List Nanna H, Jones Chey M, Martínez Todd J
Department of Chemistry and the PULSE Institute, Stanford University Stanford CA 94305 USA
SLAC National Accelerator Laboratory 2575 Sand Hill Road Menlo Park CA 94025 USA.
Chem Sci. 2021 Dec 8;13(2):373-385. doi: 10.1039/d1sc05849e. eCollection 2022 Jan 5.
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 -hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI). Its ability to undergo /-isomerization is of particular importance for super-resolution microscopy and emerging opportunities in optogenetics. Yet, key dynamical features of the underlying internal conversion process in the native HBDI chromophore remain largely elusive. We investigate the intrinsic excited-state behavior of isolated HBDI to resolve competing decay pathways and map out the factors governing efficiency and the stereochemical outcome of photoisomerization. Based on non-adiabatic dynamics simulations, we demonstrate that non-selective progress along the two bridge-torsional (, phenolate, P, or imidazolinone, I) pathways accounts for the three decay constants reported experimentally, leading to competing ultrafast relaxation primarily along the I-twisted pathway and S trapping along the P-torsion. The majority of the population (∼70%) is transferred to S in the vicinity of two approximately enantiomeric minima on the I-twisted intersection seam (MECI-Is). Despite their sloped, reactant-biased topographies (suggesting low photoproduct yields), we find that decay through these intersections leads to products with a surprisingly high quantum yield of ∼30%. This demonstrates that -isomer generation results at least in part from direct isomerization on the excited state. A photoisomerization committor analysis reveals a difference in intrinsic photoreactivity of the two MECI-Is and that the observed photoisomerization is the combined result of two effects: early, non-statistical dynamics around the less reactive intersection followed by later, near-statistical behavior around the more reactive MECI-I. Our work offers new insight into internal conversion of HBDI that both establishes the intrinsic properties of the chromophore and enlightens principles for the design of chromophore derivatives and protein variants with improved photoswitching properties.
绿色荧光蛋白（GFP）家族的功能多样性与发色团基序阴离子 - 羟基苄叉 - 2,3 - 二甲基咪唑啉酮（HBDI）竞争性光诱导转变之间的相互作用密切相关。其进行 /-异构化的能力对于超分辨率显微镜和光遗传学中的新兴机遇尤为重要。然而，天然HBDI发色团潜在内部转换过程的关键动力学特征在很大程度上仍不清楚。我们研究了孤立HBDI的本征激发态行为，以解析竞争的衰变途径，并确定控制光异构化效率和立体化学结果的因素。基于非绝热动力学模拟，我们证明沿着两条桥扭转（酚盐，P，或咪唑啉酮，I）途径的非选择性进展解释了实验报道的三个衰变常数，导致主要沿着I扭转途径的竞争超快弛豫以及沿着P扭转的S捕获。大多数粒子（约70%）在I扭转交叉缝（MECI - Is）上两个近似对映体极小值附近转移到S。尽管它们具有倾斜的、偏向反应物的地形（表明光产物产率低），但我们发现通过这些交叉点的衰变导致具有约30%惊人高量子产率的产物。这表明 - 异构体的产生至少部分源于激发态上的直接异构化。光异构化反应坐标分析揭示了两个MECI - Is的本征光反应性差异，并且观察到的光异构化是两种效应的综合结果：在反应性较低的交叉点周围的早期非统计动力学，随后是在反应性较高的MECI - I周围的后期近统计行为。我们的工作为HBDI的内部转换提供了新的见解，既确立了发色团的本征性质，又为设计具有改进光开关性质的发色团衍生物和蛋白质变体提供了原理。
