Testing for cis‘ proline with α-aminoisobutyric acid substitution

Substitution of Pro residues with AIB (α-aminoisobutyric acid) residues in peptides provides a means of evaluating the presence of cis' proline conformations both in solution and, using bioassay data, in a receptor complex. ¹ H n.m.r. has been used to probe the DMSO solution conformation of all seven of the possible AIB/Pro isomers of bradykinin. AIB substitution for Pro² and/or Pro³ appears to stabilize a type III β-turn involving the N-terminal residues, but not an incipient 3₁₀ helix suggested by model peptides. These substitutions are correlated with low biological potencies, suggesting that such conformational features may be incompatible with receptor complexation. Alternatively, AIB⁷ -bradykinin analogs exhibit a variety of long range shift perturbations relative to bradykinin. The data suggests that bradykinin can adopt several folded conformations, including β-turns involving both Ser⁶-Pro⁷-Phe⁸-Arg⁹ and Phe⁵-Ser⁶-Pro⁷-Phe⁸. The relatively high biological activities of the AIB⁷-BK suggest that the complexed form of the peptide is characterized by a cis' Pro⁷ conformation.     

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.     

Stabilization of β-turn conformations in enkephalins

Stereochemical constraints have been introduced into the enkephalin backbone by substituting α-aminoisobutyryl (Aib) residues at positions 2 and 3, instead of Gly. ¹H n.m.r. studies of Tyr-Aib-Gly-Phe-Met-NH₂, Tyr-Aib-Aib-Phe-Met-NH₂ and Tyr-Gly-Aib-Phe-Met-NH₂ demonstrate the occurrence of folded, intramolecularly hydrogen bonded structures in organic solvents. Similar conformations are also favoured in the corresponding t-butyloxycarbonyl protected tetrapeptides, which lack the Tyr residue. A β-turn centred at positions 2 and 3 is proposed for the Aib²-Gly³analog. In the Gly²-Aib³analog, the β-turn has Aib³-Phe⁴as the corner residues. The Aib²-Aib³analog adopts a consecutive β-turn or 3₁₀ helical conformation. High in vivo biological activity is observed for the Aib²and Aib²-Aib³analogs, while the Aib³peptide is significantly less active.     

Turn induction by N-aminoproline Comparison of the Gly-Pro-Gly and Glyψ[CO-NH-N]Pro-Gly sequences

The folded structure induced by the N-aminoproline residue (the hydrazino analogue of proline, denoted hPro) in the Boc-Gly¹-hPro²-Gly³-NHiPr hydrazino tripeptide has been characterized in the solid state by X-ray diffraction, and compared to the usual βII-turn structure in the Boc-Gly¹-Pro²-Gly³⁶-NHiPr cognate tripeptide. It is stabilized by a bifurcated hydrogen bond in which (Gly³)NH interacts with both (Gly¹)CO and (hPro²)N^x. This conformation is retained in CH₂Cl₂ and CHC1₃ solutions, and allows an overall folded conformation of the hydrazino tripeptide in which (iPr)NH is hydrogen-bonded to (Boc)CO. The hPro α-hydrazino acid residue appears to promote a local folded structure, and might behave as a β-turn mimic. © Munksgaard 1994.     

Peptide conformations. 33. Conformational analysis of cyclic enkephalin analogs of the type Tyr-cyclo-(-N ω -Xxx-Gly-Phe-Leu-)*

Four conformationally restricted cyclic enkephalin analogs of the type Tyr-cyclo(-Nω -Xxx-Gly-Phe-Leu-) with Xxx = l -Orn, d -Orn, l -Lys and d -Lys have been synthesized by conventional methods and the conformation of the Bocprotected, the deprotected as well as the des Tyr¹ analogs analysed by proton and nitrogen n.m.r. spectroscopy. The assignments, of the resonances were performed by two-dimensional homo- and heteronuclear correlated spectroscopy in the normal (COSY, SECSY) as well as a modified (for the detection of small couplings) version. NOE difference spectroscopy was used to distinguish the amino acid residues with aliphatic side chains. The n.m.r. parameters suggest a rather rigid conformation of the ornithine analogs with two internal NH protons whereas the lysine peptides appear to be more flexible. The structure of the Orn²-analogs can be described by a Gly³-CO HN-Leu⁵-γⁱ-turn and a hydrogen bond Orn²-CO HN -Orn². The d -lysine compound seems to include a β-turn (Gly³-CO HN-d -Lys²). The relation of the different conformational properties of the four cyclic peptides and their biological activities are briefly discussed.     

CONFORMATIONAL CHARACTERIZATION OF CYCLOPENTAPEPTIDE,LVAL-LPRO-GLY-LVAL-GLY: A REPEATING ANALOGUE OF ELASTIN

The cyclopentapeptide, ·L·Val¹-L· Pro²-Gly³-L· Val⁴-Gly⁵, was synthesized and its conformational characterization was carried out using n.m.r. and theoretical energy calculations. The n.m.r. studies indicated the existence of a cis Val¹-Pro² peptide bond in water and a very strong intramolecular H-bond between the Val¹ NH and Gly³ C=O groups. This H-bond forms a β-turn (type II) placing Val⁴ Gly⁵ residues within the turn. Two minimum energy conformations were derived, one of which agrees very well with the solution conformation.     

Influence of serine in position i on conformation and dynamics of reverse turns

NMR spectroscopy has been employed for the conformational analysis of the cyclic hexapeptide cycle(-d -Pro¹-Ala²-Ser³(Bzl)-Trp⁴-Orn⁵(Z)-Tyr⁶-) with and without protecting groups on Ser³ and Orn⁵. This peptide sequence was derived from the active loop sequence of the α-amylase inhibitor Tendamistat (HOE 467). The aim was to investigate the role of serine in position i of a standard β-turn on the conformation and stabilization of this turn. Based on distance and torsion constraints from 2D NMR spectroscopic measurements in DMSO-d₆ solution, structure refinement was accomplished by restrained molecular dynamics (MD) simulations in vacuo and in DMSO. The analysis of both structures in solution reveals a considerable effect of the unprotected serine sidechain on the adjacent β-turn conformation. While in the protected peptide with Ser³(Bzl) a βII-turn is observed between Trp⁴ and Orn⁵, the deprotected compound reveals a βI-turn in this region. The βI-turn is stabilized by a backbone-sidechain hydrogen bond from Orn⁵N_αH to Ser³O_γ. Comparisons with other NMR-derived solution structures of cyclic model peptides and in some protein structures from literature reveal a general structural motif in the stabilization of βI-turns by serine in the i position through backbone-sidechain interactions. © Munksgaard 1995.     

H N.M.R. STUDIES OF PROTECTED α-AMINOISOBUTYRIC ACID CONTAINING PEPTIDES

The benzylic methylene protons in a large number of benzyloxycarbonyl α-aminoisobutyric acid (Z-Aib) containing peptides, show chemical shift nonequivalence. The magnitude of the geminal nonequivalence is correlated with the involvement of the urethane carbonyl group, in an intramolecular hydrogen bond. Studies of the model compounds Z-Aib-Aib-Ala-NHMe, and Z-Aib-Aib-Aib-Pro-OMe clearly establish the presence of intramolecular hydrogen bonds, involving the urethane CO group. In both compounds marked anisochrony of the benzylic methylene protons is demonstrated. In Z-Aib-Aib-Pro-OMe, where a 4 → 1 hydrogen bonded β-turn is not possible, the benzylic -CH₂- protons appear as a singlet in CDCl₃ and have a very small chemical shift difference in (CD₃)₂SO. The observation of such nonequivalence is of value in establishing whether the amino terminal Aib-Pro β-turn is retained in large peptide fragments of alamethicin.     

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.     

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.     

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.     

Spectroscopic analysis of [Trp3]-β-casomorphin analogs Comparative structure conformation-activity studies

A series of [3-tryptophan]-β-casomorphin-5([Trp³]-β-CM-5) analogs were investigated by circular dichroism (CD) and fluorescence spectroscopy to explore their structure-conformation properties in solution. In addition, the comparative opioid activities of these compounds were evaluated using the in vitro guinea pig ileum (GPI) and mouse vas deferens (MVD) assays. Specifically, the pentapeptide sequence of [Trp³]-β-CM-5, H-Tyr-Pro-Trp-Pro-Gly-OH (I) was modified at Pro-2 and Pro-4 by d -Pro substitutions to provide two diastereometric analogs, [Trp³-d -Pro-4]-β-CM-5 (II) and [d -Pro^2,4,Trp³]-β-CM-5 (III). In the GPI and MVD assays, β-CM-5 effected IC₅₀ values of 1.3 μm and 8.9 μm , respectively, which confirmed its known μ/δ-selectivity on these two peripheral opioid receptor subtypes. The potencies of compounds I, II, and III were 0.2, 2.0, and < 0.005 relative to β-CM-5 on the GPI assay. Compounds I and II exhibited pronounced μ/δ-selectivities (> 18.9- and 12.4-fold respectively), whereas compound III was essentially inactive in both the GPI and MVD assays. CD studies of β-CM-5 and its [Trp³]-β-CM-5 analogs showed striking differences in their near-UV and far-UV spectra in aqueous or organic solvents. In the far UV CD spectra, weak (20%) α-helicity (maximum at 193 nm and minima at 208 and 222 nm) for β-CM-5 was obtained in trifluoroethanol (TFE); however, none of the [Trp³]-β-CM-5 analogs showed such CD bands. Of potential relevance to γ-turn or C₇ secondary structure was the observation of a strong negative band at 245 nm for compounds II and III which was not solvent-dependent in H₂O or TFE, whereas compound I showed this CD band exclusively in TFE. In the near-UV CD at 275 nm (Trp electronic transition), the relative order of intensities of this band were determined for the [Trp³]-β-CM-5 compounds to be II > I > III, which was identical to their relative biological potencies in both the GPI and MVD assays. Fluorescence energy transfer (FET) experiments of compounds I-III provided the intramolecular distances (r) between their Tyr (donor) to Trp (acceptor) side-chains, by the Förster method, and were as follows: [Trp³]-β-CM-5, r = 10.6Å; [Trp³, d -Pro⁴]-β-CM-5, r = 9.6Å; and [d -Pro^2,4,Trp³]-β-CM-5, r = 11.0Å. A rank order correlation existed between the Tyr-Trp intramolecular distances and biological activity with shorter distance corresponding to higher biological potency. Furthermore, based on the fluorescence lifetime data analysis (Globals software) of the [Trp³]-β-CM-5 analogs, which were best fitted to a double exponential decay model, the relative ranking of long (> 1.5 ns) lifetime fractions of these three compounds was II > I > III. In summary, detailed spectroscopic analysis of three [Trp³]-β-CM-5 diastereomeric analogs by CD and FET have provided intriguing data indicating a possible structure conformation-activity relationship among these μ/δ-selective opioid-mimetic compounds.     

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.     

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.     

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.     

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.     

Proton NMR Conformational Analysis of Cyclic β-Casomorphin Analogues of the Type Tyr-cyclo[-Nω-D-Orn-Xaa-Yaa-Gly-]

A conformational study of the cyclic β-casomorphin-5 analogues H-Tyr-cyclo[-D-Orn-2-Nal-Pro-Gly-] ( 1 ) (μ-selective agonist; 2-Nal = 2-naphthylalanine), H-Tyr-cyclo[-D-Orn-2-Nal-D-Pro-Gly-] ( 2 ) (mixed μ agonist/δ antagonist) and H-Tyr-cyclo[-D-Orn-Phe-D-Pro-Gly-] ( 3 ) (highly potent μ and δ agonist) has been carried out using ¹H NMR spectroscopy. A complete assignment of the proton resonances of the three pentapeptides has been achieved. Compound 1 was shown to exist in two conformations, a major one (90%) characterized by a cis amide bond between 2-Nal³ and Pro⁴, and a minor one (10%) showing cis amide bonds both between D-Orn² and 2-Nal³ and between 2-Nal³ and Pro⁴. Peptides 2 and 3 each showed only one conformer with all-trans peptide bonds in both cases. Temperature dependence studies of the amide proton chemical shifts indicated the existence of several intramolecular hydrogen bonds in the case of compounds 2 and 3 but not in the case of peptide 1. The backbone conformations of 2 and 3 were found to be similar, both being characterized by two consecutive γ turns around the D-Pro⁴ and D-Orn² residues, respectively, and by a D-Orn²-CO←HN^δ-D-Orn² hydrogen bond. Altogether, the overall backbone conformation and the preferred side chain conformation were found to be roughly similar for the three title peptides. For all three compounds a close proximity between the aromatic moiety of the 3-position residue (2-Nal or Phe) and the D(or L)-Pro⁴ residue was established on the basis of ROESY experiments. The examination of low energy conformations obtained in molecular modelling studies by taking into account the various experimentally found NMR parameters (NOEs, vicinal H,H coupling constants, torsion angles, H-bonds) led to proposals of the solution conformation for each peptide. These conformations are in close agreement with a pharmacophore model for μ opioid receptor binding compounds.     

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.     

New patterns of hydrogen bonded interactions between polypeptide chains

Crystals of glycylglycylglycine (C₆H₁₁N₃O₄), grown from an aqueous methanol solution, are triclinic, space group P1, with the unit cell dimensions (at 22 ± 3°) a= 11.656(3), b= 14.817(3), c= 4.823(2) Å, α= 88.45(3), β= 95.96(3), γ= 105.42(3)°, Z = 4 (with two molecules in the asymmetric unit) with a density of D_obs= 1.58g·cm^-3 and D_calc= 1.572g·cm^-3. The crystal structure was solved by a combination of multisolution and trial and error methods and refined with full-matrix least-squares method to a final R value of 0.036 for the observed 3021 reflections (I ≥ 2s?). The conformation of the two molecules I and II in the asymmetric unit is very similar (except around the N-terminal end); they have the fully extended trans-planar conformation, and have ω values ranging from 2 to 4°. The peptide chain repeating distances (C¹_α - C³_α) are 7.27 Å and 7.18 Å in the two molecules as compared with the value of 6.68 Å for extended β-sheets with β-carbons. There are four different interactions between these two molecules characterized by different hydrogen bonding. Molecule I is hydrogen bonded to a neighboring molecule I using four hydrogen bonds. Molecule II is hydrogen bonded to another II, using bifurcated interactions involving the peptide nitrogen. Molecule I is hydrogen bonded to two different molecules II forming distinctly different hydrogen bonding patterns from the two mentioned above. The molecules are packed in rows, in a head-to-tail fashion (C-terminal opposite N-terminal) and are held together in sheets by hydrogen bonds between carbonyl and amide groups, corresponding to the very familiar anti-parallel pleated sheet arrangement for polypeptides. The hydrogen bonds involving the amino nitrogens as donors are significantly longer and presumably weaker compared to those involving the NH⁺₃ group. The C=O distances show variations that correlated with hydrogen bonding. The N-H … O angle varies from 152 to 174° and the bent N-H … O hydrogen bonds show bifurcated interactions.     

Synthesis and NMR conformational analysis of a β-turn mimic incorporated into gramicidin S

The solution structure of a gramicidin S (GS) analog containing a β-turn mimic[BTD^4-5, Lys^2,2′]GS has been compared to that of native GS. The linear [BTD^4-5, Lys^2,2′]GS was synthesized by solid phase methodology and the cyclized peptide was analyzed by NMR. In the peptide portion of [BTD^4-5, Lys^2,2′]GS, the intramolecular hydrogen bonding pattern, inter-residue NOEs, including a transannular Hα-Hα NOE, and J^Nα coupling constants all describe a solution structure which is equivalent to that of native GS. These data confirm that the BTD group is a competent Type II' β-turn mimic since it does not disrupt the native conformation of GS. It also supports the use of GS as a conformational model in which to test β-turn mimics.