Ultrafast trans-cis photoisomerization of the neutral chromophore in green fluorescent proteins: Surface-hopping dynamics simulation.

Abstract

Reversible photoswitching fluorescent protein can reversibly switch between on-state (fluorescent) and off-state (dark). Anionic cis and neutral trans chromophores are the on- and off-states in green fluorescent proteins (GFPs), respectively. We investigated the ultrafast trans-cis photoisomerization mechanisms of the neutral GFP chromophore upon excitation to the S1 state by means of surface-hopping dynamics simulations based on the Zhu-Nakamura theory. Two trans isomers, located in the S0 state, were taken into consideration in dynamics simulation. After these two trans isomers are excited to the S1 state, the molecule moves to a excited-state minimum by increasing the imidazolinone-bridge bond length and decreasing the phenol-bridge bond length. The twist of imidazolinone-bridge bond drives the molecule toward a conical intersection, and internal conversion occurs. Then, a cis or trans conformer will be obtained in the S0 state. The torsion around the imidazolinone-bridge bond plays a key role in the ultrafast photoisomerization of a neutral chromophore. The torsional motion around the phenol-bridge bond is restricted in the S1 state, while it may occur in the S0 state. The isomerization reaction of this molecule is predicted to be not sensitive to solvent viscosity, and time-dependent density functional theory (TDDFT) calculations indicate that the fast excited-state decay from the Franck-Condon region of the trans isomer to the excited-state minimum was almost independent of solvent polarity.