The Laser-Induced Blue State of Bacteriorhodopsin:  Mechanistic and Color Regulatory Roles of Protein−Protein Interactions, Protein−Lipid Interactions, and Metal Ions

In this paper we characterize the mechanistic roles of the crystalline purple membrane (PM) lattice, the earliest bacteriorhodopsin (BR) photocycle intermediates, and divalent cations in the conversion of PM to laser-induced blue membrane (LIBM; λmax = 605 nm) upon irradiation with intense 532 nm pulses by contrasting the photoconversion of PM with that of monomeric BR solubilized in reduced Triton X-100 detergent. Monomeric BR forms a previously unreported colorless monomer photoproduct which lacks a chromophore band in the visible region but manifests a new band centered near 360 nm similar to the 360 nm band in LIBM. The 360 nm band in both LIBM and colorless monomer originates from a Schiff base-reduced retinyl chromophore which remains covalently linked to bacterioopsin. Both the PM→LIBM and monomer→colorless monomer photoconversions are mediated by similar biphotonic mechanisms, indicating that the photochemistry is localized within single BR monomers and is not influenced by BR−BR interactions. The excessively large two-photon absorptivities (≥106 cm4 s molecule-1photon-1) of these photoconversions, the temporal and spectral characteristics of pulses which generate LIBM in high yield, and an action spectrum for the PM→LIBM photoconversion all indicate that the PM→LIBM and Mon→CMon photoconversions are both mediated by a sequential biphotonic mechanism in which is the intermediate which absorbs the second photon. The purple→blue color change results from subsequent conformational perturbations of the PM lattice which induce the removal of Ca2+ and Mg2+ ions from the PM surface