Oliver Weingart, Annapaola Migani, Massimo Olivucci, Michael A. Robb, Volker Buss, and Patricia Hunt (J. Phys. Chem. A; 2004; 108(21) pp 4685 - 4693.)
DOI: 10.1021/jp049140b

Both, static and dynamic aspects of the Z-->E photoisomerization of the Z_pentadieniminium cation, a minimal model for the biologically relevant chromophore retinal, have been investigated with CASSCF methodology. A comparative study consisting of surface mappings and sampled semiclassical molecular dynamics provides detailed information about the topology of the relevant sections of the potential energy surface. A low lying intersection seam (IS) of the involved S1_S0 surfaces has been mapped along the Z-->E twist angle of the central double bond from 0¡ to 180¡. The evolution of a wave packet consisting of a statistically relevant number of zero point energy sampled trajectories towards the S1-S0 IS was examined. The results indicate that the photodynamics of the Z-->E isomerization is controlled by a small segment of the IS and that in addition to the surface topology the kinetic energy of the molecule has an important influence on the rate of the Z-->E isomerization.

Figure LHS S1 energy distribution at the hop geometries as a function of the C3-C4-C5-C6 torsion. Data refer to the trajectories that hop within the simulation time (65 over 70). The energy of the IS segment (0¡-120¡ range) relevant to the excited state dynamics of 1 is also shown as a single line as a function of the C3-C4-C5-C6 torsion. S1 energies are relative to the minimal energy intersection point CI92¡. Full circles, l, denote formation of E product , and open circles, ¡, denote regeneration of Z educt. A higher energy one-dimensional cross section of the IS is schematically indicated with a dashed line. Figure RHS: Distribution of the C3-C4-C5-C6 torsion of the hop geometries. The figure also shows the total number of hops in a set of 65 trajectories that lead to the Z educt (dark grey) or E product (light grey) as a function of the C3-C4-C5-C6 torsion.

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"Photoactivation of the Photoactive Yellow Protein: Why Photon Absorption Triggers a Trans-to-Cis Isomerization of the Chromophore in the Protein"

Gerrit Groenhof, Mathieu Bouxin-Cademartory, Berk Hess, Sam P. de Visser, Herman J. C. Berendsen, Massimo Olivucci, Alan E. Mark, and Michael A. Robb (J. Am. Chem. Soc.; 2004; 126(13) pp 4228 - 4233)
DOI: 10.1021/ja039557f

Atomistic QM/MM simulations have been carried out on the complete photocycle of Photoactive Yellow Protein, a bacterial photoreceptor, in which blue light triggers isomerization of a covalently bound chromophore. The "chemical role" of the protein cavity in the control of the photo-isomerization step has been elucidated. Isomerization is facilitated due to preferential electrostatic stabilization of the chromophore's excited state by the guanidium group of Arg52, located just above the negatively charged chromophore ring. In vacuo isomerization does not occur. Isomerization of the double bond is enhanced relative to isomerization of a single bond due to the steric interactions between the phenyl ring of the chromophore and the sidechains of Arg52 and Phe62.

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