Effect of point mutations on the ultrafast photo-isomerization of Anabaena sensory rhodopsin.

Anabaena sensory rhodopsin (ASR) is a particular microbial retinal protein for which light-adaptation leads to the ability to bind both the all-trans, 15-anti (AT) and the 13-cis, 15-syn (13C) isomers of the protonated Schiff base of retinal (PSBR). In the context of obtaining insight into the mechanisms by which retinal proteins catalyse the PSBR photo-isomerization reaction, ASR is a model system allowing to study, within the same protein, the protein-PSBR interactions for two different PSBR conformers at the same time. A detailed analysis of the vibrational spectra of AT and 13C, and their photo-products in wild-type ASR obtained through femtosecond (pump-) four-wave-mixing is reported for the first time, and compared to bacterio- and channelrhodopsin. As part of an extensive study of ASR mutants with blue-shifted absorption spectra, we present here a detailed computational analysis of the origin of the mutation-induced blue-shift of the absorption spectra, and identify electrostatic interactions as dominating steric effects that would entail a red-shift. The excited state lifetimes and isomerization reaction times (IRT) for the three mutants V112N, W76F, and L83Q are studied experimentally by femtosecond broadband transient absorption spectroscopy. Interestingly, in all three mutants, isomerization is accelerated for AT with respect to wild-type ASR, and this the more, the shorter the wavelength of maximum absorption. On the contrary, the 13C photo-reaction is slightly slowed down, leading to an inversion of the ESLs of AT and 13C, with respect to wt-ASR, in the blue-most absorbing mutant L83Q. Possible mechanisms for these mutation effects, and their steric and electrostatic origins are discussed.

Photoelectron Spectroscopy of Oppositely Charged Molecular Switches in the Aqueous Phase: Theory and Experiment.

XUV photoelectron spectroscopy (XPS) is a powerful method for investigating the electronic structures of molecules. However, the correct interpretation of results in the condensed phase requires theoretical models that account for solvation. Here we present experimental aqueous-phase XPS of two organic biomimetic molecular switches, NAIP and p-HDIOP. These switches are structurally similar, but have opposite charges and thus present a stringent benchmark for solvation models which need to reproduce the observed ΔeBE = 1.1 eV difference in electron binding energy compared to the 8 eV difference predicted in the gas phase. We present calculations using implicit and explicit solvent models. The latter employs the average solvent electrostatic configuration and free energy gradient (ASEC-FEG) approach. Both nonequilibrium polarizable continuum models and ASEC-FEG calculations give vertical binding energies in good agreement with the experiment for three different computational protocols. Counterions, explicitly accounted for in ASEC-FEG, contribute to the stabilization of molecular states and reduction of ΔeBE upon solvation.

A quasiclassical trajectory study of the OH+SO reaction: The role of rotational energy.

A full dimensional quasiclassical trajectory study of the OH+SO reaction is presented with the aim of investigating the role of the reactants rotational energy in the reactivity. Different energetic combinations with one and both reactants rotationally excited are studied. A passive method is used to correct zero-point-energy leakage in the classical calculations. The reactive cross sections, for each combination, are calculated and fitted to a capturelike model combined with a factor accounting for recrossing effects. Reactivity decreases as rotational energy is increased in any of both reactants. This fact provides a theoretical support for the experimental dependence of the rate constant on temperature.