Conformations of peptides containing 1-aminocyclohexanecarboxylic acid (Acc6). Crystal structures of two model peptides

The crystal structures of two peptides containing 1-aminocyclohexanecarboxylic acid (Acc⁶) are described. Boc-Aib-Acc⁶-NHMe · H₂O adopts a β-turn conformation in the solid state, stabilized by an intramolecular 4 → 1 hydrogen bond between the Boc CO and methylamide NH groups. The backbone conformational angles (φ_Aib = – 50.3°, ψ_Aib = – 45.8°; φ_Acc6 = – 68.4°, ψ_Acc6 = – 15°) lie in between the values expected for ideal Type I or III β-turns. In Boc-Aib-Acc⁶-OMe, the Aib residue adopts a partially extended conformation (φ_Aib = – 62.2°, ψ_Aib = 143°) while the Acc⁶residue maintains a helical conformation (φ_Acc6 = 48°, ψ_Acc⁶= 42.6°). ¹H n.m.r. studies in CDCl₃ and (CD₃)₂SO suggest that Boc-Aib-Acc⁶-NHMe maintains the β-turn conformation in solution.     

Motifs and conformational analysis of amino acid residues adjoining β-turns in proteins

Using a data set of 250 non-homologous high-resolution globular proteins, a systematic analysis of the conformations that precede and succeed (positions i and i+3) the various classical β-turn types has been carried out. The collective conformation of a specific β-turn type, including the flanking positions, termed motif, has been studied. In all the four turn types, the majority of examples are preceded and succeeded by extended conformation. Some of the other observations are: (1) In a type I β-turn, Gly at position i+ 3 has a higher favorability to occur with positive ø and does not prefer the major motif βα_R-α_R-β. (2) The left-handed alpha;-helical conformation (alpha;_L) is not preferred at both the flanking positions for type I'and II β-turns, (3) The β–β motif is favourable for all the turn types and the motif β–α_L very highly favourable for type I. © Munksgaard 1996.     

Stereochemistry of peptides containing 1-aminocycloheptane-1-carboxylic acid (Ac7c)

The crystal structures of four peptides incorporating l-aminocycloheptane-l-carboxylic acid (Ac₇c) are described. Boc-Aib-Ac₇c-NHMe and Boc-Pro-Ac₇c-Ala-OMe adopt β-turn conformations stabilized by an intramolecular 4 × 1 hydrogen bond, the former folding into a type-I/III β-turn and the latter into a type-II β-turn. In the dipeptide esters, Boc-Aib-Ac₇c-OMe and Boc-Pro-Ac₇c-OMe, the Ac₇c and Aib residues adopt helical conformations, while the Pro residue remains semi-extended in both the molecules of Boc-Pro-Ac₇c-OMe found in the asymmetric unit. The cycloheptane ring of Ac₇c residues adopts a twist-chair conformation in all the peptides studied. ¹H-NMR studies in CDCl₃ and (CD₃)₂SO and IR studies in CDCl₃, suggest that Boc-Aib-Ac₇c-NHMe and Boc-Pro-Ac₇c-Ala-OMe maintain the β-turn conformations in solution.     

Conformational investigation of αβ-dehydropeptides

Solution conformations of three series of model peptides, homochiral Ac-Pro-L-Xaa-NHCH₃ and heterochiral Ac-Pro-D-Xaa-NHcH₃ (Xaa = Val, Phe, Leu, Abu. Ah) as well as αβ-unsaturated Ac-Pro-ΔXaa-NHCH₃ [Δ Xaa =ΔVal, (Z)-ΔPhe, (Z)-ΔLeu, (Z)-ΔAbu] were investigated in CDCl₃ and CH₂Cl₂ by ¹H-, ¹³C-NMR, and FTIR spectroscopy. NH stretching absorption spectra, solvent shifts Δδ for NH (Xaa) and NHCH₃ on going from CDCl₃ to (CD₃)₂SO, diagnostic interresidue proton NOEs, and trans-cis isomer ratios were examined. These studies performed showed the essential difference in conformational propensities between homochiral peptides (L-Xaa) on the one hand and heterochiral (D-Xaa) and αβ-dehydropeptides (ΔXaa) on the other. Former compounds are conformationally flexible with an inverse γ-bend, a β-turn, and open forms in an equilibrium depending on the nature of the Xaa side chain. Conformational preferences of heterochiral and αβ-dehydropeptides are very similar, with the type-II β-turn as the dominating structure. There is no apparent correlation between conformational properties and the nature of the Xaa side chain within the two groups. The β-turn formation propensity seems to be somewhat greater in αβ-unsaturated than in heterochiral peptides, but an estimation of β-folded conformers is risky.     

Determination of structural elements related to the biological activities of a potent decapeptide agonist of human C5a anaphylatoxin

Abstract: The structural features related to the biologic activities of a potent, response-selective decapeptide agonist of human C5a, YSFKPMPLaR (C5a_65–74, Y65, F67, P69, P71, d -Ala73), were identified by NMR analysis in H₂O, DMSO and TFE. This investigation showed that the KPM residues in H₂O and the SFKPM residues in DMSO exhibited an extended backbone conformation, whereas a twisted conformation was found in this region in TFE. In H₂O, the C-terminal region (PLaR) adopted a distorted type II β-turn or a type II/V β-turn. In the type II/V β-turn, Leu72 exhibited a conformation typical of a type II β-turn, whereas d -Ala73 exhibited a conformation characteristic of a type V β-turn. Furthermore, a γ-turn involving residues LaR overlapped with the type II/V β-turn. In DMSO, the C-terminal region had the analogous turn-like motif (type II/V β-turn overlapping with γ-turn) found in H₂O. In TFE, no β-turn motifs were formed by the PLaR residues. These turn-like motifs in the C-terminal region of the peptide in both H₂O and DMSO were in agreement with the biologically important conformations predicted earlier by a structure–function analysis of a related panel of decapeptide analogs. The motifs determined by the NMR analysis of YSFKPMPLaR in H₂O and DMSO may represent structural elements important for C5a agonist activity and thus can be used to design the next generation of C5a agonist, partial agonist and antagonist analogs.     

Solution structure of a biologically active cyclic LDV peptide analogue containing a type II′β-turn mimetic

The solution structure of cyclo-[Gly-Leu-Asp-Val-BTD] (BTD=β-turn dipeptide) has been determined by two-dimensional ¹H-NMR (nuclear magnetic resonance) spectroscopy and systematic conformational searching combined with molecular dynamics studies. The structure contains two hydrogen bonds between the Gly and Val residues, and a type I β-turn with Leu and Asp at the (i+ 1) and (i+ 2) positions of the turn. The cyclic compound shows activity in a scintillation proximity assay (SPA) for the inhibition of the interaction between the integrin α₄β₁ and vascular cell adhesion molecule-1 (VCAM-1). The structure-activity relationship of the LDV sequence is discussed. © Munksgaard 1996.     

Peptides from chiral Cα,α-disubstituted glycines

The molecular and crystal structures of one derivative and three model peptides (to the pentapeptide level) of the chiral C^α,α-disubstituted glycine Cα-methyl, Cα-isopropylglycine [(αMe)Val] have been determined by X-ray diffraction. The derivative is mClAc-l -(α Me)Val-OH, and the peptides are Z-l -(αMe)Val-(l -Ala)₂-OMe monohydrate, Z-Aib-L-(αMe)Val-(Aib)₂-OtBu, and Ac-(Aib)₂-l -(αMe)Val-(Aib)₂OtBu acetonitrile solvate. The tripeptide adopts a type-I β-turn conformation stabilized by a 1 ← 4N-H . O=C intramolecular H-bond. The tetra- and pentapeptides are folded in regular right-handed 3₁₀-helices. All four L-(αMe)Val residues prefer φ, Ψ angles in the right-handed helical region of the conformational map. The results indicate that: (i) the (αMe)Val residue is a strong type-I/III β-turn and helix former, and (ii) the relationship between (αMe)Val chirality and helix screw sense is the same as that of Cα-monosubstituted protein amino-acids. The implications for the use of the (αMe)Val residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.     

Conformation—activity correlations for chemotactic tripeptide analogs incorporating dialkyl residues with linear and cyclic alkyl sidechains at position 2

Five stereochemically constrained analogs of the chemotactic tripeptide incorporating l-aminocycloalkane-l-carboxylic acid (Ac_nc) and α, α-dialkylglycines (Deg, diethylglycine; Dpg, N, N-dipropylglycine and Dbg, N, N-dibutylglycine) at position 2 have been synthesized. NMR studies of peptides For-Met-Xxx-Phe-OMe (Xxx = Ac₇c. I: Ac₈c. II: Deg, III; Dpg, IV and Dbg, V; For, formyl) establish that peptides with cycloalkyl residues, I and II, adopt folded β-turn conformations in CDCl₃, and (CD³)₂SO. In contrast, analogs with linear alkyl sidechains, III-V, favour fully extended (C₅) conformations in solution. Peptides I-V exhibit high activity in inducing β-glucosaminidase release from rabbit neutrophils, with ED₅₀ values ranging from 1.4–8.0 × 10–¹¹. M. In human neutrophils the Dxg peptides III-V have ED₅₀ values ranging from 2.3 × 10^?8 to 5.9 × 10^?10 M, with the activity order being V>IV>III. While peptides I-IV are less active than the parent. For-Met-Leu-Phe-OH, in stimulating histamine release from human basophils, the Dbg peptide V is appreciably more potent, suggesting its potential utility as a probe for formyl peptide receptors. © Munksgaard 1996.     

Synthetic and conformational studies on dehydrophenylalanine containing model peptides

Four model dipeptides containing a Z-dehydrophenylalanine residue (Δ^ZPhe) at the C-terminal, Boc-X-Δ^Z Phe-NHMe (X = Ala (1), Gly (2), Pro (3), and Val (4)), have been synthesised and their solution conformations investigated by 270 MHz ¹H n.m.r. and i.r. spectroscopy. N.m.r. studies on these peptides clearly show the presence of intramolecularly hydrogen bonded structures in CHCl₃ solutions while such structures appear to be absent in the corresponding saturated peptides. This conclusion is also supported by i.r. studies. Studies of the nuclear Overhauser effect provided evidence for the occurrence of a significant population of β-turn structures in solvents like CDCl₃ and (CD₃)₂SO. The observed NOES are consistent with a major contribution from Type II β-turn structure in CDCl₃, while in (CD₃)₂SO solutions there is evidence of a partially extended structure also.     

Probing the functional conformation of the tridecapeptide mating pheromone of Saccharomyces cerevisiae through study of disulfide-constrained analogs

Analogs of the Saccharomyces cerevisiaeα-mating factor, Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-Gly-Gln-Pro-Met-Tyr, where Lys⁷ and Gln¹⁰ were replaced with Cys, Cys(CH₃), or Ser, were synthesized using solid-phase procedures on a phenylacetamidomethyl resin. Cyclo^7,10[Cys⁷,X⁹,Cys¹⁰,Nle¹²]α-factor, where X = D-Val, D-Ala, l -Ala and Gly, were prepared by on-resin cyclization using thallic trifluoroacetate in yields of 20–30%. Linear sulfhydryl-containing peptides were generated from their corresponding cyclic peptide by treatment with dithioerythritol in basic solution. In the linear analogs, replacement of both Lys⁷ and Gln¹⁰ with a cysteine residue resulted in an over 100-fold loss of the biological activity when compared with the native pheromone. The corresponding cyclic disulfides were 5–10-fold more active than their sulfhydryl-contaihing homologs, and cyclo^7,10[Cys⁷,L-Ala⁹,Cys¹⁰,Nle¹²] α-factor was 50-fold more potent than linear analogs containing Ser or Cys(CH₃) in positions 7 and 10. Binding competition studies indicated that all analogs had low affinity for the α-factor receptor and there was a poor correlation between binding and activity in a growth arrest assay. A cyclic analog in which residues 8 and 9 were replaced by 5-aminopentanoic acid was not biologically active. Based on NMR studies, all cyclic peptides have a higher tendency to form β-turns spanning residues 7–10 than their less active linear counterparts. The results provide strong evidence that this β-turn is important for optimal signal transduction by α-factor.     

Conformational characteristics of asparaginyl residues in proteins

Backbone conformations at 1064 asparaginyl residues in 123 non-homologous, high-resolution X-ray structures of proteins were analysed. Asn adopts conformations in left-handed α-helical region and other partially allowed regions in the Ramachandran map more readily than any other non-glycyl residue. Asn conformational clusters in the (φ,ψ) regions of left-handed α-helix, right-handed 7ALPHA;-helix and extended (β) strands were investigated in detail for their occurrence in various secondary structures, especially in β-turn regions. Preferences were observed for Asn conformations in different positions in various β-turn types, including the first and fourth positions of the turn. Asparaginyl residues with extended conformations are found to occur frequently in irregular regions, although they are expected to occur predominantly in extended strands or in the third position of type II β-turns. Asn conformations at the N-cap positions of helices strongly prefer extended conformation than α₁, which seems to be characteristic of non-glycyl residues at that position. In the linkers connecting two extended strands and those connecting an α-helix and an extended strand, Asn with α¹ or α_R conformation is more favoured than Asn with the β-conformation. Analysis of Asn-Asn doublets and Asn-X-Asn triplets permitted identification of conformational families in such sequences. Results of this investigation provide useful hints in modelling Asn-rich regions in proteins such as malaria parasite coat protein.     

Conformational versatility of the Nα-acylated tripeptide amide tail of oxytocin

The synthesis, physical and analytical characterization, and crystal-state structural analysis by X-ray diffraction of three analogues of the N^α-acylated tripeptide amide tail of oxytocin, each containing a cyclic C^{α, α-} disubstituted glycine at position 2, have been performed. The peptides arc Boc-L-Pro-Ac₃c-Gly-NH₂, Z-L-Pro-Ac₅c-Gly-NH₂ and Z-L-Pro-Ac₅c-Gly-NH₂. While the former is folded in a type-II β-turn conformation at the -L-Pro-Ac₃c- sequence, the two latter tripeptides form two consecutive (type-II, type-I′) β-turns. The Ac₅c- and Ac₆c-tripeptides are the first examples of such a highly folded structural combination in a position-2 analogue of the N^α-acylated -L-Pro-L-Leu-GIy-NH² sequence.     

The evaluation of type I and type II β-turn mixtures

Circular dichroism (CD) and¹ H-{¹H}NOE spectra were obtained for Piv-Pro-Ser-NHCH₃(1),[Piv-(CH₃)₃-C-CO], Boc-Pro-Ser-NHCH₃ (2) and Boc-Val-Ser-NHCH₃ (3), to determine the solution conformation of these p-turn models. In the crystal, 1 and 3 adopt an ideal type I β-turn, while 2 is characterized by a semifolded backbone geometry incorporating a cis Boc-Pro tert-amide bond. The predominance of a β-turn conformation in solution was suggested for models 1-3 on the basis of ¹H-{¹H}NOE data. In a nonpolar solvent the prevailing trans rotamer form (>80%) of 2 has a β-turn conformation according to heteronuclear NOE measurement. Positive ¹H-{¹H} NOEs were detected between the H^α(Pro)/NH(Ser), Hα(Ser)/NH(Ser) and NH(NHCH₃3)/HN(Ser) protons in the trans Boc-Pro rotamer form of 2 at -20° in CDCl₃. Similar positive homonuclear NOE enhancements were also observed on the appropriate proton signals in other models, such as Boc-Val-Ser-NHCH₃ (3). Boc-Val-D-Ser-NHCH₃ (4) and Boc-Pro-D-Ser-NHCH₃ (5), in various solvents. The ¹H- {¹H)NOE experiments carried out in CD₃CN clearly showed that besides the type I (or III) β-turn structure, one of the main conformations of models 1-5 is close to the type II β-turn backbone geometry in a nonpolar solvent. Unexpectedly, the conformational mixture of models 1-3 were characterized by class C (helix-like) CD spectra, although class C spectra are generally only correlated with the type I β-turn conformation. These acyclic models are the first carefully investigated examples of -L-L- triamide systems, containing a significant amount of a type II β-turn, as well as the type I p-turn and, however, yielding a class C circular dichroism spectra. The CD spectra recorded for 3 and 4 in acetonitrile were ‘calibrated’ using the ¹H-{¹H}NOE data. Such a “calibration”, as well as the semi-quantitative CD and NMR comprehensive analyses, demonstrated that class C, class B, as well as class C’ CD spectra may be obtained from the linear combination of the same two-component spectra, with different conformational weights. Therefore, it is suggested that the extraction of the conformational components of such models, simply on the basis of their CD spectra, must be made with caution.     

NMR analysis of a potent decapeptide agonist of human C5a anaphylatoxin

A NMR investigation in H₂0, TFE and DMSO of a conformationally constrained, potent decapeptide agonist of human C5a, YSFKDMPLaR (C5a₆₅-₇₄, Y65, F67, P71, d -Ala73) showed that its N-terminal region (YSFKD) exhibited an extended backbone conformation in H₂O and a more twisted conformation in both TFE/H₂O (30:70, v/v; referred to as TFE) and DMSO. The C-terminal region (MPLaR) of the peptide adopted compact, turn-like structures. In H₂O, the C-terminal region adopted a type II β-turn or a distorted type V/II β-turn involving residues PLaR. In the distorted type V/II β-turn, Leu72 exhibited a conformation typical of a type V β-turn, whereas D -Ala73 exhibited a conformation typical of a type II β-turn. The distorted type V/II β-turn overlapped with an inverse γ-turn involving residues MPL. In DMSO, the C-terminal region had the analogous inverse y-turn and the V/II γ-turn found in H₂O. In many of the DMSO structures, two inverse γ-turns in the MPL and PLa positions formed a double-inverse γ-turn. None of the turns observed in H₂O were present in TFE. However, in TFE, the PLa residues formed an inverse γ-turn. Overall, the turn-like structural motifs in the C-terminal region of the peptide in both H₂O and DMSO (but not in TFE) agreed with the biologically important conformations obtained earlier by the structure-function analysis of a panel of C5a agonist peptides. These motifs may represent key structural elements important for C5a agonist activity and may be used to design the next generation of C5a agonist and antagonist analogues. © Munksgaard 1998.     

CD-n.m.r. study of the solution conformation of bradykinin analogs containing α-aminoisobutyric acid

The conformation in aqueous solution of several α-aminoisobutyric acid (AIB)-containing analogs of bradykinin (BK) has been probed by complementary CD and ¹H n.m.r. measurements. The conclusion reached is that substitution of AIB for Pro² and/or Pro³ in BK stabilizes a degree of β-turn conformation in the N-terminal tetrapeptide moiety of the resulting analogs. Changing the solvent from water to DMSO or TFE further enhances the contribution of particular hydrogen bonded structures to the time-averaged conformation of these peptides. Bradykinin and [AIB⁷]-BK adopt similar hydrogen bonded conformations in TFE, apparently with a contribution from a β-turn involving their common Arg¹-Pro²-Pro³-Gly⁴ moiety. The contrasting biological activities of BK and its AIB-analogs are considered in terms of the conformational analogy between the AIB-residue and cis¹ Pro and the propensity for a β-turn at the N-terminus of the peptide.     

Conformation and structure of linear peptides with regularly alternating l- and d-residues: structure of the blocked hexapeptide tert-butyloxycarbonyl-(d-alloisoleucyl-l-isoleucyl)3 methyl ester monohydrate

Peptides with a regular sequence of enantiomeric residues (l and d ) along the chain have received considerable attention because of their accessibility to unique conformations and because they are model compounds for the naturally occurring peptide gramicidin A, which shows monovalent cation selective transmembrane transport. The solid-state structure of the linear hexapeptide t-Boc-(d -aIle-l -Ile)₃-OMe has been determined by X-ray diffraction techniques and refined to a final R factor of 0.068. The molecule shows a bent U-shaped conformation stabilized by three intramolecular H-bonds of the N—H?O=C type: a type II β-bend (4 → 1 bend or C₁₀ ring structure) with l -Ile² and d -aIle³ at positions 2 and 3 of the bend, an α-turn (5 → 1 bend or C₁₃ ring structure) and a 1 → 5 bend or C₁₇ ring structure. The first two 10-membered and 13-membered bends are enclosed in the latter 17-membered hydrogen-bonded ring structure. This structural motif is common to hepta- and octa-peptide cyclic molecules, showing that ring closure is not required to achieve a particular topology in the molecular design of specific bended conformations. © Munksgaard 1995.     

Synthesis,crystal structure and molecular conformation of the tBuCO-D,L-Ala-Δz-Phe-NhiPr α,β-unsaturated dipeptide

The crystal structure of the tBuCO-d,l -Ala-Δ^z-Phe-NHiPr dipeptide has been solved by X-ray diffraction. The peptide crystallizes in monoclinic space group P2JC with a = 13.445 (3) Å, b = 35.088 (4) Å, c = 14.755(3) Å, β= 116.73(1)°, Z = 12 and d_c= 1.151 g.cm^?3. The three independent molecules per asymmetric unit accommodate a βII-folded conformation, but only one of them contains the typical i + 3 → i interaction characterizing a β-turn. In the other two molecules, the N…O distance exceeds 3.2 Å, a value generally considered the upper limit for hydrogen bonds in peptides. In solution, the βII-turn conformation is largely predominant.     

Molecular design of peptides

The peptide N-Boc-l -Phe-dehydro-Abu-NH-CH₃ was synthesized by the usual workup procedure. The crystals grown from methanol at 4°C belong to the space group P2₁2₁2₁ with a= 7.589(2), b= 13.690(4), c= 21.897(6) Å, Z= 4 and d_c= 1.149(5) g cm^?3 for C₁₉H₂₉N₃O₅·CH₃OH. The peptide crystals were highly sensitive to radiation. The final agreement factor R was 0.055 for 1109 observed reflections (I > 2σ) with data extending to a 2θ value of 103°. The methanol oxygen atom is split into two occupancies. Both sites are involved in identical hydrogen bonding. As a result of substitution of a dehydro-Abu residue at the (i+ 2) position the peptide adopts an ideal β-turn II′ conformation with torsion angles of corner residues as φ₁=63(1)°, ψ₁= - 127(1)°, φ₂= -66(1)° and ψ₂= - 10(1)°, and an intramolecular hydrogen bond N—H ? O of length 3.01(1) Å. This shows that the conformational constraints produced by dehydro-Abu are similar in nature to but different in magnitude than those produced by dehydro-Phe and dehydro-Leu. The methanol–peptide interactions show characteristic features of multiple hydrogen-bond formations involving polar sites of participating peptide and methanol molecules. The packing of the molecules in the unit cell is stabilized by interactions through methanol molecules with the help of several hydrogen bonds.     

Studies on β-turn of peptides

The effect of changing 1st and 4th amino acid residues on β-turn preference of tetrapeptide sequences was studied by use of CD spectra of the chromophoric derivatives, which have Dnp- and pNA-groups as the amino and carboxyl substituents, respectively. The effect was examined with the tetrapeptides having such sequences at the 2nd and 3rd positions as -L-Pro-L-Asn-, -L-Pro-Gly-, -L-Pro-D-Ala-, -L-Ala-D-Leu-, -L-Ala-L-Pro-, and -D-Ala-L-Pro-. The β-turn preferences estimated from the CD intensities of the bands due to exciton interaction were found to depend largely on the configurations of the 1st and 4th amino acid residues. When 1st and 2nd (or 3rd and 4th) residues had the same configuration, decreased intensity of the CD band was observed even if the internal sequence had high β-turn preference. Terminal Gly residues were favorable for the β-turn conformation in many of the tetrapeptide sequences examined.     

Hydration and distortion of peptide helices in crystals. α-Helical structure of a dodecapeptide,Boc-(Ala-Leu-Aib)4-OMe

The dodecapeptide Boc-(Ala-Leu-Aib)₄-OMe crystallized with two independent helical molecules in a triclinic cell. The two molecules are very similar in conformation, with a 3₁₀-helix turn at the N-terminus followed by an α-helix, except for an elongated N(7)…O(3) distance in both molecules. All the helices in the crystal pack in a parallel motif. Eleven water sites have been found in the head-to-tail region between the apolar helices that participate in peptide-water hydrogen bonds and a network of water-water hydrogen bonds. The crystal parameters are as follows: 2(C₅₈H₁₀₄N₁₂O₁₅) +ca. 10H₂O, space group P1 with a= 12.946(2), b= 17.321(3), c= 20.465(4)Å, α=103.12(2), β= 105.63(2), γ= 107.50(2)?, Z= 2, R= 10.9% for 5152 data observed > 3σ(F), resolution 1.0 Å. In contrast to the shorter sequences [Karle et al. (1988) Proc. Natl. Acad. Sci. USA 85 , 299–3031 and Boc-(Ala-Leu-Aib)₂-OMe [Karle et al. (1989) Biopolymers 28 , 773–781], no insertion of a water molecule into the helix is observed. However, the elongated N—O distance between Ala⁷ NH and Aib³ CO in both molecules (molecule A, 3.40 Å; molecule B, 3.42 Å) is indicative of an incipient break in the helices.