Optical switching of dipolar interactions on proteins.

This work shows that optical switching between the spiro (SP) and merocyanine (MC) states of different photochromes specifically labeled to G-actin can be used to rapidly and reversibly modulate specific dipolar interactions within the conjugate. Members of a common spirobenzopyran photochrome and a related spironaphthoxazine that differ only in the locations of their alkylating groups were selectively labeled to Cys-374 on G-actin. The nature of MC and SP interactions within G-actin was investigated by using optical spectroscopy. The average absorption energy of the highly polarized MC is sensitive to interactions with polar groups on solvents and G-actin; the average absorption energy of the corresponding SP state was found to be relatively constant, consistent with its lower dipole moment compared with MC (5 and 20 D, respectively). Alternate excitation of spirobenzopyran G-actin conjugates with 365 and 546 nm leads to rapid transitions from the SP to MC states and MC to SP states, respectively; optical switching within spirobenzopyran-G-actin occurs with high fidelity and the recovery of specific dipolar interactions between the protein and the MC and SP states. The difference in the free energy for specific dipolar interactions between different MC states within G-actin (6 kcal/mol) is similar to that found for complexes of G-actin and its regulatory proteins. We propose, therefore, that optical switching between SP and MC within an appropriately labeled conjugate could be used to inhibit a functional interaction with a ligand in the MC, but not the SP, state.