Photostability Mechanisms in Human γB-Crystallin: Role of the Tyrosine Corner Unveiled by Quantum Mechanics and Hybrid Quantum Mechanics/Molecular Mechanics Methodologies

The tyrosine corner is proposed as a featured element to enhance photostability in human γB-crystallin when exposed to UV irradiation. Different ultrafast processes were studied by multiconfigurational quantum chemistry coupled to molecular mechanics: photoinduced singlet–singlet energy, electron and proton transfer, as well as population and evolution of triplet states. The minimum energy paths indicate two possible UV photoinduced events: forward–backward proton-coupled electron transfer providing to the system a mechanism for ultrafast internal conversion, and energy transfer, leading to fluorescence or phosphorescence. The obtained results are in agreement with the available experimental data, being in line with the proposed photoinduced processes for the different tyrosine environments within γB-crystallin.