Origins of helix-coil switching in a light-sensitive peptide.

Intramolecular cross-linking of peptides by the light-sensitive compound diiodoacetamideazobenzene has been shown to permit reversible photocontrol of the helix-coil transition. Cross-linking between Cys residues spaced at i and i + 7 positions with the trans form of the linker was found to produce a decreased helix content compared to that of the non-cross-linked peptide. Photoisomerization to the cis form of the linker led to substantially higher helix content than in the non-cross-linked peptide. Detailed conformational analysis of the system leads to the conclusion that photocontrol of helix content does not involve specific interactions between the linker and the peptide. Instead, the change in peptide helix content caused by photoisomerization can be predicted by comparing the length ranges of the cis and trans forms of the linker with the expected distance distribution of the Cys attachment points in the intrinsic conformational ensemble of the peptide. The analysis presented here should help to guide the use of these and related linkers for the conformational control of a variety of peptide and protein systems.