Molecular Structure of Peptaibol Antibiotics: Solution Conformation and Crystal Structure of the Octapeptide Corresponding to the 2–9 Sequence of Emerimicins III and IV

Abstract

The infrared absorption and ¹H nuclear magnetic resonance analyses of chloroform solutions of the terminally-blocked segment corresponding to the 2–9 sequence of emerimicins III and IV, -(Aib)₃-L-Val-Gly-L-Leu-(Aib)_2-, are consistent with the presence of a 3₁₀-helical structure of high thermal stability. The crystal structure of the octapeptide, obtained by X-ray diffraction indicates the formation of a right-handed 3₁₀-helix, stabilized by six consecutive intramolecular N-H OC H-bonds, slightly distorted at the level of the L-Leu residue.     

Crystal structure of Boc-Ala-Aib-Ala-Aib-Aib-methyl ester,a pentapeptide fragment of the channel-forming ionophore suzukacillin

t-Buthyoxycarbonyl-L -alanyl-α-aminiosobutyryl-L -alanyl-α-aminoisobutyryl-α-aminoisobutyric acid methyl ester (t-Boc-L -Ala-Aib-L -Ala-Aib-Aib-OMe), C₂₄H₄₃N₅O₈, an end-protected pentapeptide with a sequence corresponding to the 6th through the 10th residues in suzukacillin, crystallizes in the orthorhombic space group P2₁2₁2₁ with a = 11.671, b = 14.534, c = 17.906 Å and z = 4. The molecule exists as a right-handed 3₁₀-helix with a pitch of 6.026 Å. The helix is stabilized by three 4 → 1 hydrogen bonds with the NH groups of Ala(3), Aib(4), and Aib(5) hydrogen bonding to the carbonyl oxygens of t-Boc, Ala(1), and Aib(2), respectively. The helical molecules arrange themselves in a head-to-tail fashion along the a direction in such a way that the NH groups of Ala(1) and Aib(2) hydrogen bond to the carbonyl oxygens of Aib(4) and Aib(5), respectively, of a translationally related molecule. The helical columns thus formed close-pack nearly hexagonally to form the crystal.     

Stereochemistry of Schellman motifs in peptides: Crystal structure of a hexapeptide with a C‐terminus 6 → 1 hydrogen bond

The Schellman motif is a widely observed helix terminating structural motif in proteins, which is generated when the C‐terminus residue adopts a left‐handed helical (α_L) conformation. The resulting hydrogen‐bonding pattern involves the formation of an intramolecular 6 → 1 interaction. This helix terminating motif is readily mimicked in synthetic helical peptides by placing an achiral residue at the penultimate position of the sequence. Thus far, the Schellman motif has been characterized crystallographically only in peptide helices of length 7 residues or greater. The structure of the hexapeptide Boc–Pro–Aib–Gly–Leu–Aib–Leu–OMe in crystals reveal a short helical stretch terminated by a Schellman motif, with the formation of 6 → 1 C‐terminus hydrogen bond. The crystals are in the space group P2₁2₁2₁ with a = 18.155(3) Å, b = 18.864(8) Å, c = 11.834(4) Å, and Z = 4 . The final R₁ and wR₂ values are 7.68 and 14.6%, respectively , for 1524 observed reflections [F_o ≥ 3ς(F_o)]. A 6 → 1 hydrogen bond between Pro(1)CO · · · Leu(6)NH and a 5 → 2 hydrogen bond between Aib(2)CO · · · Aib(5)NH are observed. An analysis of the available oligopeptides having an achiral Aib residue at the penultimate position suggests that chain length and sequence effects may be the other determining factors in formation of Schellman motifs. © 1999 John Wiley & Sons, Inc. Biopoly 50: 13–22, 1999     

Long, chiral polypeptide 3(10)-helices at atomic resolution

The crystal-state preferred conformation of the terminally blocked hepta- and octapeptides with the general formula -(Aib)n L-Leu-(Aib)2- (n = 4 and 5, respectively), determined by X-ray diffraction, was found to be a right-handed 3(10)-helix stabilized by five and six consecutive intramolecular NH...O = C H-bonds of the C(10)-III type, respectively. The octapeptide structure represents the first observation at atomic resolution of a regular, chiral 3(10)-helix larger than two complete turns. In both cases the right handed screw sense of the helix is dictated by the presence of the single, internal L-residue. This study confirms the propensity of short peptides rich in Aib, the prototype of the amino acid residues dialkylated at the alpha carbon, to adopt a 3(10)-helical structure and is expected to help our understanding of the conformational preferences of the membrane-active, channel-forming, ion-transporting peptaibol antibiotics.     

High-resolution solid-state 13C-NMR of peptides: a study of chain-length dependence for 3(10)-helix formation

High-resolution solid-state ¹³C-nmr spectra of two series of fully protected oligopeptides, Z-(Aib)_n-OMe (n = 3?8) and Z-(Aib)_n-L-Leu-(Aib)₂-OMe (n = 0?5), were recorded to gain insight into main-chain length dependence for 3₁₀-helix formation. We found that all the oligopeptides examined adopt an incipient or a fully developed 3₁₀-helical structure, as judged from the characteristic splitting of the C_β signals as well as the conformation-dependent displacements of the C_α and C?O peaks.     

Crystal structure of achiral nonapeptide Boc-(Aib-DeltaZPhe)4-Aib-OMe at atomic resolution: evidence for a 3(10)-helix

An x-ray crystallographic analysis was carried out for Boc-(Aib-DeltaZPhe)4-Aib-OMe (1: Boc = t-butoxycarbonyl; Aib = alpha-aminoisobutyric acid; DeltaZPhe = Z-alpha,beta-didehydrophenylalanine) to provide the precise conformational parameters of the octapeptide segment -(Aib-DeltaZPhe)4-. Peptide 1 adopted a typical 3(10)-helical conformation characterized by = +/-55.8 degrees (50 degrees -65 degrees), = +/-26.7 degrees (15 degrees -45 degrees), and = +/-179.5 degrees (168 degrees -188 degrees) for the average values of the -(Aib-DeltaZPhe)4- segment (the range of the eight values). The 3(10)-helix contains 3.1 residues per turn, being close to the "perfect 3(10)-helix" characterized by 3.0 residues per turn. NMR and Fourier transform infrared (FTIR) spectroscopy revealed that the 3(10)-helical conformation at the atomic resolution is essentially maintained in solution. Energy minimization of peptide 1 by semiempirical molecular orbital calculation converged to a 3(10)-helical conformation similar to the x-ray crystallographic 3(10)-helix. The preference for a 3(10)-helix in the -(Aib-DeltaZPhe)4- segment is ascribed to strong inducers of the 3(10)-helix inherent in Aib and DeltaZPhe residues-in particular, the Aib residues tend to stabilize a 3(10)-helix more effectively. Therefore, the -(Aib-DeltaZPhe)4- segment is useful to rationally design an optically inactive 3(10)-helical backbone, which will be of great importance to provide novel insights into noncovalent and covalent chiral interactions of a helical peptide with a chiral molecule.     

Conformational difference between diastereomers of Dnp-Val-Aib-Gly-Leu-pNA studied by x-ray crystal analyses

In order to investigate the Conformational change of the α-aminoisobutyric acid (Aib) containing peptide by the D /L replacement of an amino acid residue, single crystals of two diastereomers, Dnp-L -Val-Aib-Gly-L -Leu-pNA (L -L isomer) and Dnp-D -Val-Aib-Gly-L -Leu-pNA (D -L isomer), were prepared from aqueous methanol solutions as CH₃OH and CH₃OH · H₂O solvates, respectively, and were analyzed by the x-ray diffraction method. Molecular conformation of L -L isomer adopts consecutive two different types of β-turns, a type II′ β-turn bent at Aib-Gly, and a type III β-turn bent at Gly-Leu, stabilized by two intramolecular (Leu) NH …? O?C (Val) and (pNA) NH …? O?C(Aib) hydrogen bonds. In contrast, these two intramolecular hydrogen bonds lead the D -L isomer to a distorted 3₁₀-helix conformation consisting of consecutive two type-III β-turn of Aib-Gly-Leu sequence. The most significant structural difference between these diastereomers is the mutual orientation between the Dnp and pNA chromophores. While the extensive stacking of both the chromophores is intramolecularly formed for the folded conformation of L -L isomer, they are oriented toward an opposite direction in the open conformation of D -L isomer and are intermolecularly stacked with each other. The large separation between these diastereomers observed in the chromatography is discussed in the relation with their Conformational differences. © 1993 John Wiley & Sons, Inc.     

Solid state and solution conformations of a helical peptide with a central gly-gly segment

The influence of amino acids with contrasting conformational tendencies on the stereochemistry of oligopeptides has been investigated using an octapeptide Boc-Leu-Aib-Val-Gly-Gly-Leu-Aib-Val-OMe, which contains two helix-promoting Aib residues and a central helix-destabilizing Gly-Gly segment. Single crystal x-ray diffraction studies reveal that a 3 ₁₀-helix is formed up to the penultimate Aib residue, at which point there is a helix reversal in the backbone, reminiscent of a C-terminal 6 → I hydrogen bond. The curious feature in the crystal is the solvation of the possible 6 → 1 bond by a CH₃OH molecule, where the OH is inserted between O(3) and N(8) and participates in hydrogen bonds with both. The cell parameters are as follows: space group P2₁2₁2₁, a = 10.649(4) Å, b = 15.694(5) Å, c = 30.181(8) Å, R = 6.7% for 3427 data (| F₀| > 3σF) observed to 0.9 Å. Nuclear magnetic resonance studies in CDCl₃ using NH group solvent accessibility and nuclear Overhauser effects as probes are consistent with a 3 ₁₀-helical conformation. In contrast, in (CD₃)₂SO, unfolding of the central segment results in a multiple β-turn structure, with β-turn conformations populated at residues 1–2, 3–4, and 6–7. CD studies in methanol-2,2,2-trifluoroethanol (TFE) mixtures also provide evidence for a solvent-dependent structural transition. Helical conformations are populated in TFE, while type II β-turn structures are favored in methanol. © 1996 John Wiley & Sons, Inc.     

X-Pro peptides: Solution and solid-state conformation of benzyloxycarbonyl-(Aib-Pro)2methyl ester,a type I β-turn

The synthesis of the tetrapeptide benzyloxycarbonyl(α-aminoisobutyryl-L -prolyl)₂-methyl ester (Z-(Aib-Pro)₂-OMe) and an analysis of its conformation in solution and the solid state are reported. Stepwise synthesis using dicyclohexylcarbodiimide leads to racemization at Pro(2). Evidence for the presence of diastereomeric tetrapeptides is obtained from 270-MHz¹H-nmr and 67.89-MHz ¹³C-nmr. The all-L tetrapeptide is obtained by fractional crystallization from ethyl acetate. The NH of Aib(3) is shown to be involved in an intramo-lecular hydrogen bond by variable-temperature ¹H-nmr and the solvent dependence of NH chemical shifts. The results are consistent with a β-turn conformation with Aib(1) and Pro(2) at the corners stabilized by a 4 → 1 hydrogen bond. The molecule crystallizes in the space group P2₁2₁2₁, with a = 8.839, b = 14.938, and c = 22.015 Å. The structure has been refined to an R value of 0.051. The peptide backbone is all-trans, and a 4 → 1 hydrogen bond, between the CO group of the urethane moiety and Aib(3) NH, is observed. Aib(1) and Pro(2) occupy the corner positions of a type I β-turn with ? = ?55.4°, Ψ = ?31.3° for Aib(1) and ? = ?71.6°, Ψ = ?38° for Pro(2). The tertiary amide unit linking Pro(2) and Aib(3) is significantly distorted from planarity (Δω = 14.3°).     

Design and synthesis of novel nonpolar host peptides for the determination of the 310- and α-helix compatibilities of α-amino acid buildig blocks: An assessment of α,α-disubstituted glycines

The present work describes three novel nonpolar host peptide sequences that provide a ready assessment of the 3₁₀- and α-helix compatibilities of natural and unnatural amino acids at different positions of small- to medium-size peptides. The unpolar peptides containing Ala, Aib, and a C-terminal p-iodoanilide group were designed in such a way that the peptides could be rapidly assembled in a modular fashion, were highly soluble in solvent mixtures of triflouroethanol and H₂O for CD- and two-dimensional (2D) nmr spectroscopic analyses, and showed excellent crystallinity suited for x-ray structure analysis. To validate our approach we synthesized 9-mer peptides 79a–96 (Table IV), 12-mer peptides 99–110c (Table V), and 10-mer peptides 120a–125d and 129–133 (Table VI and Scheme 8) incorporating a series of optically pure cyclic and open-chain (R)- and (S)-α,α-disubstituted glycines 1–10 (Figure 2). These amino acids are known to significantly modulate the conformations of small peptides. Based on x-ray structures of 9-mers 79a, 80, and 87 (Figures 4–7), 10-mers 124c, 131, and 132 (Figures 9–12), and 12-mer peptide 102b (Figure 13), CD spectra of all peptides recorded in acidic, neutral, and basic media and detailed 2D-nmr analyses of 9-mer peptide 86 and 12-mer 102b, several interesting conformational observations were made. Especially interesting results were obtained using the convex constraint CD analysis proposed by Fasman on 9-mer peptides 79a–d, 80, 81, 86, and 87, which allowed us to determine the relative content of 3₁₀- and α-helical conformations. These results were fully supported by the corresponding x-ray and 2D-nmr analyses. As a striking example we found that the (S)- and (R)-β-tetralin derived amino acids (R)- and (S)-1 show excellent α-helix stabilisation, more pronounced than Aib and Ala. These novel reference peptide sequences should help establish a scale for natural and unnatural amino acids concerning their intrinsic 3₁₀- and α-helix compatibilities at different positions of medium-sized peptides and thus improve our understanding in the folding processes of peptides. © 1997 John Wiley & Sons, Inc. Biopoly 42: 575–626, 1997     

Reverse Relationship between α-Carbon Chirality and Helix Handedness in (αMe)Phe Peptides

Abstract

The crystal-state preferred conformations of two tripeptides, one tetrapeptide, and one pen- tapeptide, each containing a single residue of the chiral, C^α,α-disubstituted glycine C^α-methyl, C^α-benzylglycine [(αMe)Phe], have been determined by X-ray diffraction. The tripeptides are Z-L-(αMe)Phe-(Aib)₂-OH dihydrate and Z-Aib-D-(αMe)Phe-Aib-OtBu, the tetrapeptide is Z-(Aib)₂-D-(αMe)Phe-Aib-OtBu, and the pentapeptide is pBrBz-(Aib)₂-DL-(αMe)Phe-(Aib)₂-OtBu. While the two tripeptides are folded in a β-bend conformation, two such conformations are consecutively formed by the tetrapeptide. The pentapeptide adopts a regular 3₁₀-helix promoted by three consecutive β-bends. This study confirms the strong propensity of short peptides containing C^α-methylated α-aminoacids to fold into β-bends and 3₁₀-helical structures. Since Aib is achiral, the handedness of the observed bends and helices is dictated by the presence of the (αMe)Phe residue. In general, we have found that the relationship between (αMe)Phe chirality and helix handedness is opposite to that exhibited by protein aminoacids. A comparison with the preferred conformation of other extensively investigated C^α-methylated aminoacids is made.     

Critical Main-Chain Length for Conformational Conversion From 310-Helix to α-Helix in Polypeptides

Abstract

To assess the minimal peptide length required for the stabilization of the a-helix relative to the 3₁₀-helix in Aib-rich peptides, we have solved the X-ray diffraction structures of the terminally blocked sequential hexa- and octapeptides with the general formula -(Aib-L-Ala)_n-(n = 3 and 4, respectively). The hexapeptide molecules are completely 3₁₀-helical with four 1 ← 4 intramolecular N-H … O=C H-bonds. On the other hand, the octapeptide molecules are essentially α-helical with four 1 ← 5 H-bonds; however, the helix is elongated at the N-terminus, with two 1 ← 4 H-bonds, giving these molecules a mixed α/3₁₀-helical character. In both compounds the right-handed screw sense of the helix is dictated by the presence of the Ala residues of L-configuration. This study represents the first experimental proof for a 3₁₀ →α-helix conversion in the crystal state induced by peptide backbone lengthening only.     

Chain length dependent transition of 310- to α-helix of Boc-(Ala-Aib)n-OMe

An apolar synthetic octapeptide, Boc-(Ala-Aib)₄-OMe, was crystallized in the triclinic space group P1 with cell dimensions a = 11.558 Å, b = 11.643 Å, c = 9.650 Å, α = 120.220°, β = 107.000°, γ = 90.430°, V = 1055.889 ų, Z = 1, C₃₄H₆₀O₁₁N₈·H₂O. The calculated crystal density was 1.217 g/cm³ and the absorption coefficient ? was 6.1. All the intrahelical hydrogen bonds are of the 3₁₀ type, but the torsion angles, ? and ψ, of Ala(5) and Ala(7) deviate from the standard values. The distortion of the 3₁₀-helix at the C-terminal half is due to accommodation of the bulky Boc group of an adjacent peptide in the nacking. A water molecule is held between the N-terminal of one peptide and the C-terminal of the other. The oxygen atom of water forms hydrogen bonds with N (1) -H and N (2) -H, which are not involved in the intrahelical hydrogen bonds. The hydrogen atoms of water also formed hydrogen bonds with carbonyl oxygens of the adjacent peptide molecule. On the other hand, ¹H-nmr analysis revealed that the octapeptide took an α-helical structure in a CD₃CN solution. The longer peptides, Boc-(Ala-Aib)₆-OMe and Boc-(Ala-Aib)₈-OMe, were also shown to take an α-helical structure in a CD₃CN solution. An α-helical conformation of the hexadecapeptide in the solid state was suggested by x-ray analysis of the crystalline structure. Thus, the critical length for transition from the 3₁₀- to α-helix of Boc-(Ala-Aib)_n-OMe is 8. © 1993 John Wiley & Sons, Inc.     

Quantum chemical studies on hydrogen bonds in helical secondary structures

We present a brief review of our recent computational studies of hydrogen bonds (H-bonds) in helical secondary structures of proteins, α-helix and 3₁₀-helix, using a Negative Fragmentation Approach with density functional theory. We found that the depolarized electronic structures of the carbonyl oxygen of the ith residue and the amide hydrogen of the (i + 4)th residue cause weaker H-bond in an α-helix than in an isolated H-bond. Our calculations showed that the H-bond energies in the 3₁₀-helix were also weaker than those of the isolated H-bonds. In the 3₁₀-helices, the adjacent N–H group at the (i + 1)th residue was closer to the C=O group of the H-bond pair than the adjacent C=O group in the 3₁₀-helices, whereas the adjacent C=O group at the (i + 1)th residue was close to the H-bond acceptor in α-helices. Therefore, the destabilization of the H-bond is attributed to the depolarization caused by the adjacent residue of the helical backbone connecting the H-bond donor and acceptor. The differences in the change in electron density revealed that such depolarizations were caused by the local electronic interactions in their neighborhood inside the helical structure and redistributed the electron density. We also present the improvements in the force field of classical molecular simulation, based on our findings. Supplementary InformationThe online version contains supplementary material available at 10.1007/s12551-022-01034-5.     

Packing interactions of aib-containing helices: Molecular modeling of parallel dimers of simple hydrophobic helices and of alamethicin

α-Aminoisobutyric acid (Aib) is a helicogenic α,α-dimethyl amino acid found in channel-forming peptaibols such as alamethicin. Possible effects of Aib on helix–helix packing are analyzed. Simulated annealing via restrained molecular dynamics is used to generate ensembles of approximately parallel helix dimers. Analysis of variations in geometrical and energetic parameters within ensembles defines how tightly a pair of helices interact. Simple hydrophobic helix dimers are compared: Ala₂₀, Leu₂₀, Aib₂₀, and P20, the latter a simple channel-forming peptide [G. Menestrina, K. P. Voges, G, Jung, and G. Boheim (1986) Journal of Membrane Biology, Vol. 93, pp. 111–132]. Ala₂₀ and Leu₂₀ dimers exhibit well-defined ridges-in-grooves packing with helix crossing angles (Ω) of the order of +20°. Aib₂₀ α-helix dimers are much more loosely packed, as evidenced by a wide range of Ω values and small helix-helix interaction energies. However, when in a 3₁₀ conformation Aib₂₀ helices pack in three well-defined parallel modes, with Ω ca. ?15°, +5°, and 10°. Comparison of helix–helix interaction energies suggests that dimerization may favor the 3₁₀ conformation. P20, with 8 Aib residues, also shows looser packing of α-helices. The results of these studies of hydrophobic helix dimers are analyzed in the context of the ridges-in-grooves packing model. Simulations are extended to dimers of alamethicin, and of an alamethicin derivative in which all Aib residues are replaced by Leu. This substitution has little effect on helix–helix packing. Rather, such interactions appear to be sensitive to interactions between polar side chains. Overall, the results suggest that Aib may modulate the packing of simple hydrophobic helices, in favor of looser interactions. For more complex amphipathic helices, interactions between polar side chains may be more critical. © 1995 John Wiley & Sons, Inc.     

The polypeptide 310-helix as a template and a spacer

Summary X-ray diffraction analyses have provided detailed structural information on the 3₁₀-helices of (i) pBrBz-d-(Me)Phe-(Aib)₂-d-(Me)Phe-Aib-OtBu and Ac-(Aib)₂-l-Lys(Bz)-(Aib)₂-l-Lys(Bz)-(Aib)₂-NHMe as suitable templates for molecular recognition studies, and (ii) pBrBz-TOAC-(l-Ala)₂-TOAC-l-Ala-NHtBu as an appropriate spacer for an ESR study of side chain to side chain interactions. In addition, in Ac-TOAC-(Aib)₂-l-Trp-Aib-OMe, forming a 3₁₀-helix, the TOAC residue plays the role of an effective quencher of the fluorescence of the tryptophan residue located one turn apart.     

Molecular structure of peptaibol antibiotics: solution conformation and crystal structure of the octapeptide corresponding to the 2-9 sequence of emerimicins III and IV

The infrared absorption and 1H nuclear magnetic resonance analyses of chloroform solutions of the terminally-blocked segment corresponding to the 2-9 sequence of emerimicins III and IV, -(Aib)3-L-Val-Gly-L-Leu-(Aib)2-, are consistent with the presence of a 3(10)-helical structure of high thermal stability. The crystal structure of the octapeptide, obtained by X-ray diffraction indicates the formation of a right-handed 3(10)-helix, stabilized by six consecutive intramolecular N-H....O:C H-bonds, slightly distorted at the level of the L-Leu residue.     

Unfolding of an α-helix in peptide crystals by solvation: Conformational fragility in a heptapeptide

The structure of the peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe has been determined in crystals obtained from a dimethylsulfoxide–isopropanol mixture. Crystal parameters are as follows: C₃₈H₆₉N₇O₁₀ · H₂O · 2C₃H₇OH, space group P2₁, a = 10.350 (2) Å, b = 26.084 (4) Å, c = 10.395(2) Å, β = 96.87(12), Z = 2, R = 8.7% for 2686 reflections observed > 3.0 σ (F). A single 5 → 1 hydrogen bond is observed at the N-terminus, while two 4 → 1 hydrogen bonds characteristic of a 3₁₀-helix are seen in the central segment. The C-terminus residues, Ala(6) and Leu(7) are expended, while Val(5) is considerably distorted from a helical conformation. Two isopropanol molecules make hydrogen bonds to the C-terminal segment, while a water molecule interacts with the N-terminus. The structure is in contrast to that obtained for the same peptide in crystals from methanol-water [ I. L. Karle, J. L. Flippen-Anderson, K. Uma, and P. Balaram (1990) Proteins: Structure, Function and Genetics, Vol. 7, pp. 62–73] in which two independent molecules reveal an almost perfect α-helix and a helix penetrated by a water molecule. A comparison of the three structures provides a snapshot of the progressive effects of solvation leading to helix unwinding. The fragility of the heptapeptide helix in solution is demonstrated by nmr studies in CDC1₃ and (CD₃)₂SO. A helical conformation is supported in the apolar solvent CDCl₃, whereas almost complete unfolding is observed in the strongly solvating medium (CD₃)₂SO. © 1993 John Wiley & Sons, Inc.     

Molecular structure of t-butyloxycarbonyl-Leu-Aib-Pro-Val-Aib-methyl ester,a fragment of alamethicin and suzukacillin: A 310-helical pentapeptide

The pentapeptide Boc-Leu-Aib-Pro-Val-Aib-OMe, a fragment of alamethicin and suzukacillin, crystallizes in the space group P2₁, with a = 11.034 (2), b = 10.894 (2), c = 15.483 (2) Å, β = 104.80 (2)° and Z = 2. The crystal structure has been solved by direct methods and refined to an R value of 0.069. The peptide backbone folds into a right-handed 3₁₀-helical conformation, stabilized by two intramolecular 4 → 1 hydrogen bonds between the Leu(1) CO and Val(4) NH and Aib(2) CO and Aib(5) NH groups. The solid-state conformation is consistent with results of spectroscopic analysis in solution.     

Three complete turns of a 3(10)-helix at atomic resolution: the crystal structure of Z-(Aib)11-OtBu.

The crystal structure of the synthetic protected oligopeptide Z-(Aib)11-OtBu was determined by x-ray crystallography. The undecapeptide folds in a regular 3(10)-helix with nine consecutive 4 --> 1 hydrogen bonds. At present, this is the largest available structure of a homopeptide (including homopeptides consisting of standard amino acids) and also the longest observed regular 3(10)-helix at atomic resolution. Z-(Aib)11-OtBu crystallizes readily from hot ethanol-water mixture and is one of the crystals in which no solvent molecule is co-crystallized. In the crystal head-to-tail hydrogen bonded columns are formed in the [1 0 1] direction. Each helical column is surrounded by six others, whereby two are packed in parallel and four in antiparallel fashion. Helical columns are packed via apolar crystal contacts. The crystal structure of Z-(Aib)11-OtBu is compared with the crystal structures of Z-(Aib)10-OtBu and Z-(Aib)9-OtBu. The similarities and differences are analysed.