How Methylation Modifies the Photophysics of the Native All- trans-Retinal Protonated Schiff Base: A CASPT2/MD Study in Gas Phase and in Methanol.

A comparison between the free-energy surfaces of the all- trans-retinal protonated Schiff base (RPSB) and its 10-methylated derivative in gas phase and methanol solution is performed at CASSCF//CASSCF and CASPT2//CASSCF levels. Solvent effects were included using the average solvent electrostatic potential from molecular dynamics method. This is a QM/MM (quantum mechanics/molecular mechanics) method that makes use of the mean field approximation. It is found that the methyl group bonded to C10 produces noticeable changes in the solution free-energy profile of the S1 excited state, mainly in the relative stability of the minimum energy conical intersections (MECIs) with respect to the Franck-Condon (FC) point. The conical intersections yielding the 9- cis and 11- cis isomers are stabilized while that yielding the 13- cis isomer is destabilized; in fact, it becomes inaccessible by excitation to S1. Furthermore, the planar S1 minimum is not present in the methylated compound. The solvent notably stabilizes the S2 excited state at the FC geometry. Therefore, if the S2 state has an effect on the photoisomerization dynamics, it must be because it permits the RPSB population to branch around the FC point. All these changes combine to speed up the photoisomerization in the 10-methylated compound with respect to the native compound.

Theoretical study of the competition between intramolecular hydrogen bonds and solvation in the Cys-Asn-Ser tripeptide.

A study of the competition between intra- and intermolecular hydrogen bonds and its influence on the stability of the Cys-Asn-Ser tripeptide in aqueous solution was performed by using the averaged solvent electrostatic potential from molecular dynamics method (ASEP/MD). The model combines a DFT-B3LYP/6-311+G(d) quantum treatment in the description of the solute molecule with NVT molecular dynamics simulations in the description of the solvent. In gas phase, the most stable structure adopts a C5 conformation. Somewhat higher in energy are found the PP(II) and C7eq structures. In solution, the stability order of the different conformers is reversed: the PP(II) structure becomes the most stable, and the C5 structure is strongly destabilized. The conformational equilibrium is shifted toward conformations in which the intramolecular hydrogen bonds (IHB) have been substituted with intermolecular hydrogen bonds with the water molecules. The solvent stabilizes extended structures without IHBs that are not stable in vacuum. The effect of the protonation state on the conformational equilibrium was also analyzed.