Kawai M, Omori Y, Yamamura H, Butsugan Y, Taga T, Miwa Y
Department of Applied Chemistry, Nagoya Institute of Technology, Japan.
Biopolymers. 1993 Aug;33(8):1207-12. doi: 10.1002/bip.360330807.
The crystal structures of the isovaline (Iva) containing dipeptides, Boc-D-Iva-L-Pro-OBzl and Boc-L-Iva-L-Pro-OBzl, were determined by x-ray diffraction. The diastereomeric peptides were shown to adopt unturned conformations closely similar to each other (phi Iva 52 degrees, psi Iva 46 degrees, phi Pro -65 degrees, and psi Pro 143 degrees for D-Iva-L-Pro sequence and phi Iva 52 degrees, psi Iva 44 degrees, phi Pro -63 degrees, and psi Pro 148 degrees for L-Iva-L-Pro sequence). The Pro ring of each peptide was in C gamma-endo conformation. The unusually large angle CIva-NPro-C delta Pro values (131 degrees in both peptides) were observed, that was due to steric repulsion between the delta-methylene of Pro and the alkyl side chain of Iva residue. These conformations were essentially the same as that of the corresponding alpha-aminoisobutyric acid (Aib)-containing peptide Boc-Aib-L-Pro-OBzl. The result has demonstrated that replacement of either one of the two methyl groups of the Aib residue in Boc-Aib-L-Pro-OBzl with an ethyl group does not cause any significant change in the unturned conformation of the dipeptide.
通过X射线衍射确定了含有异缬氨酸（Iva）的二肽Boc-D-Iva-L-Pro-OBzl和Boc-L-Iva-L-Pro-OBzl的晶体结构。结果表明，非对映体肽采用彼此非常相似的未翻转构象（对于D-Iva-L-Pro序列，φIva为52°，ψIva为46°，φPro为-65°，ψPro为143°；对于L-Iva-L-Pro序列，φIva为52°，ψIva为44°，φPro为-63°，ψPro为148°）。每个肽的脯氨酸环呈Cγ-内型构象。观察到异常大的C Iva-N Pro-CδPro角度值（两种肽中均为131°），这是由于脯氨酸的δ-亚甲基与Iva残基的烷基侧链之间的空间排斥所致。这些构象与相应的含α-氨基异丁酸（Aib）的肽Boc-Aib-L-Pro-OBzl的构象基本相同。结果表明，在Boc-Aib-L-Pro-OBzl中，将Aib残基的两个甲基中的任何一个用乙基取代，不会导致二肽的未翻转构象发生任何显著变化。
