The effect of azobenzene cross-linker position on the degree of helical peptide photo-control

Photo-switchable alpha-helical peptides offer a valuable tool to probe protein-biomacromolecule interactions in a spatiotemporally controlled manner. We synthesized a series of 32 residue peptides (AA32 s) with the core structure Ac-W-(E(1)A(2)A(3)A(4)R(5))(6)-Q-NH2 and introduced the azobenzene based cross-linker BSBCA via reaction with Cys residues spaced at i, i + 7 intervals in different positions along the helix. UV/Vis measurements show that the composition of the photostationary state as well as thermal relaxation rates do not change considerably with changes in cross-linker position. CD analysis shows that photo-control of helix folding/unfolding is most effective when the cross-linker is targeted to the middle of the peptide so long as this segment has a high intrinsic helical propensity. The largest degree of photo-controlled conformational change occurred when a cross-linked central region of high intrinsic helicity was followed on the C-terminal side by a region of lower intrinsic helicity. This indicates the BSBCA cross-linker can act as a nucleation site for N-to-C propagation of a helix. These data help to guide the choice of cross-linking site in larger peptides and proteins where photo-control of conformation is desired.