexes.However, for cucurbit [6]uril systems the   constrictive binding and high-energy water molecules inside the cavity are responsible for the slow kinetics of the complexation and even kinetic trapping of the metastable complexes. 43,44This seems not to be the case for the pillar [5]arenes.Recently, Malinska reported a very detailed study of the temperature and solvent-induced transformations of the calix [6]arene complexes with pyridine, elucidating that both kinetics and thermodynamics play roles in the formation of the various crystal forms. 45Thus, a more general question arises for the host−guest solid-state community.Perhaps we are just not paying enough attention to identify the concomitant or sequential crystallization of the host−guest complexes, especially when the second (or even the third) component is present in a very small amount or the interconversion or sequential crystallization of different host−guest forms takes place over a long time?There is a strong probability that focusing attention on the complexity of host−guest crystallization will significantly expand the library of the available crystal forms of host−guest systems.
To gain more insight into the CPA5−PTM system, we studied the complexation mode in solution (CD 3 OD) via 1 H NMR spectroscopy.The inclusion of the pentanediol central chain of PTM within the CPA5 cavity was observed through the significant broadening and collapsing with the baseline of PTM methylene proton resonances (signals of the 3-, 4-, and 5-protons practically disappear) in the presence of CPA5 (Figure 10).The signal of the aromatic 2-protons is slightly shifted upfield, indicating their residence inside the extended cavity of the host.This nicely corresponds with the crystal structure of the inclusion complexes.The signal γ of the CH 2 groups of the pillar [5]arene side chains splits upon the formation of the host−guest complex with pentamidine.This split of the signal can be associated with the ring current of the proximal phenyl groups of PTM.Thus, the methylene γprotons of CPA5 directed either inside or outside the cavity are shielded or deshielded, respectively.On the basis of the titration experiments (Figure S3) we determined the association constant: K a = 1.02 × 10 3 M −1 (log K a = 3.01).
The association mode of CPA5 and PTM was additionally investigated by means of ion mobility mass spectrometry (IM-MS) supported by theoretical calculations.This combined approach enables a detailed structural description of a molecule or a molecular system to be obtained from the experimental studies of the mobility of ions in a buffer gas under the influence of an electric field.Ion mobility mass spectrometry has become a relevant method to study the structural variety of simple molecules 46 and more sophisticated biological systems 47 as well as to determine the structure and properties of larger noncovalently bonded molecules. 48,49ere, the components of the solution of CPA5 and PTM in MeOH/H 2 O were transferred to the gas phase using an electrospray ionization source and analyzed in the negative ion mode.The full-scan mass spectrum exhibits peaks corresponding to the presence of doubly and triply charged ions of the CPA5 receptor and its complex with PTM as a guest (Figure 11).This result complements the solid-state studies: namely, that CPA5 is doubly deprotonated in the crystalline complexes I and II and is triply deprotonated in the 2:3 host−guest complex III.
Ion mobility mass spectrometry measurements revealed that the CPA5−PTM supramolecular complex displays a narrow and Gaussian-shaped arrival time (AT) distribution, as is shown for its doubly charged ion in Figure 12.The drift time of the doubly charged CPA5−PTM complex was converted into a collision cross section value ( exp CCS N2 ), and this   experimental value was compared to the theoretical CCS value ( t CCS N2 ) of the complex candidate obtained from theoretical calculations (see the Supporting Information).The excellent agreement of exp CCS N2 and t CCS N2 values obtained for the CPA5-PTM complex indeed proves the inclusion nature of this associate.

■ CONCLUSIONS
The inclusion of the cationic drug pentamidine into the macrocyclic cavity of carboxylated pillar [5]arene leads to the crystallization of three forms of host−guest complexes, providing a glimpse into the vivid nature of the host−guest interactions.While strong hydrogen bonds of amidinium− carboxylate and carboxylic−carboxylate types are realized in all of the complexes, rigidifying the assembly, it seems that more subtle effects, such as local solvation/desolvation, a certain degree of flexibility of both the host and guest, and variable protonation/deprotonation states of the pillar [5]arene substituents, play significant roles in the complex crystallization behavior of the system.What can we learn from this work?It appears that the generally assumed fast and direct route from host−guest molecular recognition and self-assembly toward isolation of the single thermodynamic product (crystallization) is not always so obvious.In reality, at least some of the aqueous host−guest systems are not characterized by a single-crystal structure but by ensemble or snapshots of various host−guest crystal forms.These host−guest crystal forms can differ in the inclusion mode, HB network, solvation/hydration, conformation of partner molecules, and even host−guest stoichiometry.This brings challenges in terms of ruling out the principles and conditions for the concomitant or sequential crystallization, as well as subsequent host−guest interconversions.However, it also opens up exciting opportunities in terms of more holistic models of the host−guest systems, shifting from focusing our attention on "visible interactions" in one crystal structure to the appreciation of many subtle effects playing a combined role in the host−guest complexation and crystallization.

■ EXPERIMENTAL SECTION
Carboxylic acid substituted pillar [5]arene was synthesized according to a literature procedure and used in the carboxylic neutral form. 50,51entamidine diisethionate was purchased from Sigma-Aldrich. 1 H NMR spectra were recorded on an Agilent 400 MHz instrument at room temperature.NMR data were analyzed using MestReNova software.
Ion Mobility Mass Spectrometry Measurements.Ion mobility mass spectrometry (IM-MS) measurements were performed on a commercially available Waters quadrupole traveling-wave ion mobility time-of-flight spectrometer (Synapt G2-S HDMS).A mixture of CPA5 (c = 0.025 mM) and PTM (c = 0.015) in MeOH/H 2 O (1/1) was infused through a standard electrospray ion source into the instrument at a flow rate of 20 μL/min.The samples were analyzed in negative ion mode with a capillary voltage of 3.5 kV and a source temperature of 303 K.An ion mobility pressure of 3.65 mbar of nitrogen was used.Three sets of parameters, containing variable values of traveling wave velocity (WV) and pulse height (WH), were applied to determine the standard deviation values for the T-wavedetermined CCS values.All additional experimental details and parameters are given in Table S1.The uncertainties of the determined CCS values associated with different drift tube conditions were estimated at a >90% level of confidence.
Crystallization.A 0.45 mg portion of CPA5 and 3.4 mg of MgCl 2 were dissolved in 1 mL of a 1/1 water/ethanol mixture with gentle heating.A solution of 0.55 mg of pentamidine diisethionate in 0.5 mL of a 1/1 water/ethanol mixture was slowly added to the solution of CPA5.Diffraction-quality needlelike crystals of the host−guest complex I grew after 2 days.After 2 months the appearance of prismatic crystals of complex II among the needles of complex I was noticed.After 3 months from the start of the crystallization experiment the appearance of a few platelike crystals of complex III was noticed.Photomicrographs of the evolution of the crystal morphology with time are presented in the Supporting Information.
X-ray Crystallography.The crystals were embedded in inert perfluoropolyalkylether (viscosity 1800cSt; ABCR GmbH) and mounted using Hampton Research Cryoloops.The crystals were flash-cooled to 100.0(1)K in a nitrogen gas stream and kept at that temperature during the experiments.The X-ray data were collected on a SuperNova Agilent diffractometer using Cu Kα radiation (λ = 1.54184Å).The data were processed with CrysAlisPro. 52Structures were solved by direct methods and refined using SHELXL 53 under WinGX. 54The figures were prepared using Chimera. 55Single-crystal X-ray data and refinement details and CCDC numbers are given in the Supporting Information and Accession Codes.

■ ASSOCIATED CONTENT
* sı Supporting Information

Figure 2 .
Figure 2. Crystallization scheme of three forms of host−guest complexes depending on time.Note that crystals of complex II coexist with crystals I and crystals of complex III coexist with both crystals I and II.Complexes I and II have a 1:1 stoichiometry, but complex II is additionally solvated with ethanol (shown in violet) in addition to water molecules.Complex III has a 2:3 stoichiometry.All complexes differ in the HB pattern between pentamidine and adjacent pillar[5]arenes in the crystal lattice.

Figure 3 .
Figure 3. Host−guest complex I of carboxylated pillar[5]arene CPA5 with pentamidine PTM (in blue) highlighting (a) the inclusion mode, (b) C−H•••π and C−H•••O interactions between the guest and cavity interior, (c) the hydrogen-bonding pattern of included pentamidine with two adjacent pillar[5]arenes and water molecules.

Figure 4 .
Figure 4. (a) Self-assembly of adjacent pentamidine−pillar[5]arene supramolecules into hydrogen-bonded chains in complex I, where adjacent chains are connected by carboxylic−carboxylate hydrogen bonds between pillar[5]arenes.(b) The same two chains viewed along the direction of chain propagation.(c) The crystal packing of complex I along the [001] direction.

Figure 5 .
Figure 5. Host−guest complex II of the carboxylated pillar[5]arene CPA5 with pentamidine PTM (in yellow) and ethanol (in violet) highlighting (a) the inclusion mode, (b) N−H•••O, C−H•••π, and C− H•••O interactions between the guest and cavity interior, and (c) hydrogen-bonding pattern of the included pentamidine with the parent pillar[5]arene, three adjacent pillar[5]arenes, and a water molecule.

Figure 6 .
Figure 6.(a) Self-assembly of adjacent pentamidine−pillar[5]arene supramolecules into hydrogen-bonded chains in complex II, where adjacent chains are connected by carboxylic−carboxylic and amidinium−carboxylate HBs.(b) The same two chains viewed along the direction of chain propagation.(c) The crystal packing of complex II along the [100] direction.

Figure 7 .
Figure 7. Host−guest complex III of the carboxylated pillar[5]arene CPA5 with pentamidine PTM (in green, dark green, and sea green), highlighting (a) the inclusion mode, (b) the exo-mode binding of PTM Y (in sea green) in the lattice cavity formed by the outer walls of four CPA5 molecules, (c) hydrogen bonding pattern of exocomplexed PTM Y with adjacent pillar[5]arenes.

Figure 8 .
Figure 8. Hydrogen-bonding patterns of three crystallographically independent pentamidine guests in complex III: (a) included pentamidine X, (b) included pentamidine Z (note that the HB pattern is identical with that in the host−guest complex I), and (c) exo-complexed pentamidine Y.

Figure 9 .
Figure 9. (a) Self-assembly of adjacent pentamidine−pillar[5]arene supramolecules into hydrogen-bonded chains in complex III.(b) The same two chains viewed along the direction of chain propagation.(c) The crystal packing of complex III along the [001] direction.Three crystallographically independent pentamidines are shown in green, dark green, and sea green.

Figure 11 .
Figure 11.Negative ion mode ESI-Q1 mass spectrum of a mixture of CPA5 with pentamidine.The isotopic pattern of Complex 2− is also shown.

Figure 12 .
Figure 12. (left) Mobility separation spectrum of the selected and extracted doubly charged ion of CPA5−PTM complex recorded at a wave velocity (WV) of 600 m/s and a wave height (WH) of 25 V.(right) Structure of the complex obtained from theoretical studies (PM7) and its calculated collision cross-section value.