β-Alanine containing peptides: γ-Turns in cyclotetrapeptides

In the present paper we describe the synthesis, purification, single-crystal x-ray analysis, solution conformational characterization, and conformational energy calculations of the cyclic tetrapeptide cyclo- (β-Ala-L -Pro-β-Ala-L -Val). The peptide was synthesized by classical solution methods and the cyclization of the free tetrapeptide was accomplished in good yields in diluted methylene chloride solution using N,N-dicyclohexyl-carbodiimide. The compound crystallizes in the monoclinic space group P2₁ from ethanol with two independent molecules in the unit cell. All peptide bonds are trans. The nmr molecular conformation in the acetonitrile solution as well as that derived from the molecular dynamic simulation in vacuo is quite different from those observed in the solid state and is very similar to that previously observed for the parent compound cyclo-(β-Ala-L -Pro-β-Ala-L -Pro). © 1993 John Wiley & Sons, Inc.     

Conformational analysis of cyclolinopeptide A, a cyclic nonapeptide: nuclear Overhauser effect and energy minimization studies

The conformation of cyclolinopeptide A [cyclo(Pro-Pro-Phe-Phe-Leu-Ile-Ile-Leu-Val)], a naturally occurring cyclic nonapeptide has been investigated in dimethylsulfoxide solution by 270 MHz 1H-nmr. A complete assignment of all C alpha H and NH resonances has been accomplished using two-dimensional correlated spectroscopy and nuclear Overhauser effects (NOEs). Analysis of interresidue NOEs and JHNC alpha H values permit construction of a molecular model for the cyclic peptide backbone. The crude model derived from nmr has been used as a starting point for energy minimization, which yields a refined structure largely compatible with nmr observations. The major features of the conformation of cyclolinopeptide A are a Type VI beta-turn centered at Pro(1)-Pro(2), with a cis peptide bond between these residues and a gamma-turn (C7 structure) centered at Ile(6). Two intramolecular hydrogen bonds Val(9) CO--Phe(3)NH (4----1) and Leu(5) CO--Ile(7)NH (3----1) are observed in the low-energy conformation. The limited solvent accessibility observed for the Val(9) and Leu(5) NH groups in the nmr studies are rationalized in terms of steric shielding.     

Binding affinity difference induced by the stereochemistry of the sulfoxide bridge of the cyclic peptide inhibitors of Grb2-SH2 domain: NMR studies for the structural origin

The SAR study on a phage library-derived non-phosphorylated cyclic peptide ligand of Grb2-SH2 domain indicates that the configuration of the cyclization linkage is crucial for assuming the active binding conformation. When the thioether linkage was oxidized to the two chiral sulfoxides, the R-configured sulfoxide-cyclized peptide displayed 10-30 times more potency than the corresponding S-configured one in binding affinity to the Grb2-SH2 domain. In this paper, the solution structures of such a pair of sulfoxide-bridged cyclic peptide diastereoisomers, i.e., cyclo[CH(2)CO-Gla(1)-L-Y-E-N-V-G-NPG-Y-(R/S)C(O)(10)]-amide, were determined by NMR and molecular dynamics simulation. Results indicate that the consensus sequence of Y(3)-E(4)-N(5)-V(6) in both diastereoisomers adopt a beta-turn conformation; however, the R-configured peptide forms an extended structure with a circular backbone conformation, while the S-configured isomer forms a compact structure with key residues buried inside the molecule. The average root-mean-square deviations were found to be 0.756 and 0.804 A, respectively. It is apparent that the chiral S-->O group played a key role in the solution structures of the sulfoxide-bridged cyclic peptides. The R-sulfoxide group forms an intramolecular hydrogen bond with the C-terminal amide, conferring a more rigid conformation with all residues protruding outside except for Leu2, in which the Gla1 and Tyr3 share an overlapping function as previous SAR studies proposed. Additionally, the extended structure endows a more hydrophilic binding surface of the R-configured peptide to facilitate its capture by its targeted protein. In comparison, the S-configured sulfoxide was embedded inside the ligand peptide leading to a compact structure, in which the essential residues of Gla1, Tyr3, and Asn5 form multiple intramolecular hydrogen bonds resulting in an unfavorable conformational change and a substantial loss of the interaction with the protein. The solution structures disclosed by our NMR and molecular dynamics simulation studies provide a molecular basis for understanding how the chirality of the cyclization linkage remarkably discriminates in terms of the binding affinity, thus advancing the rational design of potent non-phosphorylated inhibitors of Grb2-SH2 domain as antitumor agents.     

Conformation and formation tendency of the cyclotetrapeptide cyclo (D-Pro-D-Pro-L-Pro-L-Pro): experimental results and molecular modeling studies

The title compound represents the smallest member of cyclic proline peptides corresponding to the general formula c(DDLL-Pro4)n with a strictly D,D,L,L double-alternating sequence of the chiral amino acid residues. The cyclopeptides with n greater than or equal to 2 could be synthesized from both DDLL-Pro4 (1) and DLLD-Pro4 (2). The cyclic monomer (n = 1) resulted only from 2, whereas not even a trace could be found by cyclization of 1. The peptide exists in a strongly strained Ci symmetrical conformation (x-ray analysis) with alternating cis and trans peptide bonds (ctct form I). The cis peptide bonds deviate from planarity (omega = 22 degrees); two of the pyrrolidine rings show a "South" conformation (phi = -94 degrees), whereas the other residues exhibit C alpha-endo puckering (phi = -124 degrees). Two of the psi angles surprisingly occur at +41 degrees (anti-cis'), the others are located in the trans' region. A quantitative ring opening occurs with trifluoroacetic acid at room temperature. In solution the existence of an isomeric ctcc sequence (form Ia) is indicated. Dreiding model studies also suggested a favorable conformation with a tctc sequence (form II). Consequently, we performed molecular mechanics calculations, based on the CHARMM force field and semiempirical quantum mechanical AM1 calculations (MOPAC program). Pronounced differences in the backbone parameters were found using these two methods. However, the theoretical studies evidenced the experimentally obtained differences in the cyclization tendencies of the linear precursors.     

Role of N-t-Boc group in helix initiation in a novel tetrapeptide.

Protecting groups in N- and C-terminal positions play a decisive role in the conformational preference of smaller peptides. Conformational analysis of tetrapeptide derivatives containing Ala, Ile and Gly residues was performed. Peptide 1, Boc-Ala-Ile-Ile-Gly-OMe (Boc: tert-butyloxycarbonyl) has a predominantly helical turn conformation in all the alcoholic solvents studied, whereas in the solid state it has a beta-sheet conformation. In contrast, peptide 2, Ac-Ala-Ile-Ile-Gly-OMe (Ac: acetyl) has a random coil conformation in solution. The FTIR spectrum of peptide 1 shows a lower frequency of urethane carbonyl, indicating involvement of the carbonyl group in hydrogen bonding in the helical turn.     

Conformational analysis of an acyclic tetrapeptide: ab‐initio structure determination from X‐ray powder diffraction,Hirshfeld surface analysis and electronic structure

A terminally protected acyclic tetrapeptide has been synthesized, and the crystal structure of its hydrated form, Boc‐Tyr‐Aib‐Tyr‐Ile‐OMe·2H₂O ( 1 ), has been determined directly from powder X‐ray diffraction data. The backbone conformation of tetrapeptide ( 1 ) exhibiting two consecutive β‐turns is stabilized by two 4 → 1 intramolecular N―H · · · O hydrogen bonds. In the crystalline state, the tetrapeptide molecules are assembled through water‐mediated O―H · · · O hydrogen bonds to form two‐dimensional molecular sheets, which are further linked by intermolecular C―H · · · O hydrogen bonds into a three‐dimensional supramolecular framework. The molecular electrostatic potential (MEP) surface of ( 1 ) has been used to supplement the crystallographic observations. The nature of intermolecular interactions in ( 1 ) has been analyzed quantitatively through the Hirshfeld surface and two‐dimensional fingerprint plot. The DFT optimized molecular geometry of ( 1 ) agrees closely with that obtained from the X‐ray structure analysis. The present structure analysis of Boc‐Tyr‐Aib‐Tyr‐Ile‐OMe·2H₂O ( 1 ) represents a case where ab‐initio crystal structure of an acyclic tetrapeptide with considerable molecular flexibility has been accomplished from laboratory X‐ray powder diffraction data. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Crystal structure and conformation of a cyclic tetrapeptide cyclo(L-Pro-Sar)2 containing all-cis peptide units

The crystal structure and conformation of the synthetic cyclic tetrapeptide, cyclo(L -Pro-Sar)₂, was determined by x-ray analysis. The peptide crystallizes in the orthorhombic space group P2₁2₁2₁ with cell parameters a = 9.277(1), b = 12.884(1), and c = 15.581(2) Å. The crystal structure was solved by the symbolic addition procedure for direct phase determination and least-squares refinement using 1796 reflections, which led to the final R value of 0.043. This structure provides the first example observed in a crystal of a cyclic tetrapeptide in which all four peptide units have been found in the cis conformation with ω angles deviating slightly by 2°–10° from the ideal value of 0°. It was also found that the two Pro C^α-CO single bonds assumed a trans′ (ψ = 159.6° and 158.4°) conformation. Adjoining average planes of the peptide groups fall at nearly right angles to each other. The pyrrolidine ring conformations of the two prolyl residues are in the envelope form, with C^γ carbon out of the least-squares planes for the remaining four atoms.     

The dominant interaction between peptide and urea is electrostatic in nature: a molecular dynamics simulation study

The conformational equilibrium of a blocked valine peptide in water and aqueous urea solution is studied using molecular dynamics simulations. Pair correlation functions indicate enhanced concentration of urea near the peptide. Stronger hydrogen bonding of urea-peptide compared to water-peptide is observed with preference for helical conformation. The potential of mean force, computed using umbrella sampling, shows only small differences between urea and water solvation that are difficult to quantify. The changes in solvent structure around the peptide are explained by favorable electrostatic interactions (hydrogen bonds) of urea with the peptide backbone. There is no evidence for significant changes in hydrophobic interactions in the two conformations of the peptide in urea solution. Our simulations suggest that urea denatures proteins by preferentially forming hydrogen bonds to the peptide backbone, reducing the barrier for exposing protein residues to the solvent, and reaching the unfolded state.     

beta-Alanyl-beta-alanine in cyclic beta-turned peptides.

In the present paper we describe the synthesis, purification, and single crystal x-ray analysis of the cyclic pentapeptide cyclo-(L-Pro-L-Pro-L-Phe-beta-Ala-beta-Ala). The peptide was synthesized by classical solution methods and the cyclization of the free pentapeptide was accomplished in good yields in diluted methylene-chloride solution using N,N-dicyclohexylcarbodiimide. The compound crystallizes in the monoclinic space group P21 from hot water with five solvent molecules. The Pro1-Pro2 peptide bond is cis and the molecular conformation is stabilized by an intramolecular hydrogen bond between the CO group of the beta-Ala5 and the NH of the Phe3 residue. The Pro1-Pro2 segment occupies the relative positions 2 and 3 of a type VIa beta-turn, while the L-phenylalanyl-beta-alanyl-beta-alanine segment is incorporated in a C13-like ring structure. The crystal packing is characterized by a network of 11 intermolecular hydrogen bonds involving all the remaining CO, NH, and the water molecules.     

Chain length effects on helix-hairpin distribution in short peptides with Aib-DAla and Aib-Aib segments

The Aib-D Ala dipeptide segment has a tendency to form both type-I'/III' and type-I/III β-turns. The occurrence of prime turns facilitates the formation of β-hairpin conformations, while type-I/III turns can nucleate helix formation. The octapeptide Boc-Leu-Phe-Val-Aib-DAla-Leu-Phe-Val-OMe (1) has been previously shown to form a β-hairpin in the crystalline state and in solution. The effects of sequence truncation have been examined using the model peptides Boc-Phe-Val-Aib-Xxx-Leu-Phe-NHMe (2, 6), Boc-Val-Aib-Xxx-Leu-NHMe (3, 7), and Boc-Aib-Xxx-NHMe (4, 8), where Xxx=DAla, Aib. For peptides with central Aib-Aib segments, Boc-Phe-Val-Aib-Aib-Leu-Phe-NHMe (6), Boc-Val-Aib-Aib-Leu-NHMe (7), and Boc-Aib-Aib-NHMe (8) helical conformations have been established by NMR studies in both hydrogen bonding (CD3OH) and non-hydrogen bonding (CDCl3) solvents. In contrast, the corresponding hexapeptide Boc-Phe-Val-Aib-DAla-Leu-Phe-Val-NHMe (2) favors helical conformations in CDCl3 and β-hairpin conformations in CD3 OH. The β-turn conformations (type-I'/III) stabilized by intramolecular 4→1 hydrogen bonds are observed for the peptide Boc-Aib-D Ala-NHMe (4) and Boc-Aib-Aib-NHMe (8) in crystals. The tetrapeptide Boc-Val-Aib-Aib-Leu-NHMe (7) adopts an incipient 3(10)-helical conformation stabilized by three 4→1 hydrogen bonds. The peptide Boc-Val-Aib-DAla-Leu-NHMe (3) adopts a novel α-turn conformation, stabilized by three intramolecular hydrogen bonds (two 4→1 and one 5→1). The Aib-DAla segment adopts a type-I' β-turn conformation. The observation of an NOE between Val (1) NH?HNCH3 (5) in CD3OH suggests, that the solid state conformation is maintained in methanol solutions.     

Conformations of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3.

A 270-MHz 1H nuclear magnetic resonance investigation of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3, and its cation complexes is reported. In CD2Cl2 and CDCl3, the peptide is proposed to occur in a C3-symmetric conformer with the N--H's of all six glycine residues intramolecularly hydrogen bonded. This conformation is different from the familiar valinomycin bracelet structure and lacks any "cavity". Cations do not bind, or bind only weakly, to the peptide in these solvents. Uncomplexed cyclo(L-Val-Gly-Gly-L-Pro)3 in acetonitrile appears to be averaging among several conformations with no evidence found for any preferred intramolecular hydrogen bonds. The strong 1:1 complexes of cyclo(L-Val-Gly-Gly-L-Pro)3 with K+ ANd Ba2+ in acetonitrile are structurally analogous to the bracelet conformation of valinomycin and involve the N--H's of the Val residues and of the Gly's preceding Pro in intramolecular hydrogen bonding. Tl+ was also found to form strong 1:1 complexes with the dodecapeptide.     

A conformational study of the tetrapeptide CH3CO-Ala-Asp-Gly-Lys-NHCH3 corresponding to a β-bend in staphylococcal nuclease

The tetrapeptide sequence Ala-Asp-Gly-Lys occurs as a type I′ β-bend at residues 94–97 in staphylococcal nuclease. We have synthesized theN-acetyl,N′-methylamide derivative of this tetrapeptide and studied its conformation in solution, using nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. In the synthesis, special attention was paid to the possibility of cyclic aspartimide formation giving rise to mixtures of α- and β-Asp-Gly products. The presence of such a mixture was excluded by infrared, NMR, and other analytical procedures applied to the products and to models for α- and β-linked aspartyl residues. The CD spectra of the protected tetrapeptide in water, methanol, and trifluoroethanol show no evidence of preferred chain conformations. In dimethylsulfoxide-d ₆ , however, the NMR spectra are consistent with the presence of a population of conformers in which the Lys and C-terminal NHCH₃ amide protons are shielded from solvent. Taken together with the observed³J_NH-C ^α _H coupling constants for all residues, this permitted the construction and energetic evaluation of possible conformations in solution. Only one such conformation was fully compatible with the NMR data; this is a type II β-bend in which the Lys and C-terminal NHCH₃ amide protons are close to the Ala C=O group and may form bifurcated hydrogen bonds with it. This conformation can be converted into the conformation existing in staphylococcal nuclease by rotating the plane of the Ala-Asp peptide group by about 120° around a line connecting the Ala and Asp C_α atoms and by making small shifts in dihedral angles elsewhere in the peptide.     

Variability in the backbone conformation of cyclic pentapeptides. Crystal structure of cyclic(Gly-L-Pro-D-Phe-Gly-L-Ala)

All the peptide bonds in cyclic(Gly-L-Pro-D-Phe-Gly-L-Ala) are in the trans conformation; however, the peptide bond C'5-N1 is twisted by 19 degrees from planarity (omega 5 = -161 degrees). A Type II beta-turn encompasses the L-Pro-D-Phe residues. Carbonyl oxygens O2, O4 and O5 are directed to the same side of the average plane through the backbone ring and they form hydrogen bonds with N3, N5 and N1, respectively, in adjacent molecules in a stacked column where the adjacent molecules are related by one translational unit. The conformation of the backbone is different from that established in other molecules with the DLDDL chirality sequence. The P21 cell contains two molecules of C21H26N5O5 with a = 4.836(2) A, b = 18.346(8) A, c = 12.464(5) A and beta = 100.05(4) degrees. The R factor for 1382 data with [F0[ greater than 1 sigma is 7.0%.     

Proline-containing beta-turns. IV. Crystal and solution conformations of tert.-butyloxycarbonyl-L-prolyl-D-alanine and tert.-butyloxycarbonyl-L-prolyl-D-alanyl-L-alanine

The conformations of the dipeptide t-Boc-Pro-DAla-OH and the tripeptide t-Boc-Pro-DAla-Ala-OH have been determined in the crystalline state by X-ray diffraction and in solution by CD, n.m.r. and i.r. techniques. The unit cell of the dipeptide crystal contains two independent molecules connected by intermolecular hydrogen bonds. The urethane-proline peptide bond is in the cis orientation in both the molecular forms while the peptide bond between Pro and DAla is in the trans orientation. The single dipeptide molecule exhibits a "bent" structure which approximates to a partial beta-turn. The tripeptide adopts the 4----1 hydrogen-bonded type II beta-turn with all trans peptide bonds. In solution, the CD and i.r. data on the dipeptide indicate an ordered conformation with an intramolecular hydrogen bond. N.m.r. data indicate a significant proportion of the conformer with a trans orientation at the urethane-proline peptide bond. The temperature coefficient of the amide proton of this conformer in DMSO-d6 points to a 3----1 intramolecular hydrogen bond. Taken together, the data on the dipeptide in solution indicate the presence (in addition to the cis conformer) of a C7 conformation which is absent in the crystalline state. The spectral data on the tripeptide indicate the presence of the type II beta-turn in solution in addition to the nonhydrogen-bonded conformer with the cis peptide bond between the urethane and proline residues. The relevance of these data to studies on the substrate specificity of collagen prolylhydroxylase is pointed out.     

Conformation of the cyclic tetrapeptide dihydrochlamydocin. Iabu-L-Phe D-Pro-LX, and experimental values for 3 leads to 1 intramolecular hydrogen bonds by X-ray diffraction.

Chlamydocin, Iabu-L -Phe-D -Pro-L X, is a naturally occurring cyclic tetrapeptide that exhibits high cytostatic activity. The conformation of the peptide ring, as well as the stereo configuration in the vicinity of the epoxide ring, have been established by a single-crystal X-ray study of dihydrochlamydocin: C₂₈H₄₀N₄O₆·H₂O. It crystallizes in the monoclinic space group P2₁ with a = 12.616(6) Å, b = 12.355(6) Å, c = 9.442(5) Å, and β = 99.5(1)°. The structure was solved by the symbolic addition procedure for phase determination followed by the tangent formula method of phase refinement. This structure represents the first cyclic tetrapeptide in which all four peptide units have been found in the trans conformation; however, each peptide unit is significantly nonplanar with ω angles deviating by 14–24° from the ideal value of 180°. This molecule contains two intramolecular 3 → 1 hydrogen bonds and experimentally determined parameters for these seven-membered turns are presented.     

Unique molecular conformation of aureobasidin A, a highly amide N-methylated cyclic depsipeptide with potent antifungal activity: X-ray crystal structure and molecular modeling studies.

A structural feature of aureobasidins, cyclic depsipeptide antibiotics produced by Aureobasidium pullulans R106, is the N-methylation of four out of seven amide bonds. In order to investigate possible relationship between the molecular conformation and the amide N-methylation, aureobasidin A (AbA), which exhibits the potent antifungal activity, was subjected to X-ray crystal analysis. The crystal, recrystallized from ether (orthorhombic, space group P2(1)2(1)2(1), a = 21.643 (3) A, b = 49.865(10) A, c = 12.427 (1) A, z= 8), contained two independent conformers per asymmetric unit and they took on a similar arrowhead-like conformation. The conformation consisted of three secondary structures of antiparallel beta-sheet, and beta- and gamma-turns, and was stabilized by three intramolecular and transannular N-H O=C hydrogen bonds. The beta-hydroxy-N-methyl-l-valine residue, which is indispensable for its bioactivity, was located at the tip of the corner. Since a nearly identical conformation has been observed for aureobasidin E, a related cyclic depsipeptide, this arrowhead-like conformation may be energetically stable and important for biological activity. The contribution of the amide N-methylation to the conformation was investigated by model building and energy calculations. The energy-minimizations of AbA analogs, in which some (one to four) of four N-methylated amide bonds were replaced with usual amide bond, led to some conformers which are fairly different from the arrowhead form of AbA, although they are stabilized by three intramolecular N-H...O=C hydrogen bonds. This result explains the reason why four out of the seven amide bonds have to be methylated to manifest biological activity, i.e. the high N-methylation of aureobasidin is necessary to form only one well-defined conformation.     

Molecular dynamics study of a complex between the human histocompatibility antigen HLA-A2 and the IMP58-66 nonapeptide from influenza virus matrix protein.

The structure of the influenza-virus-matrix-protein (IMP) 58-66 nonapeptide, bound to the major-histocompatibility-complex-encoded human leukocyte antigen (HLA) A2 protein was studied by molecular dynamics simulation. Starting from the extra electron density map of peptides co-crystallized with HLA-A2, the nonapeptide IMP58-66 was docked residue by residue in the protein binding cleft. The complex was simulated for 100 ps in a shell of 1372 water molecules. The averaged simulated HLA-A2 conformation was found to be similar to the crystal structure (0.182 nm RMS deviation, for the backbone atoms of the alpha 1-alpha 2 domain). Nine out of the 14 hydrogen bonds observed in the antigen-binding site were reproduced in the simulation. The IMP58-66 peptide exhibits an extended conformation with kinks at positions 3 and 5. The side chains of residues 2, 3 and 9 develop van der Waals' interactions with hydrophobic pockets of HLA-A2, corresponding to polymorphic residues of the major-histocompatibility-complex-encoded proteins. Both the N-terminus and C-terminus of the nonapeptide were anchored in the antigen-binding groove by hydrogen bonds with conserved amino acids. The N-terminus was more flexible and contacts four HLA-A2 conserved tyrosines (Tyr7, Tyr59, Tyr159 and Tyr171) and Glu63 by direct or water-relayed hydrogen bonds. Water intercalation occurred only around the N-terminus of the peptide, the C-terminal carboxylate forming strong hydrogen bonds with polar residues (Tyr84 and Thr143) and a salt bridge with Lys146 all over the molecular dynamics simulation. This model is fully compatible with the recently published crystal structure of the HLA-B27 protein, complexed by a mixture of self nonapeptides.     

Calcium and praseodymium complexes in solution. 1H n.m.r. conformational study of the model tetrapeptide acetyl-aspartyl-valyl-aspartyl-alanine

The synthetic tetrapeptide acetyl-aspartyl-valyl-aspartyl-alanine (Ac-DVDA) is a model of the calcium binding site of proteins such as carp parvalbumin, thermolysin and calmodulin. 1H n.m.r. spectra of the tetrapeptide are presented and assigned for D2O and DMSO solutions to determine the conformational mobility. The resonance of the two aspartyl side chains could be completely analysed and the vicinal coupling (C alpha H-C beta H and NH-C alpha H) indicated that the free peptide has considerable conformational mobility. The Ca(II) complex generates a different 1H n.m.r. spectrum for the aspartyl resonances at neutral pH. The solution conformation of Pr(III) complex of Ac-DVDA has been investigated using induced chemical shifts. The observed trends in the magnitude of the shift ratios and the rotamer population suggest that the metal ion binds predominantly to both carboxylates of two aspartyl residues in a bidentate fashion. We discuss the consistency of the differentiated spectra for aspartyl residues in the complex with the stepwise binding of Ca2+ to the carrier.     

Design of peptides using α,β-dehydro-residues: Synthesis,crystal structure and molecular conformation of Boc-L-Val- ΔPhe-ΔPhe-L-Val-OCH3

The peptide Boc-L-Val-ΔPhe-ΔPhe-L-Val-OCH₃ was synthesized by the azlactone method in solution phase, and its crystal and molecular structures were determined by x-ray diffraction method. Single crystals were grown by slow evaporation from a methanol/water solution at 6°C. The crystals belong to an orthorhombic space group P2₁2₁2₁ with a = 10.478 (6) Å, b = 13.953 (1), c = 24.347 (2) and Z = 4. The structure was determined by direct methods and refined by least squares procedure to an R value of 0.052. The structure consists of a peptide and a water molecule. The peptide adopts two overlapping β-turn conformations of Types II and I′ with torsion angles: ϕ₁ = -54.8 (6), ψ₁ = 130.5 (4), ϕ₂ = 65.8 (5), ψ₂ = 12.8 (6), ϕ₃ = 79.4 (5), ψ₃ = 3.9 (7)°. The conformation is stabilized by intramolecular hydrogen bonds involving Boc CO and NH of ΔPhe³ and CO of Val¹ and NH of Val⁴. The molecules are tightly packed in the unit cell. The crystal structure is stabilized by hydrogen bonds involving NH of ΔPhe² and CO of a symmetry related (x-½, ½ -y, -z) ΔPhe². The solvent-water molecule forms two hydrogen bonds with peptide molecule involving NH of Val¹ as an acceptor and another with CO of a symmetry related (1 -x, y-½, ½ -z) ΔPhe³ as a donor. These studies indicate that a tetrapeptide with two consecutive ΔPhe residues sequenced with valines on both ends adopts two overlapping β-turns of Types II and I′. © 1996 John Wiley & Sons, Inc.     

Impact of a micellar environment on the conformations of two cyclic pentapeptides.

In an effort to explore the influence of interfacial environments on reverse turns, we have performed a detailed analysis by nmr of the solution conformations of two cyclic pentapeptides in sodium dodecyl sulfate (SDS) micelles. The first peptide, cyclo (D-Phe1-Pro2-Gly3-D-Ala4-Pro5), adopts a single rigid conformation in solution (either chloroform or dimethylsulfoxide) and in crystals, whereas the second, cyclo (Gly1-Pro2-D-Phe3-Gly4-Val5), is much more flexible and adopts different conformations in the crystal and in solution. Both of these peptides are solubilized by SDS micelles, and nmr relaxation rates indicate that they are both partially immobilized by interaction with the micelles. Furthermore, some amide protons in both peptides participate in hydrogen bonds with water. In the presence of micelles, the former peptide retains a conformation essentially the same as that found in crystals and in solution, which consists of a beta turn and an inverse gamma turn. However, the micellar environment has a significant effect on the latter peptide. In particular, the population of a conformer containing a cis Gly-Pro peptide bond is increased significantly. The most likely conformation of the cis isomer, determined by a combination of nmr and restrained molecular dynamics, contains a Gly1-Pro2 delta turn and a gamma turn about D-Phe3. The nmr data on the trans isomer indicate that this isomer is averaging between two conformations that differ mainly in the orientation of the D-Phe3-Gly4 peptide bond.