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Two-Photon Absorption of Bacteriorhodopsin: Formation of a Red-Shifted Thermally Stable Photoproduct F₆₂₀

University of Marburg, Department of Chemistry, D-35032 Marburg, Germany

Correspondence: Address reprint requests to Prof. Dr. Norbert Hampp, University of Marburg; Tel.: 49-6421-28-25778; Fax: 49-6421-28-25798; E-mail: hampp{at}staff.uni-marburg.de.

By means of high-intensity 532 nm laser pulses, a photochemical conversion of the initial B₅₇₀ 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 F₆₂₀, 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 P₃₆₀ state reported in the literature. The photoconversion from B₅₇₀ to F₆₂₀ is most likely a resonant two-photon absorption induced step. The formation of F₆₂₀ and P₃₆₀ 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 (B₅₇₀) and product (F₆₂₀) wavelengths are strong indicators that F₆₂₀ is formed in a direct photochemical step from B_570. The chemical nature of the P₃₆₀ products probably is that of a retro-retinal containing BR, but the structural characteristics of the F₆₂₀ 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.