Coherent processes in formation of primary products of rhodopsin photolysis

Abstract

194 The absorption of the light quantum by rhodopsin (R 498 ), a visual pigment, results in the cis → trans isomerization of its chromophore, 11cis -retinal, over approximately 200 fs [1, 2] with a quantum yield of 0.65 [3]. The physical meaning of this fast and efficient photoreaction is in the maximal utilization of the absorbed light quantum energy for isomerization of 11cis -retinal rather than its dissipation into heat or loss as fluorescence. The isomerization of 11cis -retinal leads to conformational rearrangement in the protein moiety of rhodopsin molecule; as a result, rhodopsin as a G-protein binding receptor acquires the ability to interact with G-protein (transducin), thereby initiating phototransduction in the photoreceptor cell. The dynamics of the 11cis -retinal photoisomerization in rhodopsin has been studied by laser kinetic spectroscopy with a femtosecond resolution [1, 2, 4–6]. It has been demonstrated that the first reaction product, photorhodopsin (photo 570 ), is formed over 200 fs to transform into the next product, bathorhodopsin (batho 535 ), in 2–3 ps [1, 4, 5]. The oscillations of the absorption signals of photo 570 , batho 535 , and initial state R 498 , observed during several picoseconds after excitation, have been also discovered. A high rate and quantum yield of 11cis -retinal photoisomerization in rhodopsin as well as the oscillations of absorption signals of the reaction products suggested that this is a coherent reaction. This means that the photoreaction proceeds via passing of the system through nonstationary oscillation states. In terms of the model of two states [1, 7], when two potential energy surfaces (PESs) are involved in photochemical reaction of rhodopsin (Fig. 1), the corresponding processes can be represented as follows. The action of a femtosecond excitation pulse results in a coherent population of certain oscillatory states in the electron-excited molecule, the set of which is named a coherent oscillatory wave packet (hereinafter, wave packet). The coherent photoreaction can be represented as the movement of wave packet over PES S 1 of the excited state along the reaction coordinate. The reaction products are formed Coherent Processes in Formation of Primary Products of Rhodopsin Photolysis