Conformational investigation of alpha,beta-dehydropeptides. Part. IV. Beta-turn in saturated and alpha,beta-unsaturated peptides Ac-Pro-Xaa-NHCH3: NMR and IR studies.

G Pietrzyński, B Rzeszotarska, Z Kubica
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Abstract

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, Ala) as well as alpha,beta-unsaturated Ac-Pro-delta Xaa-NHCH3 [delta Xaa = delta Val, (Z)-delta Phe, (Z)-delta Leu, (Z)-delta Abu] were investigated in CDCl3 and CH2Cl2 by 1H-, 13C-NMR, and FTIR spectroscopy. NH stretching absorption spectra, solvent shifts delta delta 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 alpha,beta-dehydropeptides (delta Xaa) on the other. Former compounds are conformationally flexible with an inverse gamma-bend, a beta-turn, and open forms in an equilibrium depending on the nature of the Xaa side chain. Conformational preferences of heterochiral and alpha,beta-dehydropeptides are very similar, with the type-II beta-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 beta-turn formation propensity seems to be somewhat greater in alpha,beta-unsaturated than in heterochiral peptides, but an estimation of beta-folded conformers is risky.

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脱氢肽的构象研究。部分。饱和和α, β -不饱和肽Ac-Pro-Xaa-NHCH3的β转化:核磁共振和红外研究。
采用1H- nmr、13C-NMR和FTIR光谱研究了3个系列模型肽,即同手性Ac-Pro-L-Xaa-NHCH3和异手性Ac-Pro-D-Xaa-NHCH3 (Xaa = Val、Phe、Leu、Abu、Ala)以及α、β -不饱和Ac-Pro-delta Xaa- nhch3 [δ Xaa = δ Val、(Z)- δ Phe、(Z)- δ Leu、(Z)- δ Abu]在CDCl3和CH2Cl2中的构象。研究了nhh拉伸吸收光谱、nhh (Xaa)和NHCH3从CDCl3到(CD3)2SO的溶剂位移δ δ、诊断残基间质子NOEs和反式异构体比值。这些研究表明,同手性肽(L-Xaa)与异手性肽(D-Xaa)和α、β -脱氢肽(δ Xaa)在构象倾向上存在本质差异。前一种化合物构象灵活,根据Xaa侧链的性质,具有反γ弯、β弯和开放的平衡形式。杂手性和β -脱氢肽的构象偏好非常相似,ⅱ型β -旋为主导结构。在两个基团中,构象性质与Xaa侧链的性质之间没有明显的相关性。β -转形成倾向似乎在α, β -不饱和中比在杂手性肽中更大,但对β -折叠构象的估计是有风险的。
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