Tuzi A, Ciajolo M R, Guarino G, Temussi P A, Fissi A, Pieroni O
Department of Chemistry, University of Naples, Italy.
Biopolymers. 1993 Jul;33(7):1111-21. doi: 10.1002/bip.360330713.
The crystal and molecular structure of the peptide Boc-L-Ala-delta Phe-delta Phe-NHMe, containing two consecutive dehydro-phenylalanine (delta Phe) residues, has been solved by x-ray diffraction. Two independent molecules, X and Y, are present in the crystallographic unit. Their conformation corresponds approximately to an incipient 3(10)-helix stabilized by two intramolecular hydrogen bonds. The (phi, psi) torsion angles, however, have negative and positive signs in the two molecules X and Y, respectively. Therefore, in spite of the presence of an amino acid residue of the L configuration, the two helical molecules have opposite screw senses, even though the right-handed helix is less distorted than the left-handed one in correspondence of the L-Ala residue. The CD spectra in various solvents exhibit exciton bands originating from dipole-dipole interaction between the delta Phe side chains. Addition of DMSO to the chloroform solution produces, as a first step, a strong increasing of the CD bands, which are then progressively canceled by increasing DMSO concentration. The nmr data parallel the behavior observed in the CD spectra. In CDCl3 solution, the temperature coefficients of the NH resonances are consistent with the involvement of the last two amide protons of the sequence in intramolecular hydrogen bonds, but only negligibly small nuclear Overhauser effects (NOE) are observed. Addition of 5% DMSO-d6 allows the observation of diagnostic NOEs. CD and nmr data indicate that the solid state structure is retained in solution, and are consistent with the presence of right-handed and left-handed conformers, with a prevalence of the more stable right-handed one.
含有两个连续脱氢苯丙氨酸（δPhe）残基的肽Boc-L-Ala-δPhe-δPhe-NHMe的晶体和分子结构已通过X射线衍射解析。晶体学单元中存在两个独立的分子，X和Y。它们的构象大致对应于由两个分子内氢键稳定的初始3(10)-螺旋。然而，（φ，ψ）扭转角在两个分子X和Y中分别具有负号和正号。因此，尽管存在L构型的氨基酸残基，但这两个螺旋分子具有相反的螺旋方向，尽管对应于L-Ala残基处，右手螺旋比左手螺旋的扭曲程度小。在各种溶剂中的圆二色光谱显示出源于δPhe侧链之间偶极-偶极相互作用的激子带。向氯仿溶液中加入DMSO，第一步会使圆二色带强烈增加，然后随着DMSO浓度的增加逐渐消失。核磁共振数据与在圆二色光谱中观察到的行为一致。在CDCl3溶液中，NH共振的温度系数与序列中最后两个酰胺质子参与分子内氢键一致，但仅观察到可忽略不计的小核Overhauser效应（NOE）。加入5%DMSO-d6可以观察到诊断性的NOE。圆二色和核磁共振数据表明固态结构在溶液中得以保留，并且与右手和左手构象体的存在一致，其中更稳定的右手构象体占优势。
