Retinal, direct excitation

This may be due to a destabilization caused by twisting around the C32-C13 bond. Direct excitation isomerization yields (<)>jgo) are usually higher in systems characterized by high intersystem crossing. This suggests that a triplet isomerization mechanism plays an important role following direct excitation, a fact that was quantitatively confirmed for retinals in nonpolar solvents. With the exception of 11-cis PRSB, the lowest triplet state (T3) appears to possess a substantial isomerization barrier so that partition between cis and trans isomers takes place in higher vibronic states. An O2-induced mechanism, which is probably associated with a triplet pathway, characterizes the fluorescent derivatives ROH, RAc, and RBA. 

Direct excitation of retinal (146) to the 3 A state was studied by nonreso-nant TP excitation at 490 nm using laser pulses with an energy of 8.5 mJ and 5 ns pulse duration, resulting in formation of the 13-cis isomer as the main component and the 11,13-bis di-isomer as by-product [222]. The relative speed of TP induced isomerization depends on the geometry of the isomer (ail-trans > 1-cis 9-cis 11 -cis 13-cis > 11,13-bis cis). This was different from OP excitation. 

Fig. 3.2 Deca-picosecond time-resolved absorption spectra of the all-trans and cis isomers of retinal. The T, state was generated by direct excitation of isomeric retinal with the 355 nm, 20-25 ps pulses [5].

In order to interpret the physicochemical steps of retinal transduction as well as membrane excitability, we analyze macroscopic properties of membranes within biological components. Such membranes separate two aqueous ionic phases the chemical compositions of which are kept constant separately. The total flux through the membrane is directly deduced from the counterbalance quantities in order to maintain the involved thermodynamical affinities constant. From such measurement, we calculate the dynamical membrane permeability. This permeability depends not only on membrane structure but also on internal chemical reactions. 

Hie isomerization of 31 proceeds by a quantum chain process for several directions [125,126,130]. For example, all-trans retinal undergoes isomerization giving the 9-cis and 13-ds isomers in the excited triplet state the quantum yield of isomerization of the all-trans isomer increased from 0.13 at 1.1 X 10 M to... 

More than 30 years ago Warshel proposed, on the basis of semiempirical simulations, an isomerization mechanism that could explain how this process can occur in the restricted space of the Rh binding pocket (Warshel 1976). Since two adjacent double bonds were found to isomerize simultaneously the mechanism reveal a so-called bicycle pedal motion. Due to the concerted rotation of two double bonds in opposite directions the overall conformational change is minimized and hence this mechanism was found to be space-saving. The empirical valence bond (EVB) method (Warshel and Levitt 1976) was used to compute the excited state potential energy surface of the chromophore during a trajectory calculation where the steric effects of the protein matrix were modeled by specific restraints on the retinal atoms. Since then, Warshel and his coworkers have improved the model employing better structural data and new computational developments (Warshel and Barboy 1982 Warshel and Chu 2001 Warshel et al. 1991). The main refinement of the bicycle pedal mechanism was that the simultaneous rotation of the adjacent double bonds is aborted at a twist of 40° and leads to the isomerization of only one bond (Warshel and Barboy 1982). 

Punwong C, Owens J, Martinez TJ (2014) Direct QM/MM excited-state d5mamics of retinal protonated schrff base in isolation and methanol solution. J Phys Chem B. doi 10.1021/ jp5038798... 

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