Journal of Molecular Structure 917 (2009) 110–116

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Design of a new foldamer turn-linker-turn in acyclic hexapeptides and formationof channels through self-assembly

Arpita Dutta a, Sudeshna Kar a, Michael G.B. Drew b,*, Pradyot Koley a, Animesh Pramanik a,*

a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009, Indiab School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK

a r t i c l e i n f o

Article history:Received 6 May 2008Accepted 7 July 2008Available online 13 July 2008

Keywords:Peptideb-TurnFoldamerSelf-assemblyChannels

0022-2860/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.molstruc.2008.07.001

* Corresponding authors. Tel.: +91 33 2484 1647; fE-mail addresses: m.g.b.drew@reading.ac.uk (M.G

yahoo.co.in (A. Pramanik).

a b s t r a c t

Single crystal X-ray diffraction studies and solvent dependent NMR titration reveal that the designedpeptides I and II, Boc-Xx(1)-Aib(2)-Yy(3)-NH(CH2)2NH-Yy(3)-Aib(2)-Xx(1)-Boc, where Xx and Yy areIle and Leu in peptide I and Leu and Val in peptide II, respectively, fold into a turn-linker-turn (T-L-T) con-formation both in the solid state and in solution. In the crystalline state the T-L-T foldamers of peptide Iand II self-assemble to form a three-dimensional framework of channels. The insides of the channels arehydrophilic and found to contain solvent CHCl3 hydrogen bonded to exposed C@O of Aib located at theturn regions.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

F b-turn causes a reversal in the direction of the peptide back-bone, which was first recognized in the late 1960s by Venkatacha-lam [1]. b-Turns are common structural motifs comprising up to25% of all residues in folded proteins and peptides [2–4]. They playimportant roles in stabilizing tertiary structures, initiating foldingand facilitating intermolecular recognition [3]. The surface locali-zation of turns in proteins, and the predominance of residues con-taining potentially critical pharmacophoric information, has led tothe conclusion that turns play critical roles in a myriad of recogni-tion events [5]. Because of their critical importance there has beenconsiderable interest in designing b-turns and b-turn mimetics [6–15]. Recent interest in the conformational properties of b-turnfoldamers [16–18] generated from conformationally constrainedand conformationally flexible amino acid analogues has been stim-ulated by the knowledge that new classes of folded structures canbe formed by backbone homologation [19,20]. Specifically, incor-poration of hybrid sequences containing a- and x-amino acids isinteresting in the rational design of secondary structures [21–25].

Although there are several examples of design and stabilizationof isolated b-turns, designing peptide foldamers of more than oneb-turn structure in small acyclic peptide has not yet been observedin the literature. Therefore we are interested in designing a newpeptide foldamer turn-linker-turn (T-L-T) which will be useful in

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ax: +91 33 2351 9755..B. Drew), animesh_in2001@

designing biologically active peptides. It has been observed thatin tripeptides a fragment of chiral amino acid (1) followed by anachiral amino acid (2), helps to nucleate a b-turn structure [6–13]. Therefore tripeptides Boc-Ile(1)-Aib(2)-Leu(3)-COOH andBoc-Leu(1)-Aib(2)-Val(3)-COOH (Aib: a-amino isobutyric acid),with a helicogenic achiral amino acid Aib(2), [26] are potentiallyturn-forming subunits. To design a T-L-T foldamer, the tripeptideswere connected with a flexible linker 1,2-ethylenediamine (ED).The resulting peptides I and II (Fig. 1), Boc-Xx(1)-Aib(2)-Yy(3)-NH(CH2)2NH-Yy(3)-Aib(2)-Xx(1)-Boc (Peptide I: Xx = Ile, Yy = Leu;II: Xx = Leu, Yy = Val) were synthesized by conventional solution-phase methods and characterized by 1H NMR spectroscopy andmass spectrometry. The solid state structures of the peptides weredetermined by single-crystal X-ray diffraction studies. The solutionphase conformations of the peptides were probed by solventdependent NMR titration and CD spectroscopy.

2. Experimental

2.1. Synthesis of peptides

Peptide I and II were synthesized by conventional solutionphase methodology [27]. Couplings were mediated by dicyclohex-ylcarbodiimide/1-hydroxybenzotriazole (DCC/HOBt). Methyl esterhydrochlorides of peptides were prepared by thionyl chloride–methanol procedure. Methyl ester deprotection was performedvia saponification. All intermediates were characterized by thinlayer chromatography on silica gel and used without further puri-fication. Final peptides were purified by column chromatography

NH

NH

NH

O

O

R1

O

ONH R2

O

NH

O

NH

R2

NH

O

O

NH

OR1

O

H

H

H

H

Peptide I: R1 = -CH(CH3)CH2CH3

Peptide II : R1 = -CH2CH(CH3)2 ; R2 = -CH(CH3)2

; R2 = -CH2CH(CH3)2

Fig. 1. Schematic diagram of peptide I and II.

A. Dutta et al. / Journal of Molecular Structure 917 (2009) 110–116 111

using silica gel (100–200 mesh) as the stationary phase and an eth-ylacetate and petroleum ether mixture as the eluent. The reportedpeptides I and II were fully characterized by X-ray crystallography,NMR, IR and mass spectrometry.

2.1.1. Boc-Ile-Aib-Leu-NH-CH2-CH2-NH-Leu-Aib-Ile-Boc (peptide I)Peptide Boc-Ile(1)-Aib(2)-Leu(3)-COOH [28] (0.96 g, 2.2 mmol)

was dissolved in DMF(3 ml). 1,2-Ethylenediamine (0.06 ml,1.1 mmol) was added followed by DCC (0.68 g, 3.3 mmol) andHOBT (0.30 gm, 2.2 mmol). The reaction mixture was stirred atroom temperature for 5 days. The precipitated dicyclohexylurea(DCU) was filtered off. The filtrate was diluted with ethylacetateand washed with excess of water, 1 M HCl (3 � 30 ml), 1 M Na2CO3

solution (3 � 30 ml) and again with water. The organic layer wasthen dried over anhydrous Na2SO4 and evaporated in vacuo, givinga white solid. Yield: 0.96 g (94.1%). Purification was done usingsilica gel as stationary phase and ethyl acetate–petroleum ethermixture as the eluent. Single crystals were grown from chloro-form–methanol mixture (9:1) by slow evaporation. Mp = 215–217 �C; IR (KBr): 3312, 2965, 1662, 1526 cm�1; dY 300 VYz; CDCl3):7.30 (1,2-Ethylenediamine NH, 2H, t); 7.07 (Leu(3) NH, 2H, d,J = 7.5 Hz); 6.82 (Aib(2) NH, 2H, s); 5.18 (Ile(1) NH, 2H, d,J = 4.8 Hz); 4.37 (CaH of Leu(3), 2H, m); 3.84 (CaH of Ile(1), 2H,m); 3.36 (ACH2A of 1,2-Ethylenediamine, 4H, m); 1.87–1.78 (CbHsof Ile and Leu, 6H, m);1.52 (CbHs of Aib, 12H, s);1.44 (Boc-CH3s,18H, s); 1.12–1.2 (CcHs of Ile and Leu, 12H, m); 0.88–0.97 (CdHsof Ile and Leu, 18H, m); 13C NMR (75 MHz, CDCl3, d ppm):174.18, 172.75, 172.04, 156.67, 80.60, 60.50, 57.19, 52.30, 40.43,39.33, 36.56, 28.28, 25.83, 25.16, 24.90, 22.61, 21.25, 15.61,11.40; HRMS (M + Na+) = 905.61, Mcalcd (M + Na)+ = 906.16.

2.1.2. Boc-Leu-Aib-Val-NH-CH2-CH2-NH-Val-Aib-Leu-Boc (peptide II)Peptide Boc-Leu(1)-Aib(2)-Val(3)-COOH [29] (2.0 g, 4.8 mmol)

was dissolved in DMF (6 ml). 1,2-Ethylenediamine (0.14 ml,2.4 mmol) was added followed by DCC (1.5 g, 7.2 mmol) and HOBT(0.65 gm, 4.8 mmol). The reaction mixture was stirred at roomtemperature for 5 days. The crude product was isolated by usualworkup as it has been done in case of peptide I. Yield: 1.95 g(91.12%). Purification was done using silica gel as stationary phaseand ethylacetate–petroleum ether mixture as the eluent. Singlecrystals were grown from chloroform–methanol mixture (9:1) byslow evaporation. Mp = 181–183 �C; IR (KBr): 3322, 2966, 1660,1518 cm�1; dY 300 VYz; CDCl3, d ppm): 7.25 (1,2-EthylenediamineNH, 2H, t); 7.12 (Val(3) NH, 2H, d, J = 8.4 Hz); 7.05 (Aib(2) NH, 2H,s); 5.23 (Leu(1) NH, 2H, d, J = 6 Hz); 4.24 (CaH of Val(3), 2H, m);

4.06 (CaH of Leu(1), 2H, m); 3.37 (ACH2A of 1,2-Ethylenediamine,4H, m); 2.22 (CbHs of Val, 2H, m); 1.55–1.65 (CbHs of Leu, 4H,m);1.48 (CbHs of Aib, 12H, s);1.43 (Boc-CH3s, 18H, s); 1.2 (CcHsof Leu , 2H, m); 0.85–0.97 (CdHs of Leu and CcHs of Val, 24H, m);13C NMR (75 MHz, CDCl3, d ppm): 174.39, 173.35, 171.64, 156.27,80.56, 58.88, 57.41, 53.99, 40.58, 39.29, 29.90, 28.27, 25.38,24.82, 22.95, 21.74, 19.43, 17.45; HRMS (M + Na+) = 878.30, Mcalcd(M + Na)+ = 878.11.

2.2. FT-IR spectroscopy

IR spectra of peptides I and II were examined using Perkin-El-mer-782 model spectrophotometer. The solid state FT-IR measure-ments were performed using the KBr disk technique.

2.3. NMR experiments

All 1H and 13C NMR spectra of peptides I and II were recordedon Bruker Avance 300 model spectrometer operating at 300 and75 MHz, respectively. The peptide concentrations were 10 mM inCDCl3 for 1H NMR and 40 mM in CDCl3 for 13C NMR.

2.4. Mass spectrometry

Mass spectra of peptides I and II were recorded on HEWLETTPACKARD Series 1100MSD and Micromass Qtof Micro YA263 massspectrometers by positive mode electro spray ionization.

2.5. Circular dichroism spectroscopy

Solutions of peptides I and II in methanol (1.5 mM as final con-centration) were used for obtaining spectra. Far-UV CD measure-ments were recorded at 25 �C with a 0.5 s averaging time, a scanspeed of 50 nm/min, using a JASCO spectropolarimeter (J 720 mod-el) equipped with a 0.1 cm path length cuvette. The measurementswere taken at 0.2 nm wavelength intervals, 2.0 nm spectral bandwidth and five sequential scans were recorded for each sample.

2.6. Single crystal X-ray diffraction study

Crystals of peptides I and II suitable for X-ray diffraction wereobtained from chloroform–methanol (9:1 v/v) solutions by slowevaporation at room temperature (298 K).

Peptide I C46H95Cl6N8O15.5, M = 1223.02, tetragonal, spacegroupP43212, Z = 4, a = 17.5028(4), c = 23.2866(14) ÅA

0

, U = 7133.8(5) ÅA03,

dcalc = 1.139 gcm�3.Peptide II C44H80Cl6N8O9, M = 1101.88, orthorhombic, space-

group P212121, Z = 4, a = 17.3300(14), b = 18.6703(15),c = 19.2785(16) ÅA

0

, U = 6237.7(9) ÅA03, dcalc = 1.173 gcm�3.

10284, 15633 independent data were collected with MoKaradiation at 150 K using the Oxford Diffraction X-Calibur CCD Sys-tem. The crystals were positioned at 50 mm from the CCD. Frames(321) were measured with a counting time of 10 s. Data analyseswere carried out with the Crysalis program [30]. The structureswere solved using direct methods with the Shelxl97 program[31]. The non-hydrogen atoms were refined with anisotropic ther-mal parameters. The hydrogen atoms bonded to carbon and nitro-gen were included in geometric positions and given thermalparameters equivalent to 1.2 times those of the atom to which theywere attached. In peptide I a solvent CHCl3 was located and refinedwith full occupancy. One water molecule was given full occupancyand its hydrogen atoms were located in a difference Fouriermap and refined with distance constraints. The centralNHACH2ACH2ANH link was disordered over a twofold axis andtwo possible positions of the nitrogen and carbon atoms were re-fined with 50% occupancy. Seven additional peaks were observed

112 A. Dutta et al. / Journal of Molecular Structure 917 (2009) 110–116

in the difference Fourier map and refined as oxygen atoms fromwater molecules with occupancy factors of 0.25. Their hydrogenatoms were not included. Many of these oxygen atoms are withinbonding distance of each other and may be part of solvent metha-nol molecules though this could not be proven. In peptide II therewere three solvent molecules, one water molecule whose hydro-gen atoms could not be located and two chloroform molecules.Both structures were refined on F2 using Shelx97 to R1 0.0940,0.1058; wR2 0.2311, 0.3018 for 5642, 7300 reflections withI > 2r(I). The data have been deposited at Cambridge Crystallo-graphic Data Center with reference numbers CCDC 670977 and683673.

3. Results and discussion

3.1. Peptide conformations in the solid state

The crystal structures of the peptides I and II reveal that theyadopt T-L-T conformations (Figs. 2–4). In peptide I two b-turnsgenerated from each tripeptide subunit (T) are connected withthe extended conformation of a centrally placed 1,2-ethylenedia-mine linker (Fig. 2). This linker is disordered over a crystallo-graphic twofold axis with two alternative positions A and B forboth N(12) and C(13). It seems likely that each molecule con-tains an N12A-C13A-C13B_$1-N12B_$1 link across the twofoldaxis ($1 = y, x, �z) as illustrated in Fig. 2 where A and B repre-sent alternative positions, refined with 50% occupancy. The crys-tal structure of peptide II shows no such disorder with all atomsin general positions (Fig. 3). To facilitate a comparison of theconformations of the two peptides, the numbering scheme ofpeptide II has been chosen to be comparable with that of pep-tide I, with two parts A and B. Torsion angles in the backbonesof the two structures are given in Table 1.

In peptide I, the residues Ile(1) and Aib(2) occupy the i + 1 andi + 2 positions of the turns with the torsion angles at Ile(1) and

Fig. 2. Structure of peptide I with ellipsoids at 25% probability. Hydrogen bonds are showare shown, but not the solvent water molecules with 25% occupancy.

Aib(2) as u1: �63.1(5)�, w1: �26.6(5)� and u2: �59.7(5)�, w2:�31.2(5)�, respectively (Fig. 2 and 4, Table 1), values characteristicof a type III b-turn structure[32]. The terminal BocACO groups ofthe peptide I act as double acceptors in intramolecular hydrogenbonding with HN-Leu(3) (4 ? 1) and HN-ED (5 ? 1) withN9� � �O2 and N12B� � �O2 distances as 3.131(4) and 2.823(8) ÅA

0

,respectively (Table 2). The alternative position N12A forms hydro-gen bonds to O2 at 3.351(8) ÅA

0

. The resulting T-L-T conformation isfurther stabilized by two water mediated hydrogen bonds whichbridge between the two T sections as shown in Fig. 2. These hydro-gen bonds are formed to CO-Ile(1) of one turn and CO-Leu(3) of an-other turn with O100(W)� � �O5 (y, x, �z) and O100(W)� � �O11distances as 2.860(4) and 2.911(4) ÅA

0

, respectively (Table 2).The crystal structure of peptide II shows that it also folds into a

T-L-T conformation, where the b-turns generated from each tripep-tide subunit are connected with the extended conformation of acentrally placed 1,2-ethylenediamine (Table 1, Fig. 3). The confor-mation of peptide II shows an approximate 2-fold axis runningthrough the central C(13A)AC(13B) bond although as can be seenfrom Table 1 there are minor differences in the backbone torsionangles of parts A and B of up to 13�. It is also apparent from Table1 that the torsion angles in the backbone of peptide II are similar tothose in peptide I but major differences are found in the 1,2-ethy-lenediamine link (Fig. 4). In peptide I, the disorder has been inter-preted as giving a link in which the three torsion angles CANACAC,NACACAN and CACANAC are 91.7(5)�, �174.1(7)�, �70.1(10)�,respectively while in peptide II these angles are 88.2(8)�,169.8(5)� and 98.9(6)�. Thus paradoxically peptide II conforms toapproximate C2 symmetry in the link while peptide I, which crys-tallises over a C2 axis, does not and is disordered over that axis.This difference in the ED link is highlighted by the fact that twosymmetry related water molecules are associated with peptide Ithough hydrogen bonds to O(5) and O(11)$1 and O(5)$1 andO(11), while in peptide II one water molecule is found which formshydrogen bonds to O(11A) and O(11B).

n as dotted lines. The solvent water with 100% occupancy and chloroform molecules

Fig. 3. Structure of peptide II with ellipsoids at 25% probability. Hydrogen bonds shown as dotted lines. The solvent water and the one chloroform molecule that formhydrogen bonds are shown, but the chloroform molecule that does not form hydrogen bonds is omitted.

Fig. 4. Showing the backbones of peptides I and II in the crystal structures. Forclarity only one intramolecular hydrogen bond is shown in each b-turn subunit.

Table 1Selected back-bone torsion angles (deg) in peptides I and II

Torsion angles Peptide I Peptide II

Part A Part B

O(1)AC(2)AN(3)AC(4) (x0) �176.0(3) �172.2(4) �174.0(7)C(2)AN(3)AC(4)AC(5) (u1) �63.1(5) �62.3(6) �75.2(8)N(3)AC(4)AC(5)AN(6) (w1) �26.6(5) �27.6(6) �19.5(8)C(4)AC(5)AN(6)AC(7) (x1) �179.1(4) �177.7(5) �175.9(6)C(5)AN(6)AC(7)AC(8) (u2) �59.7(5) �52.1(7) �52.4(9)N(6)AC(7)AC(8)AN(9) (w2) �31.2(5) �31.5(7) �38.0(7)C(7)AC(8)AN(9)AC(10) (x2) �175.1(4) �175.2(5) �174.3(5)C(8)AN(9)AC(10)AC(11) (u3) �105.3(5) �107.9(6) �99.0(6)N(9)AC(10)AC(11)AN(12A) (w3) 5.3(7) 17.5(7) 11.5(7)C(10)AC(11)AN(12A)AC(13A) �174.3(6) 180.0(6) 175.0(5)C(11)AN(12A)AC(13A)AC(13B)x 91.7(5) 88.2(8) 98.9(6)N(12A)AC(13A)AC(13B)xAN(12B)x �174.1(7) 169.8(5) –C(13A)AC(13B)xAN(12B)xAC(11)x �70.1(10)C(13B)xAN(12B)xAC(11)xAC(10)x �174.4(5)N(12B)xAC(11)AC(10)AN(9) �49.9(5)

x refers to symmetry element $1 in peptide I but is not applicable to peptide II.$1 is symmetry element y, x, �z.

A. Dutta et al. / Journal of Molecular Structure 917 (2009) 110–116 113

In peptide II, the residues Leu(1) and Aib(2) occupy the cornerpositions of b-turns in the T subunits. The torsion angles atLeu(1), Aib(2) and Val(3) in the two halves A and B of the moleculewere found to be u1: �62.3(6)�, �75.2(8)�, w1 :�27.6(6)�,�19.5(8)�; u 2: �52.1(7)�, �52.4(9)�, w2: �31.5(7)�, �38.0(7)�and u3 : �107.9(6)�, �99.0(6)�, w3 : 17.5(7)�, 11.5(7)�, thus very

similar to those in the backbone of peptide I (Figs. 3 and 4, Table1). The b-turn in part A is stabilized by two 10 membered intramo-lecular hydrogen bonds between Val(3)-NH and BocAC@O(N9A� � �O2A, 3.298(7) ÅA

0

) and ED-NH and Leu(1)C@O (N12A� � �O5A,2.927(7) ÅA

0

) while that of in part B is stabilized by one intramolec-ular hydrogen bond N12B� � �O5B, 2.817(6) ÅA

0

(Table 2) while theN9B� � �O2A distance has increased to 3.726 ÅA

0

and can not be con-sidered as a hydrogen bond.

3.2. Solution phase conformations

The 1H NMR spectra of the peptides I and II in CDCl3 indicatesymmetrical conformations. In order to investigate the existenceof intramolecular hydrogen bonding and peptide conformations

Table 2Hydrogen bonding parameters of peptides I and II

DAH� � �A H� � �A/ÅA0

D� � �A/ ÅA0

DAH� � �A/�

Peptide IIntramolecularN12BAH� � �O2 2.32 2.823(8) 115N9AH9� � �O2 2.35 3.131(4) 151N12AAH12A� � �O5 2.56 3.057(8) 116N12AAH12B� � �O2 2.47 3.351(8) 168

IntermolecularO100AH100� � �O5 2.00 2.860(4) 171O100AH100� � �O11a 2.07 2.911(4) 174N3AH3� � �O11b 2.05 2.891(4) 167N6AH6� � �O100b 2.09 2.925(4) 164C1AH1� � �O8 2.12 3.081(5) 167

Peptide IIIntramolecularN9AAH9A� � �O2A 2.46 3.298(7) 165N12AAH12A� � �O5A 2.12 2.927(7) 157N12BAH12B� � �O5B 1.99 2.817(6) 160

IntermolecularN3BAH3B� � �O11Bc 2.11 2.951(7) 166N6AAH6A� � �O100d 2.33 2.975(8) 132N3AAH3A� � �O11Ad 2.19 3.033(7) 166O100� � �O11B * 2.774(6 *O100� � �O11A * 2.776(6) *C1BAH1B� � �O8B 2.19 2.968(8) 143

*Hydrogen atoms bonded to the water molecule O(100) were not located.Symmetry equivalents: ay, x,�z; bx�0.5, 1.5�y, 0.25�z; c0.5�x, 0.5 + y, �z; d1�x,1.5�y, 0.5 + z.

114 A. Dutta et al. / Journal of Molecular Structure 917 (2009) 110–116

in the solution phase the solvent dependence of the NH chemicalshifts was examined by NMR titration [29]. In peptide I theIle(1)-NH appears as a doublet in the upfield region(d = 5.18 ppm in CDCl3) among the four NH resonances resultingfrom the urethane functional group(Boc-group). The Aib(2)-NH ap-pears at d = 6.82 ppm, as a singlet due to the lack of an a-hydrogenatom. The triplet at d = 7.30 ppm corresponds to ED-NH. The onlyremaining doublet at d = 7.07 ppm was assigned to Leu(3)-NH.The solvent titration shows that by increasing the percentage of(CD3)2SO in CDCl3 from 0% to 12% (v/v) the changes in the chemicalshift (Dd) values for Ile(1)-NH, Aib(2)-NH, Leu(3)-NH and ED-NHare 0.59, 0.35, �0.15 and �0.11 ppm, respectively (Fig. 5). The Ddvalues demonstrate that while the Ile(1)-NH and Aib(2)-NH aresolvent exposed, the other two NH groups are solvent shielded,

0 2 4 6 8 105.0

5.5

6.0

6.5

7.0

7.5

Ile(1)-NH Aib(2)-NH Leu(3)-NH Ethylene diamine-NHδ

(pp

m)

% of d6 DMSO in CDCl3

Fig. 5. NMR solvent titration curve for NH protons in peptide I (initial concentrationof I, 10 mM in 0.5 ml CDCl3).

indicating a T-L-T structure as that of in the crystalline state (Fig.2), where the Leu(3)-NH and ED-NH are involved in intramolecularhydrogen bonding.

In NMR solvent titration of peptide II the changes in the chem-ical shift (Dd) values for Leu(1)-NH, Aib(2)-NH, Val(3)-NH and ED-NH are found to be 0.54, 0.30, �0.30 and �0.11 ppm, respectively(Fig. 6). The result shows that the Leu(1)-NH and Aib(2)-NH aresolvent exposed and Val(3)-NH and ED-NH are intramolecularlyhydrogen bonded, indicating a structure similar to that found inthe crystal structure (Fig. 3). The CD spectra of peptides I and IIin methanol (at 25 �C) show a negative band centered at 203 nm(Fig. 7).

3.3. Crystal packing

The peptides I and II show similar type of crystal packing. The T-L-T foldamers of peptides I and II self-assemble to form three-dimensional framework of channels (Figs. 8 and 9). Six peptidemolecules of I are interconnected by two types of intermolecularhydrogen bonds, between Aib(3)-NH and water (N6� � �O100,2.925(4) ÅA

0

) and between Ile(1)-NH and CO-Leu(3) (N3� � �O11,2.891(4) ÅA

0

) to complete each circular subunit (Fig. 10, Table 2).These subunits are further connected in three directions (a, b,and c axis) to create a three-dimensional network of channels withan internal dimension of 3.68 � 6.26 ÅA

0

(Fig. 10). Two turns of eachself-assembling T-L-T foldamer are shared by two neighbouringcircular subunits with one each. Remarkably, in the supramolecu-lar assemblage of T-L-T foldamers all the turns are localized on theinner surface of the three-dimensional channels. The inside of thechannels is hydrophilic and found to contain solvent CHCl3 hydro-gen bonded to exposed C@O of Aib located at the turn region of thepeptides (Fig. 10). This hydrogen bond with a C1� � �O8 distance of3.081(5) ÅA

0

is unusually strong for a solvent CHCl3 interaction.There were seven weak peaks in the final difference Fourier mapwhich were refined as water molecules with 25% occupancy. Whilethese atoms are situated in the channels, these atoms do not formany close contacts that could be considered as hydrogen bondswhich presumably accounts for their partial occupancy and/or highthermal motion.

Like peptide I the T-L-T foldamer of peptide II also self-assem-bles into a three-dimensional framework of channels with an inter-nal dimension of 4.90 � 4.03 ÅA

0

(Fig. 9). Here also six peptidemolecules are interconnected by two types of intermolecular

0 2 4 6 8 105.0

5.5

6.0

6.5

7.0

7.5

Leu(1)-NH Aib(2)-NH Val(3)-NH Ethylenediamine-NHδ

(pp

m)

% of d6 DMSO in CDCl3

Fig. 6. NMR solvent titration curve for NH protons in peptide II (initial concentra-tion of II, 10 mM in 0.5 ml CDCl3).

200 210 220 230 240-30

-25

-20

-15

-10

-5

0

MeOH

Peptide I Peptide II

θ(m

deg

)

λ (nm)

Fig. 7. CD curve of peptide I and II in methanol (1.5 mM).

ab

c

Fig. 8. View showing the cylindrical channels with solvent CHCl3 parallel to the baxis formed by peptide I through the self-assembly. Similar views are observedparallel to a and c axis also.

a

b

c

Fig. 9. View showing the channels with solvent CHCl3 parallel to the c axis formedby peptide II through self-assembly.

Fig. 10. Showing a circular subunit of the channels where six peptide I moleculesare interconnected through hydrogen bonds. Solvent CHCl3 molecules are hydrogenbonded to C@O of Aib. The side chains of the amino acids are omitted for clarity.

Fig. 11. Showing an elliptical subunit of the channels where six peptide IImolecules are interconnected through hydrogen bonds. Solvent CHCl3 moleculesare hydrogen bonded to C@O of Aib. The side chains of the amino acids are omittedfor clarity.

A. Dutta et al. / Journal of Molecular Structure 917 (2009) 110–116 115

hydrogen bonds, between Aib(3)-NH and water (N6A� � �O100,2.975(8) ÅA

0

) and between Leu(1)-NH and CO-Val(3) (N3A� � �O11A,3.033(7) ÅA

0

; N3B� � �O11B, 2.951(7) ÅA0

) to complete each ellipticalsubunit (Fig. 11, Table 2). The channels are solvated with CHCl3

molecules which are hydrogen bonded to C@O of Aib (C1B� � �O8B,2.968(8) ÅA

0

, Table 2).

4. Conclusions

In the present study, it has been shown that the rationally de-signed acyclic peptides I and II with the centrally placed flexiblelinker 1,2-ethylenediamine fold into T-L-T conformations both inthe solid state and in solution. The result demonstrates that a com-bination of appropriate core inserts and proper selection of turnforming subunits can lead to a T-L-T conformation. The T-L-T folda-mers of peptides I and II also show the important property of fab-ricating three-dimensional framework of channels in the solidstate through the self-assembly. The inner surfaces of the channelsare hydrophilic and form hydrogen bonds with solvent CHCl3

through the exposed C@O of Aib at turns. The pattern of self-assembly shows that by manipulating the turn regions of a T-L-Tfoldamer it is possible to introduce suitable binding sites in thechannels. Functionalization of the inner surfaces of channels andpores is important for applications in gas storage, selective chiralabsorption and catalysis [33].

116 A. Dutta et al. / Journal of Molecular Structure 917 (2009) 110–116

Acknowledgements

A.D. and S.K. thank CSIR, New Delhi, India, for a senior researchfellowship (SRF). The financial assistance of UGC, New Delhi isacknowledged [Major Research Project, No.32-190/2006(SR)]. Wethank EPSRC and the University of Reading, UK for funds for OxfordDiffraction X-Calibur CCD diffractometer.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2008.07.001.

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