Photo-control of peptide helix content by an azobenzene cross-linker: steric interactions with underlying residues are not critical.

Photo-control of protein conformation could prove useful for probing function in diverse biological systems. Recently, we reported photo-switching of helix content in a short peptide containing an azobenzene cross-linker between cysteine residues at positions i and i + 7 in the sequence. In the original sequence, underlying residues at positions i + 3 and i + 4 were made bulky as preliminary modelling suggested that this would enhance photo-control of helix content. To test this hypothesis, peptides with Val, Aib; Ile, Aib; and Ala, Ala at positions i + 3 and i + 4 were synthesized, cross-linked and characterized. Before cross-linking, the peptides show distinct conformational behaviours: two with differing helix/coil mixtures whereas the other has a circular dichroism (CD) spectrum characteristic of beta-sheet and a tendency to aggregate. However, upon cross-linking the peptides have very similar CD spectra: predominantly random coil in the dark but predominantly helical upon irradiation. These results refute the original hypothesis. Steric interactions between the linker and underlying residues do not appear to be critical for photo-switching behaviour. When the cross-linking bridge is lengthened by replacing the i, i + 7 cysteine residues with homocysteine, a lower degree of photo-control of helicity is observed. Furthermore, a non-cross-linking version of the azobenzene reagent is shown not to produce any photo-control of helicity. We conclude that the intramolecular cross-link is essential for photo-switching and that it should be applicable to a wide range of peptides and proteins.