Peptides from chiral C alpha,alpha-disubstituted glycines. Crystallographic characterization of conformation of C alpha-methyl, C alpha-isopropylglycine [(alpha Me)Val] in simple derivatives and model peptides

The molecular and crystal structures of one derivative and three model peptides (to the pentapeptide level) of the chiral C alpha,alpha-disubstituted glycine C alpha-methyl, C alpha-isopropylglycine [(alpha Me)Val] have been determined by X-ray diffraction. The derivative is mClAc-L-(alpha Me)Val-OH, and the peptides are Z-L-(alpha Me)Val-(L-Ala)2-OMe monohydrate, Z-Aib-L-(alpha Me)Val-(Aib)2-OtBu, and Ac-(Aib)2-L-(alpha Me)Val-(Aib)2OtBu acetonitrile solvate. The tripeptide adopts a type-I beta-turn conformation stabilized by a 1----4N--H...O = C intramolecular H-bond. The tetra- and pentapeptides are folded in regular right-handed 3(10)-helices. All four L-(alpha Me)Val residues prefer phi, psi angles in the right-handed helical region of the conformational map. The results indicate that: (i) the (alpha Me)Val residue is a strong type-I/III beta-turn and helix former, and (ii) the relationship between (alpha Me)Val chirality and helix screw sense is the same as that of C alpha-monosubstituted protein amino-acids. The implications for the use of the (alpha Me)Val residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.     

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.     

Circular dichroism studies of α-aminoisobutyric acid-containing peptides: Chain length and solvent effects in alternating Aib-L-Ala and Aib-L-Val sequences

The CD spectra of the peptides Boc-X-(Aib-X)_n-OMe (n = 1, 2, 3) and Boc-(Aib-X)₅-OMe, where X = L -Ala or L -Val have been examined in several solvents. The X = Ala and Val peptides behave similarly in all solvents, suggesting that the Aib residues dominate the folding preferences of these peptides. The decapeptides adopt helical conformations in methanol and trifluoroethanol, with characteristic negative CD bands at 222 and 205 nm. In the heptapeptides, similar spectra with reduced intensities are observed. Comparison with nmr studies suggest that estimates of helical content in oligopeptides by CD methods may lead to erroneous conclusions. The pentapeptides yield solvent-dependent spectra indicative of conformational perturbations. Peptide association in dioxane results in an unusual spectrum with a single negative band at 210 nm for the decapeptides. Disaggregation is induced by the addition of methanol or water to dioxane solutions. Aggregation of the heptapeptides is less pronounced in dioxane, suggesting that a critical helix length may be necessary to promote association stabilized by helix dipole–dipole interactions.     

Controlling the helical screw sense of peptides with C‐terminal L‐valine

One chiral L ‐valine (L ‐Val) was inserted into the C‐terminal position of achiral peptide segments constructed from α‐aminoisobutyric acid (Aib) and α,β‐dehydrophenylalanine (Δ^ZPhe) residues. The IR, ¹H NMR and CD spectra indicated that the dominant conformations of the pentapeptide Boc‐Aib‐ΔPhe‐(Aib)₂‐L ‐Val‐NH‐Bn (3) and the hexapeptide Boc‐Aib‐ΔPhe‐(Aib)₃‐L ‐Val‐NH‐Bn (4) in solution were both right‐handed (P) 3₁₀‐helical structures. X‐ray crystallographic analyses of 3 and 4 revealed that only a right‐handed (P) 3₁₀‐helical structure was present in their crystalline states. The conformation of 4 was also studied by molecular‐mechanics calculations. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.     

Inversion of 310-helix screw sense in a (D-αMe)Leu homotetrapeptide induced by a guest D-(αMe)val residue

The terminally blocked tetrapeptide pBrBz-[D -(αMe)Leu]₂-D -(αMe)Val-D -(αMe)Leu-OtBu is folded in the crystal state in a left-handed 3₁₀-helical structure stabilized by two consecutive 1 ← 4 C?O ?H? N intramolecular H-bonds, as determined by X-ray diffraction analysis. A CD study strongly supports the view that this conformation is also that largely prevailing in MeOH solution. A comparison with the published conformation of pBrBz-[D -(αMe)Leu]₄-OtBu indicates that incorporation of a single internal β-branched (αMe)Val guest residue into the host homo-tetrapeptide from the γ-branched (αMe)Leu residue is responsible for a dramatic structural perturbation, i.e. an inversion of the 3₁₀ screw sense from right to left-handed.     

Terminal effect of chiral residue on helical screw sense in achiral peptides

To understand the terminal effect of chiral residue for determining a helical screw sense, we adopted five kinds of peptides I – V containing N‐ and/or C‐terminal chiral Leu residue(s): Boc–L ‐Leu–(Aib–ΔPhe)₂–Aib–OMe ( I ), Boc–(Aib–ΔPhe)₂–L ‐Leu–OMe ( II ), Boc–L ‐Leu–(Aib–ΔPhe)₂–L ‐Leu–OMe ( III ), Boc–D ‐Leu–(Aib–ΔPhe)₂–L ‐Leu–OMe ( IV ), and Boc–D ‐Leu–(Aib–ΔPhe)₂–Aib–OMe ( V ). The segment –(Aib–ΔPhe)₂– was used for a backbone composed of two “enantiomeric” (left‐/right‐handed) helices. Actually, this could be confirmed by ¹H‐nmr [nuclear Overhauser effect (NOE) and solvent accessibility of NH resonances] and CD spectroscopy on Boc–(Aib–ΔPhe)₂–Aib–OMe, which took a left‐/right‐handed 3₁₀‐helix. Peptides I – V were also found to take 3₁₀‐type helical conformations in CDCl₃, from difference NOE measurement and solvent accessibility of NH resonances. Chloroform, acetonitrile, methanol, and tetrahydrofuran were used for CD measurement. The CD spectra of peptides I – III in all solvents showed marked exciton couplets with a positive peak at longer wavelengths, indicating that their main chains prefer a left‐handed screw sense over a right‐handed one. Peptide V in all solvents showed exciton couplets with a negative peak at longer wavelengths, indicating it prefers a right‐handed screw sense. Peptide IV in chloroform showed a nonsplit type CD pattern having only a small negative signal around 280 nm, meaning that left‐ and right‐handed helices should exist with almost the same content. In the other solvents, peptide IV showed exciton couplets with a negative peak at longer wavelengths, corresponding to a right‐handed screw sense. From conformational energy calculation and the above ¹H‐nmr studies, an N‐ or C‐terminal L ‐Leu residue in the lowest energy left‐handed 3₁₀‐helical conformation was found to take an irregular conformation that deviates from a left‐handed helix. The positional effect of the L ‐residue on helical screw sense was discussed based on CD data of peptides I – V and of Boc–(L ‐Leu–ΔPhe)_n–L ‐Leu–OMe (n = 2 and 3). © 1999 John Wiley & Sons, Inc. Biopoly 49: 551–564, 1999     

Synthesis and Solid State Conformation of Tetrapeptide Amides Containing two Aib and two (αMe)Phe Residues – Use of Enantiomerically Pure 2‐Benzyl‐2‐methyl‐2H‐azirin‐3‐amines as (αMe)Phe‐Synthons

A series of tetrapeptide amides containing two aminoisobutyric acids (Aib) and two α‐methylphenylalanine ((αMe)Phe) units were prepared through the ‘azirine/oxazolone method’. New 2‐benzyl‐2‐methyl‐2H‐azirin‐3‐amines have been used for the selective introduction of (S)‐ and (R)‐(αMe)Phe, respectively. The solid‐state conformations of five tetrapeptide amides were determined by X‐ray crystallography. In all cases, two β‐turns stabilize 3₁₀‐helical conformations and it was confirmed that, in contrast to proteinogenic amino acids, the configuration of (αMe)Phe does not determine the screw sense of the helix.     

Structures of peptides from α-amino acids methylated at the α-carbon,

The structural preferences of peptides (and depsipeptides) from the achiral MeAib and Hib residues, and the chiral Iva, (αMe) Val, (αMe) Leu, and (αMe) Phe residues, as determined by conformational energy computations, x-ray diffraction analyses, and ¹H-nmr and spectroscopic studies, are reviewed and compared with literature data on Aib-containing peptides. The results obtained indicate that helical structures are preferentially adopted by peptides rich in these α-amino acids methylated at the α-carbon. Intriguing experimental findings on the impact of the chirality of Iva, (αMe) Val, and (αMe) Phe residues on helix screw sense are illustrated. © 1993 John Wiley & Sons, Inc.     

Ribbon structure stabilized by C10 and C12 turns in αγ hybrid peptide

The present study describes the synthesis and crystallographic analysis of αγ hybrid peptides, Boc‐Gpn‐L‐Pro‐NHMe ( 1 ), Boc‐Aib‐Gpn‐L‐Pro‐NHMe ( 2 ), and Boc‐L‐Pro‐Aib‐Gpn‐L‐Pro‐NHMe ( 3 ). Peptides 1 and 2 adopt expanded 12‐membered (C₁₂) helical turn over γα segment. Peptide 3 promotes the ribbon structure stabilized by type II β‐turn (C₁₀) followed by the expanded C₁₂ helical γα turn. Both right‐handed and left‐handed helical conformations for Aib residue are observed in peptides 2 and 3 , respectively Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.     

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     

The crystal structure of Z‐(Aib)10‐OH at 0.65 Å resolution: three complete turns of 310‐helix

The synthetic peptide Z‐(Aib)₁₀‐OH was crystallized from hot methanol by slow evaporation. The crystal used for data collection reflected synchrotron radiation to sub‐atomic resolution, where the bonding electron density becomes visible between the non‐hydrogen atoms. Crystals belong to the centrosymmetric space group P . Both molecules in the asymmetric unit form regular 3₁₀‐helices. All residues in each molecule possess the same handedness, which is in contrast to all other crystal structure determined to date of longer Aib‐homopeptides. These other peptides are C‐terminal protected by OtBu or OMe. In these cases, because of the missing ability of the C‐terminal protection group to form a hydrogen bond to the residue i‐3, the sense of the helix is reversed in the last residue. Here, the C‐terminal OH‐groups form hydrogen bonds to the residues i‐3, in part mediated by water molecules. This makes Z‐(Aib)₁₀‐OH an Aib‐homopeptide with three complete 3₁₀‐helical turns in spite of the shorter length it has compared with Z‐(Aib)₁₁‐OtBu, the only homopeptide to date with three complete turns.     

Association studies of N-acetyl-amino acid N,N-dimethylamides in carbon tetrachloride

The self-association of N-acetylglycine N,N-dimethylamide, N-acetyl-L -valine N,N-dimethylamide, and N-acetyl-L -phenylalanine N,N-dimethylamide in carbon tetrachloride was investigated by using ir and ¹H-nmr methods. It was concluded from ir measurements that the associated species is the dimer formed as a result of the simultaneous formation of two intermolecular hydrogen bonds. This is supported by the results of ¹H-nmr measurements. Thermodynamic quantities for the association were determined from the temperature and concentration dependence of the NH proton chemical shifts of the sample solutions. Compared with the Gly derivative, L -Val and L -Phe derivatives have larger values of ?ΔH for association, which shows good correlation with Δv_NH values, the difference between the maxima of the monomer and dimer bands, obtained from ir spectra. This is due to the less stable monomer conformation and to the stronger intermolecular hydrogen bonding of the dimers in L -Val and L -Phe derivatives. The line shapes of both methyl proton resonances of L -Val residue and methylene proton resonances of L -Phe residue were found to vary with concentration and temperature of the sample solutions. These data indicate that the rotation about the C^α—C^β bond is restricted by the steric hindrance present in the associated dimers. All these experimental results can be related to the fact that L -Val and L -Phe derivatives have a warped framework because of the bulky side chains, whereas the Gly derivative has a planar framework.     

Sweet and bitter taste: Structure and conformations of two aspartame dipeptide analogues

The synthesis and X-ray diffraction analysis of two dipeptide taste ligands have been carried out as part of our study of the molecular basis of taste. The compounds L -aspartyl-D -α-methylphenylalanine methyl ester [L -Asp-D -(αMe)Phe-OMe] and L -aspartyl-D -alanyl-2,2,5,5-tetramethylcyclopentanyl ester [L -Asp-D -Ala-OTMCP] elicit bitter and sweet taste, respectively. The C-terminal residues of the two analogues adopt distinctly different conformations in the solid state. The aspartyl moiety assumes the same conformation found in other dipeptide taste ligands with the side-chain carboxylate and the amino groups formaing a zwitterionic ring with a conformation defined by ψ,χX1 = 157.7°, ?61.5° for L -Asp-D -Ala-OTMCP and 151.0°, ?68.8° for L -Asp-D -(αMe)Phe-OMe. In the second residue, a left-handed helical conformations is observed for the (αMe)Phe residue of L -Asp-D -(αMe)Phe-OMe with ?₂ = 49.0° and ψ₂ = 47.9°, while the Ala residue of L -Asp-D -Ala-OTMCP adopts a semi-exextended conformation characterized by dihedral angles ?₂ = 62.8° and ψ₂ = ?139.9°. The solid-state structure of the bitter L -Asp-D -(αMe)Phe-OMe is extended; while the crystal structure of the sweet L -Asp-D -OTMCP roughly adopts the typical L-shaped structure shown by other sweeteners. The data of L -Asp-D -(αMe)Phe-OMe are compared with those of its diastereoisomer L -Asp-L -(αMe)Phe-OMe. Conformational analysis of the two taste ligands in solution by NMR and computer simulations agrees well with our model for sweet and bitter tastes.     

Design, synthesis, and crystal structure of self-assembling norbornene (NBE)-supported two-helix bundles: a unique example of Janus helicity in the solid-state structure of NBE(Aib(5))(2)

Norbornene-supported bis-helical peptides with the general structure NBE(Aib(n) )(2) (NBE: 2,3-trans-norbornene dicarbonyl unit; Aib: alpha,alpha'-dimethyl glycine unit; n = 4,5) have been synthesized and examined for self-assembly preferences in the solid state. An x-ray study has revealed a phenomenon of Janus helicity in the solid state structure of NBE(Aib(5))(2). The lower homologue NBE(Aib(4))(2), however, shows an identical screw sense for both the helical arms. The difference in the handedness of left and right arms is reflected in the self-assembly patterns. Thus, while the NBE(Aib(4))(2) molecule self-assembles to form an infinite hydrogen-bonded superhelical ladder, the Janus molecule NBE(Aib(5))(2) crystallizes as individual units surrounded by water molecules. The structures of Z-Aib(4)-OMe and Z-Aib(5)-OMe are also presented to compare their conformations with the helical arms of the title compound and also to the already known structures of other X-Aib(n) -Y compounds. The helices in all the molecules are the 3(10)-type.     

Structural versatility of peptides from Cα, α-disubstituted glycines: Crystal-state conformational analysis of homopeptides from Cα-methyl,Cα-benzylglycine [(αMe)Phe]n

The molecular and crystal structures of one derivative and three homopeptides (from the di-to the tetrapeptide level) of the chiral, C^{α, α}-disubstituted glycine C^α-methyl, C^α-benzylglycine [(αMe)Phe], have been determined by x-ray diffraction. The derivative is mClAc-D -(αMe)Phe-OH, and the peptides are pBrBz-[D -(αMe)Phe]₂-NHMe, pBrBz-[D -(αMe)Phe]₃-OH hemihydrate, and pBrBz-[D -(αMe)Phe]₄-OtBu sesquihydrate. All (αMe)Phe residues prefer ?,ψ torsion angles in the helical region of the conformational map. The dipeptide methylamide and the tripeptide carboxylic acid adopt a β-turn conformation with a 1 ← 4 C?O…?H? N intramolecular H bond. The structure of the tripeptide carboxylic acid is further stabilized by a 1 ← 4 C?O…?H? O intramolecular H bond, forming an “oxy-analogue” of a β-turn. The tetrapeptide ester is folded in a regular (incipient) 3₁₀-helix. In general, the relationship between (αMe)Phe chirality and helix screw sense is opposite to that exhibited by protein amino acids. A comparison is made with the conclusions extracted from published work on homopeptides from other C^α-methylated α-amino acids. © 1993 John Wiley & Sons, Inc.     

α-Hydroxymethylserine as a peptide building block: synthetic and structural aspects

A synthetic methodology has been developed for peptide bond formation with α-hydroxmethylserine as the carboxyl or amino component and also for the preparation of homo-sequences. The key intermediate, O,O-protected α-hydroxymethylserine in the form of an isopropylidene derivative, is easily accessible and represents the first example of a heterocyclic C^α,α-disubstituted amino acid containing an 1,3-dioxane ring. The use of this intermediate facilitates protection of the sterically hindered amino and carboxyl groups and is advantageous for the coupling and deprotection steps. X-ray structure determination of Z-HmS(Ipr)–Ala–OMe revealed that the two crystallographically independent molecules present in the asymmetric unit adopt an S-shaped conformation. In the one molecule the achiral HmS(Ipr) residue has the torsion angle values (ϕ==61.4°,ψ=40.8°) in the left-handed helical region of the Ramachandran map, while in the second molecule the negative torsion angles (ϕ=−60.1°, ψ=–44.4°) are associated with the right-handed helix. © 1998 European Peptide Society and John Wiley & Sons, Ltd.     

A quantum mechanical study of the intrinsic helix-forming tendency of α-aminoisobutyric acid and dehydroalanine residues

A quantum mechanical study to compare the ability of α-aminoisobutyric acid (Aib), de-hydroalanine (ΔAla), and alanine (Ala) residues to stabilize helical conformations has been performed. To address the study, the oligopeptides X_n (X = Aib, ΔAla and Ala), where n varies from 1 to 6, were computed with the AM1 semiempirical method. The results show that the residues modified at the C^α carbon atom, Aib and ΔAla, are better helical formers than Ala. Thus, a cooperative energy effect was found for both residues, and especially for ΔAla. These terms permit the understanding the different conformational behaviors between Ala and its C^α-modified residues Aib and ΔAla. This trend is important for de novo protein design, where Aib and ΔAla must be considered useful residues in the design of synthetic helical motifs. © 1994 John Wiley & Sons, Inc.     

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.     

Mixed conformation in Cα,α-disubstituted tripeptides: X-ray crystal structures of Z-Aib-Dph-Gly-Ome and Bz-Dph-Dph-Gly-Ome

We report here the synthesis and molecular structure in the solid state of fully protected tripeptides containing C^α,α-diphenylglycine (Dph), namely Z-Aib-Dph-Gly-OMe (Aib: C^α,α-dimethylgrycine) and Bz-Dph-Dph-Gly-OMe. The molecular conformation around the Dph residue, containing two bulky substituents, is fully extended, while the Aib residue, containing two smaller groups on the C^α atom, adopts the typical 3₁₀/α-helical conformation. Gly residues, without substituents on the C^α atom, show different conformational preferences. Each residue seems to behave, from a conformational point of view, independently from the presence of the other residues, and thus mixed local conformations (folded and extended) are present in the crystals. The nonconventional peptide synthesis, using the Ugi reaction, is also reported. © 1994 John Wiley & Sons, Inc.     

Conformational Preferences of Heterochiral Peptides. Crystal Structures of Heterochiral Peptides Boc-(D) Val-(D) Ala-Leu-Ala-OMe and Boc-Val-Ala-Leu-(D) Ala-OMe-Enhanced Stability of β-sheet Through C-H…O Hydrogen Bonds

Abstract

The crystal structures of Boc-(D) Val-(D) Ala-Leu-Ala-OMe (vaLA) and Boc-Val-Ala-Leu-(D) Ala-OMe (VALa) have been determined. vaLA crystallises in space group P2₁2₁2₁ with a = 9.401 (4), b = 17.253 (5), c = 36.276 (9)Å, V = 5884 (3) ų, Z = 8, R = 0.086. VALa crystallises in space group P2₁ with a = 9.683 (9), b = 17.355 (7), c = 18.187 (9) Å, β = 95.84 (8)°, V = 3040(4) ų, Z = 4, R = 0.125. There are two molecules in the asymmetric unit in antiparallel β-sheet arrangement in both the structures. Several of the C^α hydrogens are in hydrogen bonding contact with the carbonyl oxygen in the adjacent strand.

An analysis of the observed conformational feature of D-chiral amino acid residues in oligopeptides, using coordinates of 123 crystal structures selected from the 1998 release of CSD has been carried out. This shows that all the residues except D-isoleucine prefer both extended and α_L conformation though the frequence of occurence may not be equal. In addition to this, D-leucine, valine, proline and phenylalanine have assumed α_R conformations in solid state. D-leucine has a strong preference for helical conformation in linear peptides whereas they prefer an extended conformation in cyclic peptides.