s-Cis and s-trans isomerism in acylproline analogs

N-Acetyl-2,3-dehydroproline, N-acetyl-5-oxo-L-proline, N-acrylyl-L-proline, N-acetyl-L-azetidine-2-carboxylic acid and N-acetyl-D,L-pipecolic acid have been examined in ²H₂O by ¹H and ¹³Cn.m.r. for the purpose of finding s-cis or s-trans locked acylprolines. Conformationally locked acylprolines could be incorporated into proline-containing peptide hormones such as angiotensin and thyroliberin in order to determine the rotational state of the peptide bond to proline in the hormone receptor complex. The populations of trans and cis rotational isomers were determined as a function of p² H in order to assign the trans and cis isomers and to compare the populations in all the acylprolines at neutral p² H, where the cis isomer is normally present. Proton spectra were also recorded at from 7° to 75° in order to qualitatively determine the exchange rate between the isomers. The majority of these analogs exhibit a cis-trans isomerization similar to that of N-acetyl-L-proline in the ratio of trans to cis rotational isomer found at neutral p² H (about 1:1), the temperature dependence of the population ratio (none), and the coalescence temperature for proton resonances (greater than 75°). However, N-acetyl-5-oxo-L-proline was found to be greater than 98% s-trans at neutral pH, compared to 50% s-trans in N-acetyl-L proline, and therefore a good candidate for synthesis of an s-trans locked peptide hormone. N-Acetyl-2,3-dehydroproline rapidly exchanges between s-cis and s-trans in contrast to all other proline analogs examined and exhibits coalescence of the β-proton cis and trans resonances at 45°. Titration with the shift reagent Pr⁺⁺⁺ was employed to confirm the assignments of the cis and trans methyl resonances of all of the N-acetyl compounds except N-acetyl-5-oxo-L-proline.     

Structural characteristics of diproline: a new crystal form of tBoc-Pro-Pro-OH

The structure of a new crystalline form of tBoc-Pro-Pro-OH (C₁₅ H₂₄ N₂ O₅) has been determined. The crystals were monoclinic, P2₁ a = 14.667(5), b = 16.600(4), c = 15.502(3) Å, β= 117.84(2)?, V= 3337.2 ų and Z= 8, D_c= 1.24g/cm³. There are four molecules in the asymmetric unit, each displaying polyproline-type structure but differing in the proline pucker. All four molecules display a twist conformation in the first proline ring, with molecules A, B and C being ^β_γT (P ～ 183?, τ 33 for A and B, t～18 for C) and molecule D between ^β_γT and _γE (P= 10°, τ38). The second residue of all four molecules has an envelope conformation. Molecules A and B display an ^αE conformation (P～126?, t～25) and molecules C and D display a ^βE conformation (P～168?, τ37). The molecules are hydrogen-bonded (O…OH), forming helical channels along the a-axis.     

Ten-membered cyclotripeptides

The crystal structure of a conformationally restricted cyclotripeptide containing N-methylanthranilic acid, l -phenylalanine and l -proline has been determined. The 10-membered ring of cyelo(-MeAnt-Phe-Pro-) is characterized by a pseudosymmetry plane and by three cis amide bonds. The MeAnt aromatic ring makes an angle of 63.3° with the best plane of the backbone and is nearly perpendicular to the plane of the two adjacent amide bonds. The proline ring adopts ^β_αT twist conformation (Ps = 144°) and the benzylic side chain is extended towards the nitrogen. An analysis of i.r. and n.m.r. spectral data indicates that the solution conformation retains the features found in the crystals. Crystal data: orthorhombic, P2₁2₁2₁ with a= 10.153(3), b= 11.700(3), c= 16.402(3) Å and Z = 4. The final R and R_w are 0.047 and 0.067, respectively.     

s-cis,s-trans Isomerism about the Ala-Pro peptide bond in Nα-phospho- and Nα-phosphono-l-alanyl-l-prolines by 31P n.m.r.

Nα-(Phenethylphosphono)-l -alanyl-l -proline 1, a potent inhibitor of angiotensin converting enzyme, exhibits two ³¹P n.m.r. resonances (intensity ratio one to one), which exchange with a constant (k₁) of about 1 s^?1 and a free energy of activation ΔG*=～ 20kcal/mol at 23° in deuterated dimethylsulfoxide. Two resonances in exchange are also observed in deuterium oxide at pH 7.5. Thus the exchanging ³¹P resonances report the s-cis, s-trans conformational equilibrium about the alanyl-proline peptide bond. Similar results were observed with Nα-[(O-phenyl)-phenethylphosphono]-l -alanyl-l -proline 2. Nα -(O-phenylphospho)-l -alanyl-l -proline, 3 Nα-(O, O′-diphenylphospho)-l -alanyl-l -proline 4, and Nα-[2-(2-oxo-1, 3, 2-dioxaphosphiranyl)]-l -alanyl-l -proline 5 in deuterated dimethyl sulfoxide, deuterium oxide, and deuterochloroform. A ¹³C n.m.r. spectrum of 5 confirmed the presence of s-cis and s-trans resonances for the proline carbons in the same intensity ratio observed by ³¹P n.m.r.     

Crystal structure and molecular conformation of achatin-I (H-Gly-d-Phe-Ala-Asp-OH), an endogenous neuropeptide containing a d-amino acid residue

In order to investigate the active conformation of achatin-I (H-Gly-d -Phe-Ala-Asp-OH), an endogenous neuropeptide from the Achatina fulica ganglia, its crystal structure and molecular conformation were analysed by the X-ray diffraction method. Crystals from methanol/dioxane are monoclinic, space group P2₁ with a=5.083(1), b= 9.125(1), c= 20.939(3) Å, β=94.73(1)° The structure was solved by direct methods and refined to R = 0.051 for 1714 independent reflections with Fo > σ(Fo). The molecule exists as a zwitterion with the Gly N-terminal end protonated and Asp β-carboxyl deprotonated; the C-terminal of Asp is in a neutral state. The molecule takes a kind of β turn structure with the d -Phe-Ala residues at the corner of the bend. This turn conformation is primarily formed by the strong intramolecular hydrogen bonds of NH(Gly)—O^δ1 (Asp) and NH(Asp)- O^δ1(Asp) pairs, thus forming a 15-membered ring structure. Judging from the published data concerning the structure-activity relationship, this turn conformation may reflect an important feature related to the neuroexcitatory activity of achatin-I.     

The pseudo-βI-turn

Synthesis and conformational analysis of three cyclic hexapeptides cyclo(-Gly¹-Pro²-Phe³-Val⁴-Xra⁵-Phe⁶), Xaa= Phe (I), D-Phe (II) and D-Pro (III), were carried out to examine the influence of proline on the formation of reverse turns and the dynamics of hydrophobic peptide regions. Assignment of all ¹H and ¹³C resonances was achieved by homo- and heteronuclear 2D-NMR techniques (TOCSY, ROESY, HMQC, HMQC-TOCSY and HMBCS-270). The conformational analysis is based on interproton distances derived from ROESY spectra and homo- and heteronuclear coupling constants (E.COSY, HETLOC and HMBCS-270). For structural refinements restrained molecular dynamics (MD) simulations in vacuo and in DMSO were performed. Each peptide exhibits two conformations in DMSO solution due to cis-trans isomerism about the Gly-Pro peptide bond. Surprisingly the cis-Gly-Pro segment in the minor isomers is not involved in a βVI-turn, but forms a turn structure with cis-Gly-Pro in the i and i+ 1 positions. Although no stabilizing hydrogen bond is found in this turn, the φ and ψ-angles closely correspond to a βI-turn [Pro²:φ(i+ 1) -60°, ψ(i+ 1) -30° Phe³: φ(i+ 2) -100°, ψ(i+ 2) -50°]. Hence we call this structural element a pseudo-βI-turn. As expected, in the dominating all-trans isomers proline occupies the i+ 1 position of a standard βI-turn. Therefore, cis-trans isomerization of the Gly¹-Pro² amide bond only induces a local conformational rearrangement, with minor structural changes in other parts of the molecule. However, the geometry of the other regions is affected by the chirality of the i+ 1 amino acid for both isomers (βI for Phe⁵, βII′ for D-Phe⁵ or D-Prp⁵).     

N-acyl-diketopiperazines.

N-(N-phenylacetyl-L-alanyl)-cyclo-(L-phenylalanyl-D-prolyl) (I) was synthesized in one step starting from the linear precursor phenylacetyl-L-alanyl-L-phenyl-alanyl-L-proline. X-ray crystallographic analysis of (I) shows that the diketopiperazine ring adopts a boat conformation appreciably more puckered than that found in the unacylated cyclo(L-Pro-D-Phe). The side chain of Phe residue is in quasi-axial flagpole orientation with the aromatic ring folded over the diketopiperazine ring. ¹H-n.m.r. data indicate that the same conformation is preferred in chloroform solution. The proline ring assumes a β-envelope conformation. No intramolecular interactions between the diketopiperazine system and the aromatic ring of the N-phenylacetyl-alanyl side chain have been evidenced. Crystals: space group P2₁ with a= 9.956(3), b= 8.809(2), c= 13.615(2) Å, β= 111.0(1)° and Z = 2. The final R and R_w are 0.037 and 0.052, respectively.     

Crystal structure and conformation of two N-tosyl-protected dipeptides containing amino acids with polar side-chains

X-Ray diffraction studies and energy-minimization calculations were carried out on two dipeptides, N-tosyl-l -Ser-Gly-OMe monohydrate (C₁₃H₁₈N₂O₆S·H₂O, compound A) and N-tosyl-l -Thr-Gly-OMe (C₁₄H₂₀N₂O₆, compound B). Compound A crystallized in the monoclinic system, space group P2₁ with unit cell parameters a= 4.915(1), b= 15.625(4), c= 11.003(1) Å, β= 91.28(1)°, V= 844.8 ų. M_r= 348.4, d= 1.37(2) g cm^?3, Z = 2, λ(Cu Kα) = 1.5418 Å, μ= 1.99 mm^?1, T=293 K. R= 0.032 for 1451 unique reflections with I > 2σ(I). Compound B crystallized in the orthorhombic system, space group P2₁2₁2₁, with unit cell parameters a= 5.050(2), b= 16.483(3), c= 20.769(5) Å, V= 1729.3 ų, Z = 4. M_r= 344.4, d= 1.32(2) g cm^?3, μ(Cu Kα)= 1.90 mm^?1. R= 0.040 for 1060 unique reflections with I > 2σ(I). The major difference in the backbone conformation of the two compounds is in their glycine residues, with the glycine residue in compound A adopting an extended conformation with φ= - 132.6(3)° and ψ= 175.3(3)° and that in compound B having a folded conformation with φ=?56.3(6)° and ψ=?42.6(7)°. In compound A the oxygen atom of the Ser side-chain and the carbonyl oxygen atom of glycine are bridged by the water of crystallization through O—H ··· O hydrogen bonds, resulting in the relatively rare trans conformation [χ=? 175.7(2)°] for this side-chain. The Thr side-chain in compound B is in the sterically preferred (tg^?) conformation [χ^1,1=? 179.4(4)° and χ^1,2=?62.3(5)°]. The conformations were found to be in general agreement with those obtained by an energy-minimization procedure. The energy-minimized structure of N-tosyl-l -Ser-Gly-Ome (anhydrous) showed a strong hydrophobic interaction between the methyl substituents of the tosyl group and the methyl ester (C—C = 4.08 Å).     

Crystal structure of tert.-butyloxycarbonyl-L-prolyl-D-alanyl-D-alanyl-N-methylamide Dimeric β-sheet formation

The crystal structure of the tripeptide t-Boc-L-Pro-D-Ala-D-Ala-NHCH₃, monohydrate, (C₁₇H₃₀N₄O₅·H₂O, molecular weight = 404.44) has been determined by single crystal X-ray diffraction. The crystals are mono-clinic, space group P2₁, a = 9.2585(4), b = 9.3541(5). c = 12.4529(4) Å, β= 96.449(3)°, Z = 2. The peptide units are in the trans and the tBoc-Pro bond in the cis orientation. The first and third peptide units show significant deviations from planarity (Δω=5.2° and Δω=3.7°, respectively). The backbone torsion angles are: φ¹, = -60°, ψ_1/= 143.3°, ω₁= -174.8°, φ₂= 148.4°, ψ₂= -143.1°, ω₂= -179.7°, φ₃= 151.4°, ψ₃= -151.9°, ω₃= -176.3°. The pyrrolidine ring of the proline residue adopts the C₂— C^γ conformation. The molecular packing gives rise to an antiparallel β-sheet structure formed of dimeric repeating units of the peptide. The surface of the dimeric β-sheet is hydrophobic. Water molecules are found systematically at the edges of the sheets interacting with the urethane oxygen and terminal amino groups. Surface catalysis of an L-Ala to D-Ala epimerization process by water molecules adsorbed on to an incipient β-sheet is suggested as a mechanism whereby crystals of the title peptide were obtained from a solution of tBoc-Pro-D-Ala-Ala-NHCH₃.     

5,5-Dimethylthiazolidine-4-carboxylic acid (DTC) as a proline analog with restricted conformation

Spectroscopic evidence is presented for the lack of intramolecular hydrogen bonding in a simple peptide derivative of 5,5-dimethylthiazolidine-4-carboxylic acid (Dtc). The infrared spectrum of Boc-Pro-Ile-OMe 1 in nonpolar solvents displays two N-H stretching bands at 3419 and 3330 cm^-1 in CCl₄ and one at 3417 and 3328cin^-1 in CHCl₃. The low frequency band at 3328–3330cm^-1 may be assigned to conformations with an intramolecular hydrogen bond between the Ile N-H and Boc C=O. The band at 3417-3419 cm^-1 is the normal Ile N-H stretch. In the polar solvent CH₃ CN only one NH stretching band at 3365 cm^-1 is observed. The IR spectrum of Boc-Dtc-Ile-OMe 2, on the other hand, displays one N-H stretching band at 3423cm^-1 in CCI, and one at 3418cm^-1 in CHCI₃. The IR spectrum of 2 does not display the N-H stretching band that would arise from intramolecular hydrogen bonding between the Boc C=O and Ile N-H. The lack of intramolecular hydrogen bonding for Boc-Dtc-Ile-OMe 2 was evident also in the NMR spectra in nonpolar solvents. The ¹H-NMR spectrum of the Pro dipeptide 1 in 50% CDCl₃/C₆D₆ at 20° displayed two Ile-NH signals at 6.58 and 7.74 ppm. The latter signal corresponds to the intramolecularly hydrogen bonded Ile-NH in the trans-Boc isomer of 1 (60% of the total population), while the former signal corresponds to the nonhydrogen bonded Ile-NH in the cis-Boc isomer. The ¹H-NMR spectrum of the Dtc dipeptide 2 displayed two slowly exchanging cis- and trans-Boc amide isomers as well, but both amide proton resonances were observed upfield at 6.67 and 6.74 ppm, which correspond only to a nonhydrogen bonded Ile N-H. The X-ray crystal structure of Boc-Dtc displays only a cis-Boc-Dtc urethane amide group and two conformations for the Dtc ring, one in which the beta carbon atom is anti to the carboxyl group and the other in which the gamma sulfur atom is anti to the carboxyl group. Conformational analysis of Ac-Dtc-NHMe suggests that in the hydrogen bonded C7 conformation steric interaction between the syn-beta methyl group and carbonyl group of Dtc adds nonbonded and angle strain energies to counteract the stabilizing coulombic interaction between the Boc C=O and terminal amide N-H. Whereas the C7 conformation is a prominent conformation for peptide derivatives of proline, other conformations are favored in peptide derivatives of Dtc (ψ -ñ 110-150° or ñ 320-360°). These results suggest that, in peptides where substitution of Pro appears to maintain or enhance biological activity, the substitution of Dtc for Pro may test the functional importance of the C7 conformation in that position of the peptide sequence.     

Role of azaamino acid residue in β‐turn formation and stability in designed peptide

Abstract: The structural perturbation induced by C^αH→N^α exchange in azaamino acid‐containing peptides was predicted by ab initio calculation of the 6‐31G* and 3‐21G* levels. The global energy‐minimum conformations for model compounds, For‐azaXaa‐NH₂ (Xaa = Gly, Ala, Leu) appeared to be the β‐turn motif with a dihedral angle of φ = ± 90°, ψ = 0°. This suggests that incorporation of the azaXaa residue into the i + 2 position of designed peptides could stabilize the β‐turn structure. The model azaLeu‐containing peptide, Boc‐Phe‐azaLeu‐Ala‐OMe, which is predicted to adopt a β‐turn conformation was designed and synthesized in order to experimentally elucidate the role of the azaamino acid residue. Its structural preference in organic solvents was investigated using ¹H NMR, molecular modelling and IR spectroscopy. The temperature coefficients of amide protons, the characteristic NOE patterns, the restrained molecular dynamics simulation and IR spectroscopy defined the dihedral angles [ (φ_i+1, ψ_i+1) (φ_i+2, ψ_i+2)] of the Phe‐azaLeu fragment in the model peptide, Boc‐Phe‐azaLeu‐Ala‐OMe, as [(?59^°, 127^°) (107^°, ?4^°)]. This solution conformation supports a βII‐turn structural preference in azaLeu‐containing peptides as predicted by the quantum chemical calculation. Therefore, intercalation of the azaamino acid residue into the i + 2 position in synthetic peptides is expected to provide a stable β‐turn formation, and this could be utilized in the design of new peptidomimetics adopting a β‐turn scaffold.     

C9 conformation of N-(Nα-[(tert.-butyloxy)-carbonyl]-L-alanyl]-N,N'-dicyclohexylurea in solid and solution

An X-ray diffraction study was carried out on a single crystal of N-(N^α-[(tert.-butyloxy)-carbonyl)-l -alanyl)-N,N^α-dicyclohexylurea belonging to the tetragonal space group P4₁2₁2, having cell dimensions a = b= 10.102(3) Å, c = 46.067(7) Å, V = 4701.2 ų, Z = 8. The crystal structure was solved by direct methods and refined to an R value of 0.056 for 1602 unique reflections with I<2.5 σ(I). Crystal structure analysis shows the presence of an intramolecular N–H O=C H-bond stabilizing the molecule in a folded form similar to that of a β turn, forming a nine-membered ring. IR and ¹H-NMR studies in CDCI₃ solution confirm the stable folded conformation found in the crystalline state, as well as the existence of N–H O=C H-bonds in the title compound, as in peptides.     

Conformation and mobility of tyrosine side chain in tetrapeptides: Specific effects of cis- and trans -proline in Tyr-Pro- and Pro-Tyr-segments

We examined the properties of tyrosine in four free tetrapeptides: Ala-Ala-Tyr-Ala (AATA), Ala-Pro-Tyr-Ala (APTA), Ala-Tyr-Ala-Ala (ATAA) and Ala-Tyr-Pro-Ala (ATPA) by CD, n.m.r. and energy calculations. Experimental data (the aromatic ¹L_b signal, rotamer populations around the C_α-C_β bond (x¹), rotations around C_β- C_γ(χ²), chemical shifts of ortho- and meta-protons in the phenolic ring (in aqueous and Me₂SO solutions), NH proton temperature coefficients and vicinal coupling constants ³J_NH-CαH in the backbone (Me₂SO solution) were compared with calculated minimum energy conformations. We find qualitative agreement between the results of the different techniques with respect to global tendencies of conformational behaviour: we present experimental evidence showing that the presence of proline in the sequence has a more pronounced effect on the side chain organization of the residues preceding it than on one succeeding it. This steric influence of proline on its immediate neighbor is even stronger in the cis isomer than in the more common trans isomer. The strong preference for Rotamer II (χ¹ = 180°) over Rotamer I (χ¹ = - 60°) in ATPA (cis -form) concomitant with a noticeable deviation of χ² is a striking example.     

Influence of hydrophobic interactions on the conformational adaptability of the β‐Ala residue

Abstract: The chemical synthesis and X‐ray crystal structure analysis of a model peptide incorporating a conformationally flexible β‐Ala residue: Boc‐β‐Ala‐Pda, 1 (C₂₃H₄₆N₂O₃: molecular weight = 398.62) have been described. The peptide crystallized in the crystal system triclinic with space group P2₁: a = 5.116(3) Å, b = 5.6770(10) Å, c = 21.744(5) Å; α = 87.45°, β = 86.87°, γ = 90.0°; Z = 1. An attractive feature of the crystal molecular structure of 1 is the induction of a reasonably extended backbone conformation of the β‐Ala moiety, i.e. the torsion angles φ ≈ ?115°, µ ≈ 173° and ψ ≈ 122°, correspond to skew^?, trans and skew⁺ conformation, respectively, by an unbranched hydrophobic alkyl chain, Pda, which prefers an all‐anti orientation (θ₁ ≈ ?153°, θ₂ ≈ … θ₁₄ ≈ ±178°). The observation is remarkable because, systematic conformational investigations of short linear β‐Ala peptides of the type Boc‐β‐Ala‐Xaa‐OCH₃ (Xaa = Aib or Acc⁶) have shown that the chemical and stereochemical characters of the neighboring moieties may be critical in dictating the overall folded and/or unfolded conformational features of the β‐Ala residue. The overall conformation of 1 is typical of a ‘bar’. It appears convincing that, in addition to a number of hydrophobic contacts between the parallel arranged molecules, an array of conventional N‐H…O=C intermolecular H‐bonding interactions stabilize the crystal molecular structure. Moreover, the resulting 14‐membered pseudo‐ring motif, generated by the amide–amide interactions between the adjacent molecules, is completely devoid of nonconventional C?H…O interaction. The potentials of the conformational adaptation of the β‐Ala residue, to influence and stabilize different structural characteristics have been highlighted.     

Mechanism of solvent-induced thermal stabilization of α-amylase from Bacillus amyloliquefaciens

The transition temperature of irreversible thermal inactivation of α-amylase from Bacillus amyloliquefaciens was estimated to be 60°C. At this temperature, the enzyme inactivation followed first-order kinetics, having a half-life (t_1/2) of 12 min with a rate constant (k) of 0.06 min^?l. Conformational change was a prerequisite for this thermal inactivation. This is governed by stepwise temperature-dependent phenomena. Among the solvent stabilizers tested, the enzyme was thermally stable in presence of DMSO and PEG 300 and the stabilizing efficiency of these cosolvents was concentration-dependent. The enzyme was partially stabilized by 5.0 M DMSO and 1.9 M PEG 300 up to 78°C. However, above 78°C the enzyme was inactivated in these cosolvents also. The mechanism of stabilization has been explained by preferential hydration of the enzyme in these structure stabilizing solvents by exclusion from the protein surface and interface by measurement of partial specific volume in these cosolvents. The data suggest a high value of preferential interaction parameter, (δ_g3/δ_g2)_T, _μ1,_μ3 being -0.606 g/g in 40% DMSO and a low value of -0.025 g/g in 5% glycerol. The preferential interaction parameters in sucrose and glycerol suggests that (δ_g3/δ_g2)_T, _μ1,_μ3 is highest of -0.420 g/g in 10% glycerol than any other cosolvent. © Munksgaard 1995.     

Intramolecular H-bonds and thioamide rotational isomerism in thiopeptides

Mono- and dithionated N-acyl amino acid and dipeptide N′-methylamides were synthesized using Lawesson's reagent and 5-thioacetyl thioglycolic acid. The conformation of the thionated models was characterized by IR, ¹³C, and ¹H NMR spectroscopy, including NOE experiments. The formation of —C=S…H—N—C=X (X = O or S) intramolecular H-bonds of the type 2 → 2. 1 → 3 and 1 → 4 was evidenced by the characteristic shifts of the IR stretching frequencies of the NH group. Act-Pro-NHCH₃ (4) and Act-Prot-NHCH₃ (5) were found to be present as mixtures of rotational isomers about the CS—N bond. ¹³C chemical shifts of the γ- and β-carbons of the proline ring elucidated the conformation (Z or E) of the tertiary thioamide group. Our results suggest that the conformation of thiopeptides is determined by two factors: 1) the H-bond donating and accepting ability of the thioamide group and 2) the repulsion between the thiocarbonyl sulfur atom and the side chain groups of the neighbouring amino acid residues.     

Structure and conformation of peptides involving prolyl residue

The dipeptide, L-prolyl-L-leucine monohydrate (C₁₁ H₂₀ N₂ O₃· H₂O, molecular weight, 246.3) crystallizes in the monoclinic space group P2_1/, with cell constants: a = 6.492(2)Å b = 5.417(8)Å c = 20.491(5)Å, β= 96.59(2)°, Z = 2, Do = 1.15g/cm³, and Dc = 1.142g/cm³. The structure was solved by SHELX-86 and refined by full matrix least squares methods to a final R-factor of 0.081 for 660 unique reflections (I > 2σ (I)) measured on an Enraf Nonius CAD-4 diffractometer (CuK_x, λ= 1.5418 4AR, T = 293 K). The peptide linkage exists in the trans conformation. The pyrrolidine ring exists in the envelope conformation. The values of the sidechain torsion angles are: ψ₁= -59.3(13)°, ψ₂₁= -63.1(16)° and ψ₂₂= 174.8(15)° for leucine (C-terminal). The crystal structure is stabilised by a three-dimensional network of N—H… O, O—H… O, and C—H…O hydrogen bonds.     

CONFORMATION OF CYCLO-(l-THREONINE)2 AND CYCLO-(l-ALLO THREONINE)2

The diketopiperazines cyclo-(l -Thr)₂ and cyclo-(l -allo Thr)₂ in water and in dimethyl sulfoxide were studied by proton and carbon-13 nuclear magnetic resonance, and the dominant conformations were deduced from proton-proton and proton-carbon coupling constants. In cyclo-(l -Thr)₂ the χ¹ = 60°, hydroxyl over the ring, side chain conformation is favored; this conformation is also favored for cyclo-(l -Ser)₂ and cyclo-(l -Ser-d -Ser). However, the important side chain conformation for cyclo-(l -allo Thr)₂ is χ¹ = -60°, methyl group over the diketopiperazine ring. The determining factors are apparently steric. The diketopiperazine ring of cyclo-(l -Thr)₂ is puckered to hold the side chains more nearly axial than is that of cyclo-(l -allo Thr)², although the degree of ring folding is probably not large.     

A solution NMR study of the selectively 13C, 15N-labeled peptaibol chrysospermin C in methanol

Abstract: The conformation of the 19-residue peptaibol chrysospermin C in methanol has been investigated by NMR spectroscopy using selective ¹⁵N and ¹³C labeling of the α-aminoisobutyric acid (Aib) residues. Complete ¹H and ¹³C sequential assignments, including stereospecific assignments for the heavily overlapped resonances from the two C^β methyl groups of the eight Aib residues, are reported for a peptaibol for the first time. An Aib residue followed by a Pro is an exception to previous suggestions regarding stereospecific assignment of the two C^β methyl groups of Aib residues. Local nuclear Overhauser effects and ³J_HNC and ³J_HNCβ scalar couplings indicate that the φ angles of the Aib residues are restricted sterically to local conformations consistent with right-handed helices. Despite these constraints on the eight Aib residues, the NMR data for chrysospermin C in methanol are generally most consistent with an ensemble of transient conformations, including backbone conformations inconsistent with helical structures. Initial NMR measurements for chrysospermin C bound to micelles suggest structural and dynamic differences relative to alamethicin bound to micelles which may be related to differences in gating voltages for formation of ion channels.