REPRINT - ICN Center

REPRINT

z Organic & Supramolecular Chemistry

Cleft-Induced Ditopic Binding of Spherical Halides with aHexaurea ReceptorBobby Portis,[a] Ali Mirchi,[a] Mohammad H. Hasan,[b] Maryam Emami Khansari,[a]

Corey R. Johnson,[a] Jerzy Leszczynski,[a] Ritesh Tandon,[b] and Md. Alamgir Hossain*[a]

A tripodal-based hexaurea receptor with two clefts (inner cleftand outer cleft) has been studied for spherical halides by1HNMR, UV-Vis titrations, and theoretical calculations usingdensity functional theory (DFT). As demonstrated from exper-imental and computational results, the receptor exhibits cleft-induced binding for halides in a 1 :2 binding mode, showingthe binding strength in the order of fluoride > chloride >

bromide > iodide. The strongest affinity for fluoride isattributed due to the best fit of two fluoride anions at the twoclefts within the host‘s cavity. This is also supported by DFTcalculations, suggesting that each anion is stabilized by sixNH⋅⋅⋅F bonds - one at the inner cleft and other at the outer cleft.In addition, the synthesized receptor, as examined on HeLacells, has been found to show good biocompatibility.

Introduction

Since anions are crtical species in the environment and life, thestudy of anion binding with chemical receptors remains anactive area of research in supramolecular chemistry.[1–8] Inparticular, molecular compounds functionalized with ureagroups have been shown to effectively bind inorganic anionsunder neutral conditions,[9–16] allowing them to be utilized forpotential applications such as extractions,[17–19] transport,[20]

catalysis,[21] and antiangiogenic activities relating to certainliving cells.[22] A number of tripodal-based ureas derived fromtris(2-aminoethyl)amine (tren) have been reported recently,showing complementary binding for various anions in solutionand solid states.[14–17] Halides are of special interest because oftheir relevance in biological and environmental systems.[23–28]

For instance, chloride is known as a major electrolyte thatregulates the flux of metabolites in living cells and maintainsosmotic balance in human body.[23] The misregulation ofchloride transport across the cell membranes may cause severehuman diseases, such as cystic fibrosis.[24] Fluoride is utilized intoothpaste as well as in city water to benefit the dental health.However, an excess accumulation of fluoride in the humanbody may cause severe health-related problems includingcollagen breakdown, bone disorder, and thyroidmalfunctions.[25] A high level of chloride in drinking water is

linked to the higher risk of lymphoma.[26] During the waterrefinement process, an elevated level of bromide in water mayturn into bromate that is suspected to cause gastrointestinalcancer.[27] In biological processes, iodide blocks the discharge ofthyroid hormones, thus making it beneficial for the treatmentof patients with hyperthyroidism.[28] Although, a number ofsynthetic receptors containing neutral NH groups as bindingsites are available for halide binding,[29–31] comprehensive studyestablishing the correlation between experimental and theoret-ical aspects in host-guest interactions have not been ad-equately explored.[32]

As demonstrated in the literature, the variation of bindingsites, cavities, and linking groups in synthetic receptors largelyimpact on the selectivity and binding mechanism in anioncomplexes.[1,29,30] In particular, increasing the binding sites to areceptor could lead to an improved binding affinity for aguest.[31] In order to increase the binding sites within a singlehost, we recently synthesized hexaurea-based receptors thatshow an excellent selectivity for sulfate.[15] It was structurallyshown that the hexaurea receptor 1 functionalized with m-nitrophenyl substituents forms a C3 symmetric sulfate complexin which the tetrahedral sulfate is encapsulated with twelveNH⋅⋅⋅O bonds. Because of the presence of two distinct clefts(inner and outer) within the framework of 1, we have beeninterested to explore its cleft-induced ditopic binding forspherical halides with variable sizes. Herein we report thehalide binding studies of 1 that binds two halides – eachoccupying a single cleft, exhibiting the cleft-induced bindingstrength as fluoride > chloride > bromide > iodide. Further-more, cytotoxicity studies of 1 have been performed on HeLacells to understand its cell viability.

Results and Discussion

Synthesis: The receptor was synthesized by following thestrategy reported earlier.[15] Attempts to obtain suitable crystalsfor X-ray analysis were unsuccessful.

[a] B. Portis, Dr. A. Mirchi, Dr. M. E. Khansari, C. R. Johnson,Prof. Dr. J. Leszczynski, Prof. Dr. Md. Alamgir HossainDepartment: Chemistry and BiochemistryInstitution: Jackson State University1400 J R Lynch Street, Jackson, MS 39217, USAE-mail: [email protected]

[b] M. H. Hasan, Prof. Dr. R. TandonDepartment: Microbiology and Immunology, Institution: University ofMississippi Medical CenterJackson, MS 39216, USA

Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/slct.201903950

Full PapersDOI: 10.1002/slct.201903950

1401ChemistrySelect 2020, 5, 1401–1409 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Wiley VCH Montag, 27.01.20202004 / 156935 [S. 1401/1409] 1

NMR Studies: 1HNMR studies were performed to evaluatethe interactions of 1 with halides (F@, Cl@, Br@, and I@) as theirtetrabutylammonium salts in DMSO-d6 at room temperature.The receptor exhibited four distinct NH resonances [9.66 (H1),8.18 (H2), 7.97 (H3) and 6.56 ppm (H4)] that are consistent withits C3 symmetric conformation. An initial screening of 1 wasperformed with the addition of 5 equiv. of different halides. Asshown in Figure 2, a noticeable downfield shift of NHresonances was observed after the addition of chloride to 1,while the aromatic protons shifted upfield. The addition offluoride to 1 caused to the broadening of NH resonances,which might be the result of the deprotonation of NH group tohighly basic fluoride anion, resulting into the formation of [email protected] effect is also supported by the sharp colour change of thereceptor in the presence of fluoride (discussed later). Thetitrations of 1 with bromide provided small shift change (Δδ=

0.025 ppm) for one NH proton (H4) of the inner cleft, indicatingan interaction of the host molecule with the guest. Similarly, anegligible change in the proton resonances was observed afterthe addition of iodide to 1. Disappointedly, the calculation ofbinding constants was hampered due to the small and irregularshift change (Fig S8 and S9). We were also unsuccessful in

identifying any noticeable change on the NOESY experimentsin the presence of bromide or iodide. However, UV-Vis titrationsshowed sufficient spectral change to allow calculating bindingconstants for all halides (discussed in the subsequent section).

Figure 3 shows the stacking of 1HNMR titration spectra of 1upon the systematic addition of chloride, showing a progres-sive change in proton resonances. The change in the chemicalshift of NH resonances during the titration was attempted to fitboth a 1 :1 and a 1 :2 binding model separately, and we foundthe best fit of the experimental data using a 1 :2 bindingmodel.[33] This suggests the formation of both a 1 :1 complexand a 1 :2 complex in solution. The calculated bindingconstants are: log K11=2.55 and log K12=3.06. In the case offluoride, CH (aromatic) signals were used to calculate thebinding constants, yielding log K11=2.06 and log K12=3.82. Theoverall binding constants (in log β2) of 1 are 6.67 and 5.58 forfluoride and chloride, respectively. A cyclic hexaurea consistingof four xanthene and two diphenyl ether groups prepared byBöhmer and coworkers was reported to form a 1 :2 complexwith chloride.[34] We previously reported a paraxylene-basedmacrocyclic amine that was shown to form a 1 :2 complex,providing the binding constants (log β2) of 4.05 and 4.00 forfluoride and chloride, respectively in D2O.

[35] However, thenegligible change in the proton resonances was observedduring the titrations with bromide or iodide, hampering thedetermination of binding constants.

UV-Vis Studies: The binding interactions of the receptorwere further studied by UV–Vis titration experiments in DMSOusing halide salts of [n-Bu4N]

+. Previous work has shown thatthe nitrophenyl groups functionalized to urea/thiourea recep-tors could be effective chromophores for optical sensing ofanions.[29,36,37] The receptor 1, due to the presence of nitro-phenyl groups on the outer cleft, showed an absorption bandat λmax=351 nm. After the addition of each of halides to 1, theabsorption was changed with respect to both the intensity andwave length (Figure 4). This change is attributed to theinteraction of a halide ion with 1, suggesting the formation ofan anion complex.

Figure 1. Receptor 1: (a) chemical structure, and (b) electrostatic potentialmap calculated at the M06-2X/6-31G(d,p) level of theory (red=negativepotential, and blue=positive potential).

Figure 2. Partial 1HNMR spectra of 1 (2 x 103 M) in the presence of 5 equiv. ofhalides in DMSO-d6 at room temperature, showing the changes in chemicalshifts.

Figure 3. Partial 1HNMR titration spectra of 1 (2 x 103 M), showing changes inthe NH chemical shifts with an increasing amount of Cl@ in DMSO-d6 at roomtemperature.

Full Papers



The progressive addition of fluoride to 1 in DMSO resultedin a bathochromic (red) shift in the absorption band (351 to358 nm), exhibiting a gradual decrease in the absorptionintensity. An isosbestic point was displayed at 368 nm (Fig-ure 4a). Whereas with other halides, the absorption intensityincreased without shifting the absorption band or yielding anyisosbestic point. An obvious color change from light yellow tored also occurred with the addition of fluoride to 1 (Figure 5),

agreeing the red shift of the absorbance (λmax). However, theaddition of other halides did not make any significant colorchange to the receptor. The change in the absorption intensityof 1 with the gradual addition of a halide was again consistentwith a 1 :2 binding model instead of a 1 :1 binding model.[33] Aslisted in Table 1, the association constants of 1 for 1 :1 bindingare 2.11, 2.58, 2.61, and 2.92 (in log K11) for fluoride, chloride,bromide, and iodide, respectively. While the associationconstants for 1 :2 binding are 4.66, 3.12, 2.71 and 1.12 (in logK12) for fluoride, chloride, bromide, and iodide, respectively. Thehigher binding constants for the second step (K12) as comparedto that for the first step (K11) for fluoride, chloride and bromidedemonstrate that 1 :2 binding is preferred for a small anionthrough a “cooperative” binding process.16 In contrast, thebinding for iodide occurs through a “non-cooperative” proc-ess.16 The overall binding trend (log β2) for 1 :2 complexesfollows the order: fluoride > chloride > bromide > iodide. Thissuggests that the strength of the ditopic binding is primarilydepended on the compatibility of two halides within the cavityas well as the relative basicity of the respective halide, beingthe strongest for fluoride due to the best fitting of two tinyfluoride ions at the two clefts, agreeing with the results of DFTcalculations (discussed later). Furthermore, this binding orderindicates that two larger anions in a single cavity could

Figure 4. UV-Vis titraion spectra of 1 (1.5 x 104 M) for fluoride (a), bromide (b), chloride (c), and iodide (d) in DMSO at room temperature. The titration curvesare shown in insets (λmax=320 nm).

Figure 5. Colorimetric studies of 1 (2 mM) with one equivalent of differenthalides in DMSO, showing a colour change for fluoride.

Full Papers



experience a stronger electrostatic repulsion, thereby weaken-ing the stability of a 1 :2 complex with a larger guest. Incontrast, the receptor forms a 1 :1 complex, exhibiting thebinding order as iodide > bromide > chloride > fluoride. Thisdemonstrates that the 1 :1 binding mode is preferred for alarger anion. The overall binding constants (log β2 =

log11*log12) of 1 for halides that are 6.77 (fluoride), 5.58(chloride), 5.32 (bromide) and 4.04 (iodide), are in goodagreement with the results of DFT calculations.[38,39]

Computational Studies: In order to predict the energiesand the structural aspects in the halide-bound complexes, wecarried out computational calculations using density functionaltheory (DFT) and excited-state time-dependent density func-tional theory (TD-DFT). All theoretical calculations were per-formed using DFT method with hybrid meta-exchange correla-tion functional M06-2X,[40–42] available in the Gaussian 09package.[43] It has been shown that the M06-2X functional isuseful to predict the non-covalent interactions.[15,16,32,35] For thecomplexes of fluoride, chloride and bromide, the double-ζbasis set, 6–31G(d,p) was used, while the pseudopotential basisset LANL2DZ was used for iodide complex. The Barone-Tomasipolarizable continuum model (PCM) was used to calculateenergies in the environment of DMSO solvent. In order tocompare the experimental results, we continued to optimizeanion complexes by placing one halide and two halidesseparately within the cavity of the optimized receptor, andobtained structures of both 1 :1 and 1 :2 complexes for halides.The H-bonding interactions for 1 :1 stoichiometric complexes([1(F)] @, [1(Cl)] @, [1(Br)] @ and [1(I)]@) and 1 :2 complexes([1(F)2]

2 @, ([1(Cl)2]2 @, ([1(Br)2]

2 @ and ([1(I)2]2 @) are provided in

supporting information. The comparative binding energieswere calculated with zero-point correction for gas phase andsolvent-phase (DMSO) energies based on the single pointcalculations. We have performed calculations on both 1 :1 and1 :2 complexes (receptor:halide) for each halide (X@=F@, Cl@,Br@ or I@) both in gas and solvent phases. The binding energiesof 1 were calculated for both 1 :1 binding and 1 :2 bindingusing the Eq. 1 and Eq. 2, respectively shown below:

DE11 ¼ EðcomplexÞ - ½EðreceptorÞ þ EðhalideÞ� (1)

DE12 ¼ EðcomplexÞ - ½EðreceptorÞ þ 2EðhalideÞ� (2)

Receptor 1: As demonstrated by the optimized structure of1 with an empty cavity, three identical arms (ortho-phenylene-bridged) linked to the tren backbone, lead to the receptoradopting an ideal C3 symmetric cone shape consisted of aninner cleft and an outer cleft. As shown in Figure 1b, the innercleft is decorated with three NH⋅⋅⋅O intra-molecular H-bondinginteractions with an equidistance of NH⋅⋅⋅O=3.14 Å. Within theinner cleft, each oxygen atom of one arm is hydrogen bondedto a NH group of an adjacent arm, with NH protons pointinginside the cavity, thus leading the receptor preorganized foranion binding. Similar intra-molecular H-bonding interactionswere characterized crystallographically[44] or predictedtheoretically[20] for tren-based urea receptors. However, suchinteractions are absent on the outer cleft. The electrostaticpotential surface of the free 1 was calculated at the M06-2X/6-31G(d,p) level of theory, exhibiting that the most electrostaticpositive potential is created at the core of each cleft (Figure 1b),thereby making the molecule potential as a ditopic anionreceptor.

1:1complexes: The structures of 1 : 1 halide complexes of 1,as obtained from the DFT calculations, are shown in Figure 6.H-bonding parameters of the corresponding complexes areprovided in Supporting Information. The structural geometriesof each complex reveal that the receptor is organized toencapsulate each of halides retaining its C3 conformation. Withthe exception of the fluoride complex, all NH binding sites ineach of the three complexes (chloride, bromide and iodidecomplexes) are pointed towards the center forming a total oftwelve H-bonds. [dH⋅⋅⋅Cl =2.281–2.701Å, dN⋅⋅⋅Cl =3.283–3.598Å,dH⋅⋅⋅Br =2.416–2.818Å, dN⋅⋅⋅Br =3.413–3.669 3.598Å, dH⋅⋅⋅I =2.623–3.207 Å and dN⋅⋅⋅I =3.633–3.857Å]. The higher number of H-bonds with anions was previously reported in literature.[45,46] Forthe fluoride complex, the encapsulated fluoride locating at thecenter of the cavity is held with six H-bonds from thesurrounding NH binding sites (three from the inner cleft andthree from the outer cleft) [dH⋅⋅⋅F =1.717–1.884Å, dN⋅⋅⋅F =2.721–2.826Å] instead of twelve H-bonds revealed for the other halideions. Due to the smaller size of fluoride, other six NH groups(N2, N6, N10, N5, N9 and N13) linked at the far end of the o-xylene spacers are not involved in H-bonding interactions. Ineach of the halide complexes, the terminal aromatic groups arestacked to each other through CH⋅⋅⋅π interactions.

In gas phase, the calculated binding energies for 1 :1complexes are @168, @96, @93 and @59 kcal/mol for fluoride,chloride, bromide and iodide, respectively. While, in solvent,the corresponding energies are found to be @99, @44, @16,@12 kcal/mol, which is due to the polarity included in thecalculations. Thus the binding energies (ΔE11) for 1 :1 com-plexes in both gas phase and solution follow the order offluoride > chloride > bromide > iodide, correlating with therelative basicity of the respective halide (Table 2).

1:2complexes: The optimized structures of 1 :2 halidecomplexes of 1 are displayed in Figure 7. In the fluoridecomplex, each cleft (inner or outer) is occupied by a singlefluoride with six H-hydrogen bonds in an ideal C3 symmetricconformation. The hydrogen bonding parameters (SupportingInformation) for the complex are in the range of dH⋅⋅⋅F =1.661–

Table 1. Binding data of 1 for halides

Anion Log K11 Log K12 Log β2 (β2 = K1K2)

Fluoride 2.06(3) bc 4.61(4) bc 6.67(4) bc

2.11(3) 4.66(3) 6.77(3)Chloride 2.55(2) b 3.03(2) b 5.58(2) b

2.58(2) c 3.12(3) c 5.70(3) c

Bromide 2.61(3) c 2.71(3) c 5.32(3) c

Iodide 2.92(4) c 1.12(2) c 4.04(3) c

a uncertainties of the binding constants are based on different spectraldata; b 1HNMR titrations; c UV-Vis titrations.

Full Papers



1.969Å, dN⋅⋅⋅F =2.648–2.875Å and are comparable to thatobserved theoretically (NH⋅⋅⋅F=2.677 to 2.822 Å)[32,47] or exper-imentally (NH⋅⋅⋅F=2.584(3) to 2.724(3) Å)[48] for fluoride com-plexes. The binding of two fluorides by a single host waspreviously reported in an azacryptand[48] or a quinoline-basedthiourea receptor.[47] The H-bonding patterns in the 1 :2complexes of chloride, bromide and iodide are different thanthat found in the fluoride complex. In the complexes of

chloride and bromide, the inner halide is bound by ninehydrogen bonds with NH groups, six from the inner cleft andthree from the outer cleft [dH⋅⋅⋅Cl =2.303–2.775Å, dN⋅⋅⋅Cl =3.220–3.649 Å, dH⋅⋅⋅Br = 2.461–2.843Å and dN⋅⋅⋅Br =3.367–3.724Å]. Thedistances between the halides and the tertiary nitrogen (N1)are 4.909, 4.870 and 4.135 Å in the chloride, bromide andfluoride complexes, respectively. The longer distance from N1to the chloride or bromide, as compared to the corresponding

Figure 6. Perspective views of the optimized structures of 1 : 1 complexes: (a) [1(F)]@, (b) [1(Cl)]@, (c) [1(Br)]@ and (d) [1(I)]@.

Full Papers



distance observed in the fluoride complex, could be the effectof the relative anionic size and the additional three NH⋅⋅⋅anioninteractions from the outer cleft. On the other hand, the secondhalide is bound by six H-bonds from three NH⋅⋅⋅anion

interactions of the outer cleft [dH⋅⋅⋅Cl =2.171–2.250Å,dN⋅⋅⋅Cl =3.194–3.243Å, dH⋅⋅⋅Br = 2.349–2.433Å and dN⋅⋅⋅Br =3.368 –3.421Å] and three CH⋅⋅⋅anion interactions of the terminalaromatic groups [dH⋅⋅⋅Cl =2.721–2.766Å, dC⋅⋅⋅Cl =3.636–3.657Å,dH⋅⋅⋅Br = 2.795–2.908Å and dC⋅⋅⋅Br =3.737–3.811Å]. The CH⋅⋅⋅aniondistances (dC⋅⋅⋅anion) are shorter than the sum of the van derWaals radii of CH and chloride (4.12Å) or bromide (4.27 Å),[49]

demonstrating the CH⋅⋅⋅halide interactions in the anion-com-plex. The iodide complex is found to be structurally deformedwhere each iodide is unsymmetrically bound. The inner iodideis bound by eight NH⋅⋅⋅I bonds, while the outer iodide is boundwith three NH⋅⋅⋅I and two CH⋅⋅⋅I bonds.

The overall binding energies (ΔE12) for 1 :2 complexes arefound to be @258, @112, @113 and @76 kcal/mol for fluoride,chloride, bromide and iodide, respectively in gas phase (zeropoint energy), which are higher than those for 1 :1 complexes(Table 2). In PMC model, the corresponding binding energiesfor 1 :2 complexes are @169, @59, @36, @33 kcal/mol, ascompared to those for 1 :1 complexes with @99, @44, @16,@12 kcal/mol for fluoride, chloride, bromide and iodide,respectively. The trend of total binding energies (ΔE12) for 1 :2complexes which is in the order of fluoride > chloride >

Table 2. Thermodynamic parameters for 1 : 1 and 1 :2 binding mode of thecomplexes in the gas phase and binding energies for PCM model.a

Complex ΔE, kcal/mol (gasmodel)

ΔE(ZPE),a

kcal/mol(gas model)

ΔH, kcalmol@1 (gasmodel)

ΔΔG,kcalmol@1

(gasmodel)

ΔE, kcal/mol (PCMmodel)

[1(F)]@ -169 -168 -169 -159 -99[1(Cl)]@ -96 -96 -97 -86 -44[1(Br)]@ -93 -93 -93 -85 -16[1(I)]@ -59 -59 -59 -54 -12[1(F)2]2@ -260 -258 -259 -245 -169[1(Cl)2]

2@ -112 -112 -113 -98 -59[1(Br)2]

2@ -111 -113 -112 -96 -36[1(I)2]2@ -75 -76 -75 -62 -33

a For comparison, the binding energies were also calculated at ZPE (zeropoint energy) in the gas phase.

Figure 7. Perspective views of the optimized structures of 1 : 2 complexes: (a) [1(F)2]2@, (b) [1(Cl)2]2@, (c) [1(Br)2]2@ and (d) [1(I)2]2@.

Full Papers



bromide > iodide, is in agreement with the overall bindingconstants (β2, where β2 = K1K2) observed experimentally. Thehigher binding energies for 1 :2 complexes than thosepredicted for 1 :1 complexes with halides, demonstrate that a1 :2 complex is stronger than a corresponding 1 :1 complex,supporting experimental data. However, the successive bindingenergies (ΔE12 - ΔE11) which are @70, @15, @20 and @21 kcal/mol for fluoride, chloride, bromide and iodide, respectively inPMC model, do not directly correlate with the solution data forthe stepwise binding constants, K2 (Table 1). This could be dueto the fact that the optimized 1 :1 and 1 :2 complexes weremodeled by placing one halide and two halides separatelywithin the receptor, thus the binding mechanism in theoptimized complexes may be different than those observedexperimentally where anions are added progressively in orderto complete the stepwise binding process.

In order to understand if the binding energies correlate tothe Gibbs energy change (ΔΔG) and enthalpy change (ΔH) dueto the complexation, we also performed further calculations onboth 1 :1 and 1 :2 binding modes in the gas phase. As listed inTable 2, both the Gibbs free energy and the enthalpy for 1 :1and 1 :2 complexes nicely correlate to the binding energies(ΔE), as well as to the respective overall binding constants(Table 1). The receptor shows the highest Gibbs free energychange and enthalpy change for the complexation withfluoride for both 1 :1 and 1 :2 binding mode; while upon thebinding with iodide, it exhibits the lowest Gibbs free energychange and enthalpy change for 1 :1 and 1 :2. As shown inTable 2, the Gibbs free energy change (ΔΔG) for 1 :1 bindingare found to be @159, @86, @85 and @54 kcal/mol, while thatfor 1 :2 binding are @245, @98, @96 and @62 kcal/mol forfluoride, chloride, bromide and iodide, respectively. Interest-ingly, the calculated stepwise Gibbs energy change for thesecond step reaction (ΔΔG12 - ΔΔG11) which are @86, @12,@11 and @8 kcal/mol for fluoride, chloride, bromide and iodide,respectively, directly correlate to the solution data for thestepwise binding constants (K2) displayed in Table 1, as theGibbs energy change links to the entropy change uponcomplexation. Furthermore, the cooperative effect is based onthe preorganization of the receptor by the binding of the firstanion, which effectively decreases the entropy loss upon thebinding of the second one.

Time-dependent density functional theory (TD-DFT): Wealso performed TD-DFT calculations to understand the colordeveloped for the fluoride complex during the colorimetricstudies. We previously showed that a complete exchange atasymptotic distances is essential to predict excitations in anioncomplexes.[37,50] Figure 8 shows the absorption spectra for thefree receptor 1, [1(F)]@ and [1(F)2]2@ calculated for the lowest10 excited states in DMSO solvent.

Examining the absorption spectra of the free receptor andthe complexes, it is evident that the 1 :2 complex has thehighest absorption wavelength (red shift) with the strongestoscillator strength in comparison with the free receptor or the1 :1 complex. The highest wavelength for the 1 :2 complexcompared to that for 1 and 1 :1 fluoride complex could be dueto the increased number of H-bonding interactions of the

receptor with two fluorides. To gain further insight into thechanges in charges, a charge-density difference analysis wasperformed for the first electronic excited state of [1(F)]@ and[1(F)2]2@. Figure 9 shows the charge-density differences for[1(F)]@ and [1(F)2]

2@, illustrating that the complexation isaccompanied by the charge transfer between the nitro groupsand the aromatic rings. Furthermore, due to the higher chargein the 1 :2 complex (provided by two encapsulated F@), a largerpositive potential is created in the 1 :2 complex (Figure 9b)compared to the observed positive potential in the 1 :1complex (Figure 9a)

Live Cell Imaging and Cytotoxicity Assay: The receptor 1was evaluated for its biocompatibility using HeLa cells.[51] In ourprevious work, we have shown that most of the cytotoxiceffects on living cells with synthetic receptors can be achievedby maintaining an optimal duration of 24 h.[37,52] During thisexperiment, HeLa cells were seeded in a 12-well plate. Afterincubating for 24 h, the media was replaced with freshlyprepared media. The receptor was then dissolved in DMSO andfilter-sterilized. The monolayer of HeLa cells was then treatedwith 1 varying the concentrations from 10 to 500 μM for 24 h.During this assay, the treated HeLa cells absorbed about 0.1%of DMSO. Thus, as a control, the cells were treated with 0.1%DMSO for 24 h. Bright-field images were acquired for livingcells using an inverted Evos-FL microscope. As displayed inFigure 10, no dead cells were found up to 250 μM of 1.

Figure 8. Absorption spectra calculated with TD-DFT for 1, [1(F)]@ and[1(F)2]2@ in DMSO solvent.

Figure 9. Charge density difference of (a) 1 : 1, and (b) 1 :2 fluoride complexesof 1.

Full Papers



However, when the cells were treated with 500 μM of 1,significant numbers of dead cells were observed under themicroscope (a single dead cell is shown on a 500 μM panelwith an arrow in Figure 10).

To further evaluate the biocompatibility of the receptor,quantitative cell viability studies were performed on HeLa cellsthat were treated with 1 varying the concentration from 10 to500 μM for 24 h. The cell viability was determined using atrypan blue exclusion assay[53] and quantified on a TC20automated cell counter[54] (BioRad Inc). As shown in Figure 11,the treated cells remained alive up to 250 μM concentration,suggesting that the probe 1 has no significant impact on cell

viability up to this concentration. However, significant cytotox-icity was observed at a higher concentration of 1 (500 μM).These results confirm that the receptor exhibits no cytotoxicityin mammalian cells in the concentration range of 10–250 μM,demonstrating good biocompatibility of the probe that couldbe used in living cells.

Conclusions

We have reported that a hexaurea-based tripodal receptorappended with m-nitrophenyl groups effectively binds each ofhalides, forming both 1 :1 and 1 :2 complexes. Specially, thereceptor exhibits strong selectivity for fluoride over otherhalides, displaying a visible color change. Results from thetitrations studies demonstrate that the receptor exhibits cleft-induced binding for halides to form 1 :2 complexes, showingassociation constants (K12) for halides in the order of fluoride >chloride > bromide > iodide. The highest affinity for fluoride isdue to the best compatibility of two smaller anions within asingle cavity. Furthermore, the higher basicity of fluoride ascompared to other halides also plays a role in the fluoridecomplex being the strongest in this series.

In contrast, 1 : 1 association constants (K11) increase withincreasing of the anionic sizes. The overall association constants(β2) follow the same order as observed for 1 :2 association,reflecting that the binding occurred at the two clefts, eachoccupying a single cavity. The formation of halide complexes isalso supported by DFT calculations, showing the highestbinding energy for the complex of 1 with two fluoride ions,each residing at one cleft (inner or outer) through six NH⋅⋅⋅Fbonds. The receptor was also found to exhibit an attractive cellviability on HeLa cells, showing almost no toxicity up to250 μM. Taken together, these results obtained with a hexa-

Figure 10. Bright-field images of HeLa cells treated with 1. Cells were either mock-treated (0.1% DMSO-treated control) or treated with 1 for 24 h at theconcentrations specified. The black arrow in 500 μM panel points towards a single dead cell.

Figure 11. Viability of HeLa cells treated with increasing concentrations of 1(0-500 μM) for 24 h. Error bars represent the standard error of a mean fromthree independent experiments.

Full Papers



functional urea receptor for halide binding using both exper-imental and theoretical tools could serve as a prototype for thedesign of new multifunctional receptors for selective binding ofanions.

Supporting Information Summary

Experimental section, spectroscopic data of 1, additionaltitration plots, DFT calculated hydrogen bonding interactionsof 1 with halides and Cartesian coordinates are provided in thesupporting information.

Acknowledgements

The project described was supported by the US Department ofDefense (Grant Number W911NF-19-1-0006). Computational workdescribed in this paper is supported by the grants from NationalScience Foundation (NSF/CREST HRD-1547754 and DMR110088).Analytical core facility at Jackson State University was supportedby the National Institutes of Health (G12RR013459). Biologicalwork described in this paper is supported from American HeartAssociation (Award No. 14SDG20390009).

Keywords: DFT calculations · Halide binding · Hexaurea · Host-guest chemistry · Molecular modelling

[1] K. Bowman-James, A. Bianchi, E. García-España, Anion coordinationchemistry, John Wiley & Sons, New York, 2011; pp 321–361.

[2] P. A. Gale, E. N. W. Howe, X. Wu, Chem. 2017, 1, 351–422.[3] M. M. Rhaman, D. R. Powell, M. A. Hossain, ACS Omega 2017, 2, 7803–

7811.[4] K. I. Assaf, M. S. Ural, F. Pan, T. Georgiev, S. Simova, K. Rissanen, D. Gabel,

W. M. Nau, Angew. Chem. Int. Ed. 2015, 54, 6852–6856; Angew. Chem.2015, 127, 6956–6960.

[5] B. A. Moyer, R. Custelcean, B. P. Hay, J. L. Sessler, K. Bowman-James,V. W. Day, S.-O. Kang, Inorg. Chem. 2012, 52, 3473–3490.

[6] S. A. Haque, M. A. Saeed, A. Jahan, J. Wang, J. Leszczynski, M. A. Hossain,Comments Inorg. Chem. 2016, 36, 305–326.

[7] N. Busschaert, C. Caltagirone, W. Van Rossom, P. A. Gale, Chem. Rev.2015, 115, 8038–8155.

[8] J. W. Steed, Chem. Soc. Rev. 2009, 38, 506–519.[9] V. Amendola, L. Fabbrizzi, L. Mosca, Chem. Soc. Rev. 2010, 39, 3889–

3915.[10] M. E. Khansari, K. D. Wallace, M. A. Hossain, Tetrahedron Lett. 2014, 55,

438–440.[11] P. Bose, R. Dutta, S. Santra, B. Chowdhury, P. Ghosh, Eur. J. Inorg. Chem.

2012, 2012, 5791–5801.[12] R. Custelcean, B. A. Moyer, B. P. Hay, Chem. Commun. 2005, 5971–5973.[13] I. Ravikumar, P. Lakshminarayanan, M. Arunachalam, E. Suresh, P. Ghosh,

Dalton Trans. 2009, 4160–4168.[14] H. Xie, S. Yi, S. Wu, J. Chem. Soc. Perkin Trans. 2 1999, 2751–2754.[15] B. Portis, A. Mirchi, M. Emami Khansari, A. Pramanik, C. R. Johnson, D. R.

Powell, J. Leszczynski, M. A. Hossain, ACS Omega 2017, 2, 5840–5849.[16] M. E. Khansari, A. Mirchi, A. Pramanik, C. R. Johnson, J. Leszczynski, M. A.

Hossain, Sci. Rep. 2017, 7, 6032.[17] B. Akhuli, I. Ravikumar, P. Ghosh, Chem. Sci. 2012, 3, 1522–1530.[18] D. J. White, N. Laing, H. Miller, S. Parsons, P. A. Tasker, S. Coles, Chem.

Commun. 1999, 2077–2078.[19] N. Busschaert, L. E. Karagiannidis, M. Wenzel, C. J. Haynes, N. J. Wells,

P. G. Young, D. Makuc, J. Plavec, K. A. Jolliffe, P. A. Gale, Chem. Sci. 2014,5, 1118–1127.

[20] M. E. Khansari, C. R. Johnson, I. Basaran, A. Nafis, J. Wang, J. Leszczynski,M. A. Hossain, RSC Adv. 2015, 5, 17606–17614.

[21] S. Lin, E. N. Jacobsen, Nat. Chem. 2012, 4, 817–824.[22] A. Pantho, M. Price, A. Ashraf, U. Wajid, M. Khansari, A. Jahan, S. Afroze,

M. Rhaman, C. Johnson, T. Kuehl, Int. J. Environ. Res. Public Health 2017,14, 517.

[23] A. P. Davis, D. N. Sheppard, B. D. Smith, Chem. Soc. Rev. 2007, 36, 348–357.

[24] M. H. Akabas, J. Biochem. 2000, 275, 3729–3732.[25] J. Colquhoun, Perspect. Biol. Med. 1997, 41, 29–44.[26] R. D. Morris, Environ. Health Perspect. 1995, 103, 225–231.[27] T. P. Bonacquisti, Toxicology 2006, 221, 145–148.[28] M. Andersson, M. B. Zimmermann, in Thyroid Function Testing, Springer,

New York, 2010, pp. 45–69.[29] M. Cametti, K. Rissanen, Chem. Commun. 2009, 2809–2829.[30] M. A. Hossain, R. A. Begum, V. W. Day, K. Bowman-James in

Supramolecular Chemistry: From Molecules to Nanomaterials, Vol 3 (Eds.:J. W. Steed, P. A. Gale), Willey, West Sussex, 2012, pp 1153–1178

[31] M. J. Langton, C. J. Serpell, P. D. Beer, Angew. Chem. Int. Ed. 2016, 55,1974–1987; Angew. Chem. 2016, 128, 2012–2026.

[32] A. Pramanik, D. R. Powell, B. M. Wong, M. A. Hossain, Inorg. Chem. 2012,51, 4274–4284.

[33] M. J. Hynes, J. Chem. Soc. Dalton Trans. 1993, 311–312.[34] D. Meshcheryakov, V. Böhmer, M. Bolte, V. Hubscher-Bruder, F. Arnaud-

Neu, H. Herschbach, A. Van Dorsselaer, I. Thondorf, W. Mögelin, Angew.Chem. Int. Ed. 2006, 45, 1648–1652; Angew. Chem. 2006, 118, 1679–1682.

[35] L. Ahmed, M. M. Rhaman, J. S. Mendy, J. Wang, F. R. Fronczek, D. R.Powell, J. Leszczynski, M. A. Hossain, J. Phys. Chem. A 2015, 119, 383–394.

[36] C. R. Bondy, P. A. Gale, S. J. Loeb, J. Am. Chem. Soc. 2004, 126, 5030–5031.

[37] M. Emami Khansari, M. H. Hasan, C. R. Johnson, N. A. Williams, B. M.Wong, D. R. Powell, R. Tandon, M. A. Hossain, ACS Omega 2017, 2, 9057–9066.

[38] M. Boiocchi, L. Del Boca, D. E. Gómez, L. Fabbrizzi, M. Licchelli, E.Monzani, J. Am. Chem. Soc. 2004, 126, 16507–16514.

[39] K. R. Dey, B. M. Wong, M. A. Hossain, Tetrahedron Lett. 2010, 51, 1329–1332.

[40] Y. Zhao, D. G. Truhlar, Chem. Phys. Lett. 2011, 502, 1–13.[41] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215–241.[42] Y. Zhao, D. G. Truhlar, Acc. Chem. Res. 2008, 41, 157–167.[43] M. J. Frisch Gaussian 09, Gaussian, Inc., Wallingford CT, 2009.[44] A. Pramanik, B. Thompson, T. Hayes, K. Tucker, D. R. Powell, P. V.

Bonnesen, E. D. Ellis, K. S. Lee, H. Yu, M. A. Hossain, Org. Biomol. Chem.2011, 9, 4444–4447.

[45] K. Bowman-James, Acc. Chem. Res. 2005, 38, 671–678.[46] G. Shi, Z. A. Tehrani, D. Kim, W. J. Cho, I.-S. Youn, H. M. Lee, M. Yousuf, N.

Ahmed, B. Shirinfar, A. J. Teator, Sci. Rep. 2016, 6, 30123.[47] I. Basaran, M. E. Khansari, A. Pramanik, B. M. Wong, M. A. Hossain,

Tetrahedron Lett. 2014, 55, 1467–1470.[48] M. A. Hossain, M. A. Saeed, A. Pramanik, B. M. Wong, S. A. Haque, D. R.

Powell, J. Am. Chem. Soc. 2012, 134, 11892–11895.[49] B. S. Van der Waals Radii, Inorg. Mater. 2001, 37, 871–885.[50] S. A. Haque, R. L. Bolhofner, B. M. Wong, M. A. Hossain, RSC Adv. 2015, 5,

38733–38741.[51] R. I. Freshney, Culture of animal cells: a manual of basic technique„ John

Wiley and Sons, Hoboken, New Jersey, 2005, pp 37–56.[52] M. M. Rhaman, M. H. Hasan, A. Alamgir, L. Xu, D. R. Powell, B. M. Wong,

R. Tandon, M. A. Hossain, Sci. Rep., 2018, 8, 286.[53] W. Strober, Curr. Protoc. Immunol. 2001, 21, A.3B.1–A.3B.2.[54] M. A. Archer, T. M. Brechtel, L. E. Davis, R. C. Parmar, M. H. Hasan, R.

Tandon, Sci. Rep. 2017, 7, 46069.

Submitted: October 21, 2019Accepted: January 16, 2020

Full Papers



REPRINT - ICN Center

Documents

Transcript of REPRINT - ICN Center