Molecular dynamics of excited state intramolecular proton transfer: 2-(2′-hydroxyphenyl)-4-methyloxazole in gas phase, solution, and protein environments

The ultrafast enol - keto photoisomerization in the lowest singlet excited state of 2-(2′-hydroxyphenyl)-4-methyloxazole (HPMO) is investigated using classical molecular dynamics in conjunction with an empirical valence bond potential. This process is studied in four different environments: the gas phase, dimethyl sulfoxide, water, and human serum albumin protein. The effects of the environment on the proton transfer time and the promoting-mode motions are analyzed. The ring - ring in-plane bending of HPMO is identified as the dominant low-frequency vibrational mode that decreases the proton donor - acceptor distance to facilitate proton transfer. The mean proton transfer times are 100-200 fs in all of the environments. The population decay of the enol tautomer in the S₁ state is significantly slower for the reaction in water than in DMSO and protein. The slower population decay in water is found to arise from configurations with intermolecular hydrogen bonds between HPMO and water molecules, leading to a disruption of the intramolecular hydrogen bond in HPMO. All of the condensed-phase environments are found to dampen the donor - acceptor vibrational mode after the proton transfer process, thereby stabilizing the keto tautomer. In the gas phase, the donor - acceptor mode oscillations continue to facilitate the forward and reverse isomerization processes.