Miick S M, Martinez G V, Fiori W R, Todd A P, Millhauser G L
Department of Chemistry and Biochemistry, University of California, Santa Cruz 95064.
Nature. 1992 Oct 15;359(6396):653-5. doi: 10.1038/359653a0.
Short alanine peptides, containing 16 or 17 residues, appear to form alpha-helices in aqueous solution. But the main spectroscopic analyses used on helical peptides (circular dichroism and nuclear magnetic resonance) cannot distinguish between an alpha-helix (in which the ith residue is hydrogen-bonded to residue i + 4; ref. 9) and the next most common peptide helix, the 3(10)-helix10 (i-->i + 3 hydrogen-bonding). To address this problem we have designed single and doubly spin-labelled analogues of alanine-based peptides in which the nitroxide spin label forms an unbranched side chain extending from the sulphur atom of a cysteine residue. Here we report the circular dichroism, Fourier-transform infrared and electron-spin resonance spectra of these peptides under helix-forming conditions. The infrared absorbance gives an amide I' band with a frequency that is substantially different from that observed for alpha-helices. The electron-spin resonance spectra of doubly labelled helices show that the ranking of distances between side chains, around a single turn (residues 4-8), is inconsistent with an alpha-helical structure. Our experiments suggest that the more likely peptide geometry is a 3(10)-helix.
含有16或17个残基的短丙氨酸肽似乎在水溶液中形成α螺旋。但是用于螺旋肽的主要光谱分析方法（圆二色性和核磁共振）无法区分α螺旋（其中第i个残基与第i + 4个残基形成氢键；参考文献9）和下一个最常见的肽螺旋，即3(10)-螺旋10（i→i + 3氢键结合）。为了解决这个问题，我们设计了基于丙氨酸的肽的单自旋标记和双自旋标记类似物，其中氮氧自旋标记形成从半胱氨酸残基的硫原子延伸的无分支侧链。在此我们报告了这些肽在螺旋形成条件下的圆二色性、傅里叶变换红外光谱和电子自旋共振光谱。红外吸收给出了一个酰胺I'带，其频率与α螺旋所观察到的频率有很大不同。双标记螺旋的电子自旋共振光谱表明，在单一圈（残基4 - 8）周围侧链之间距离的排序与α螺旋结构不一致。我们的实验表明，更可能的肽几何结构是3(10)-螺旋。
