Photoinduced Proton Transfer as a Possible Mechanism for Highly Efficient Excited-State Deactivation in Proteins

CASSCF//CASPT2 pathways for a two-glycine minimal model system show that photoinduced electron-driven forward and backward proton transfer could play an important role for the stability of proteins against damage by UV radiation, when a hydrogen bond is located between the two amino acids. The overall photoinduced process involves two electron and proton transfer processes (forward and backward) and results in the reformation of the initial closed-shell electronic structure of the system

Modulating Nitric Oxide Release by <i>S</i>-Nitrosothiol Photocleavage: Mechanism and Substituent Effects

The photochemistry and photophysics of a series of <i>S</i>-nitrosothiols (RSNOs) have been studied computationally. The photocleavage mechanism of the model compound CH₃SNO to release CH₃S· and ·NO was studied at the CASPT2 level resulting in a barrierless process when irradiating in the visible region (S₁), in the near UV region (S₂) and for photosensitized (T₁) reaction. The absorption energy required to initiate photocleavage was calculated at the CASPT2 and B3P86 levels showing the possibility of the modulation of NO release by RSNO photoactivation as a function of the substituent R. Good correlations between the wavelengths of the lowest energy ¹(<i>n</i>,π*) and ¹(π,π*) transitions of aryl <i>S</i>-nitrosothiols and the corresponding Hammett constants of the substituents have been obtained

Synthesis, Optical Properties, and Regioselective Functionalization of 4a-Aza-10a-boraphenanthrene

4a-Aza-10a-boraphenanthrene has been synthesized in only four steps from commercially available materials with a remarkable overall yield of 62%. In contrast to other BN-isosteres of phenathrene, this isomer is weakly fluorescent, which has been explained by means of computational studies that found a low energy conical intersection for the nonradiative deactivation of the excited state. Moreover, a completely regioselective functionalization of 4a-aza-10a-boraphenanthrene at C-1 by reaction with activated electrophiles has been achieved