Transmembrane segment peptides with double D-amino acid replacements: helicity, hydrophobicity, and antimicrobial activity.

The adoption of a helical conformation in a membrane environment effectively increases the "apparent hydrophobicity" of a peptide segment by satisfying the backbone H-bonding potential, thus stabilizing it in this environment. Here we sought to explore whether destabilizing the helical conformation would have a measurable effect on the apparent hydrophobicity of such segments in both aqueous and membrane-mimetic environments. In order to uncouple peptide hydrophobicity from helicity, we used the prototypic KKAAAAAAAAAAAAWAAAAAAKKKKNH(2) peptide as a template, and performed pairwise DD-scanning mutagenesis over the length of the sequence. Studies on this library of 13 peptides show that the DD replacements at positions near the center of peptide sequence had the most significant effects on the peptides' retention time in high performance liquid chromatography experiments. Decreased retention times correlate well with decreased helicity as measured by CD spectroscopy in the aqueous environment. Trp fluorescence measurements indicated that the peptides displayed a significant red shift in LPC (but not LPG) with peptides having DD replacements near the middle of the peptide sequence, emphasizing the importance of the anionic membrane in promoting peptide insertion. When tested against a laboratory strain of Escherichia coli, antimicrobial activity of the DD-peptides correlated with the apparent hydrophobicity but not with the overall micelle-based helical content of the peptides per se. Further analysis of the DD-positional dependence of the antimicrobial activity suggests that the presence of a local, uninterrupted stretch of helical structure (10-12 residues) may be a prerequisite for peptide biological activity. The overall findings support the notion that one should distinguish between the hydrophobicity of individual residues and the apparent hydrophobicity of the peptide as a whole, as the latter will ultimately have a greater influence on the properties of the full-length species.