Two-Photon Absorption of Bacteriorhodopsin: Formation of a Red-Shifted Thermally Stable Photoproduct F 620

Abstract

By means of high-intensity 532 nm laser pulses, a photochemical conversion of the initial B 570 state of bacteriorhodopsin (BR) to a stable photoproduct absorbing maximally at ≈620 nm in BR suspensions and at ≈610 nm in BR films is induced. This state, which we named F 620, is photochemically further converted to a group of three products with maximal absorptions in the wavelength range from 340 nm to 380 nm, which show identical spectral properties to the so-called P 360 state reported in the literature. The photoconversion from B 570 to F 620 is most likely a resonant two-photon absorption induced step. The formation of F 620 and P 360 leads to a distinguished photo-induced permanent optical anisotropy in BR films. The spectral dependence of the photo-induced anisotropy and the anisotropy orientations at the educt (B 570) and product (F 620) wavelengths are strong indicators that F 620 is formed in a direct photochemical step from B 570. The chemical nature of the P 360 products probably is that of a retro-retinal containing BR, but the structural characteristics of the F 620 state are still unclear. The photo-induced permanent anisotropy induced by short laser pulses in BR films helps to better understand the photochemical pathways related to this transition, and it is interesting in view of potential applications as this feature is the molecular basis for permanent optical data storage using BR films.