Imamura Takeyoshi, Konishi Kazue, Konishi Katsutoshi
Department of Biophysical Chemistry, Dokkyo University School of Medicine, Mibu, Tochigi 321-0293, Japan.
J Pept Sci. 2006 Jun;12(6):403-11. doi: 10.1002/psc.741.
The free energies of transfer for indole and tryptophan derivatives and pentapeptides having single tryptophan residues from aqueous to sodium dodecyl sulfate (SDS) micellar phases have been systematically studied using the conventional method of ultraviolet absorption spectrophotometry. The free energies for the position isomers of methyl indoles varied depending on the substitution positions. Thus, the contribution of the methyl group to the binding affinity of the 4-methyl indole to the micelle was about twice that of the 2- and 7-methyl indoles. The free energy changes with the introduction of halogen groups to the indole rings were correlated to the nonpolar water-accessible surface area (DeltaA(np)) of the halogen moieties, which were regarded as hydrophobic. The relationships followed straight lines passing through the origins. Position dependence having tendencies similar to the methyl indoles was observed among the magnitudes of the slopes of the straight lines. These results strongly suggest that the indole rings of the derivatives residing in the micellar interface regions direct their imino moieties --NH-- toward the micellar surfaces. Experiments using model tryptophan pentapeptides showed that the magnitude of free energy change per methylene unit of an alkyl amino acid residue in the pentapeptide increased with elongation of the alkyl moiety and was not a constant value as reported for various alkyl compounds. When the peptides distribute to the SDS micelles, the peptide backbones are anchored in aqueous phases and the amino acid side chains in the interfaces extend their alkyl groups toward the micellar centers. Thus, the free energy changes can be connected to the positions of the alkyl groups of the amino acid residues in the micelles.
采用传统的紫外吸收分光光度法，系统研究了吲哚和色氨酸衍生物以及含有单个色氨酸残基的五肽从水相转移至十二烷基硫酸钠（SDS）胶束相的转移自由能。甲基吲哚位置异构体的自由能因取代位置而异。因此，4-甲基吲哚与胶束的结合亲和力中甲基的贡献约为2-甲基吲哚和7-甲基吲哚的两倍。吲哚环引入卤素基团时的自由能变化与卤素部分的非极性水可及表面积（ΔA(np)）相关，卤素部分被视为疏水基团。这些关系呈过原点的直线。直线斜率大小之间观察到与甲基吲哚类似的位置依赖性。这些结果有力地表明，位于胶束界面区域的衍生物的吲哚环将其亚氨基部分——NH——指向胶束表面。使用模型色氨酸五肽的实验表明，五肽中烷基氨基酸残基每个亚甲基单元的自由能变化幅度随烷基部分的延长而增加，并非如各种烷基化合物报道的那样是一个恒定值。当肽分布到SDS胶束中时，肽主链锚定在水相中，界面中的氨基酸侧链将其烷基伸向胶束中心。因此，自由能变化可与胶束中氨基酸残基烷基的位置相关联。
