Internal Conversion of the Anionic GFP Chromophore: In and Out of the I-twisted S1/S0 Conical Intersection Seam

The functional diversity of the green fluorescent protein (GFP) family is intimately connected to the interplay between competing photo-induced transformations of the chromophore motif, anionic p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI^–). Its propensity to undergo Z/E photoisomerization following excitation to the S₁(pp^*) state is of particular importance for super-resolution microscopy and emerging opportunities in optogenetics. However, key dynamical aspects of this process and its range of tunability still remain elusive. Here, we investigate the internal conversion behavior intrinsic to HBDI^– with focus on competing deactivation pathways, rate and yield of photoisomerization. Based on non-adiabatic dynamics simulations, we confirm that non-selective progress along the two bridge-torsional (i.e., phenolate, P, or imidazolinone, I) pathways can account for the three decay constants reported experimentally, leading to competing ultrafast relaxation along the I-twisted pathway and S₁ trapping along the P-torsion. The majority of the population (~70%) is transferred to S₀ in the vicinity of two near-symmetry-related minima on the I-twisted intersection seam (MECI-Is). Despite their reactant-biased topographies, our account of inertial effects suggests that isomerization not only occurs as a thermal process on the vibrationally hot ground state but also as a direct photoreaction with a total quantum yield of ~40%.

By comparing the non-adiabatic dynamics to a photoisomerization committor analysis, we provide a detailed mapping of the intrinsic photoreactivity and dynamical behavior of the two MECI-Is. Our work offers new insight into the internal conversion process of HBDI^– that enlightens principles for the design of chromophore derivatives and protein variants with improved photoswitching properties.