ИСТИНА

Green Fluorescent Protein (GFP) and its derivatives have revolutionized biology through a diverse range of applications including advanced fluorescence imaging and biosensing. The molecular light machine at the heart of GFP is an extended π-system based on 4-hydroxybenzylidene-2,3-dimethylimidazolinone (pHBDI), which is covalently bound to the protein. The fluorescence is lost, when the native GFP chromophore is taken out of the protein environment, and it can only be restored when cooled down to 100 K. Here, by using high-level quantum chemistry calculations, we explore the origin of the ultrafast excited-state dynamics of the bare GFP chromophore anion and compare it to that of mHBDI. In pHBDI, ultrafast internal conversion through a conical intersection occurs on a timescale of picoseconds and results in efficient deexcitation coupled to cis-trans photoisomerization. Interestingly, the nuclear rearrangements along the reaction coordinate in the excited state can compete with vibrationally assisted electron emission. A non-adiabatic nature of the excited-state dynamics bridges the gap between inherent timescales of the nuclear and electronic motion and unexpectedly results in co-existing mutual energy-borrowing mechanisms between nuclei and electrons. We show that the energy exchange is remarkably fast and, importantly, mode-specific. This finding paves the way for direct ultrafast control of the functioning of light-sensitive proteins via selective vibrational pre-excitation and highlights the importance and ubiquity of non-adiabatic processes in nature. In contrast to pHBDI, the first excited state of mHBDI is optically dark in the Franck-Condon region, and photoexcitation in the visible range results in the S0S2 transition. Three conical intersections are found that interconnect the S0, S1 and S2 states. The initial relaxation from the S2 state occurs barrierless and results in the population of the S1 state. The excited-state population is shown to be trapped in S1 due to the presence of the energy barrier along the minimum-energy pathway from the planar equilibrium geometry in S1 to the highly twisted structure of the S1/S0 conical intersection. The excited-state lifetime of mHBDI is estimated to be several orders of magnitude longer than that of pHBDI. Despite this long lifetime, mHBDI is non-fluorescent. We show that the excited-state dynamics of mHBDI is fully governed by the dark state of a charge transfer character.