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Abstract：
The photoinduced all-trans to 13-cis isomerization of the retinal Schiff base represents the ultrafast first step in the reaction cycle of bacteriorhodopsin (BR). Extensive experimental and theoretical work has addressed excited-state dynamics and isomerization via a conical intersection with the ground state. In conflicting molecular pictures, the excited state potential energy surface has been modeled as a pure S[Formula: see text] state that intersects with the ground state, or in a 3-state picture involving the S[Formula: see text] and S[Formula: see text] states. Here, the photoexcited system passes two crossing regions to return to the ground state. The electric dipole moment of the Schiff base in the S[Formula: see text] and S[Formula: see text] state differs strongly and, thus, its measurement allows for assessing the character of the excited-state potential. We apply the method of ultrafast terahertz (THz) Stark spectroscopy to measure electric dipole changes of wild-type BR and a BR D85T mutant upon electronic excitation. A fully reversible transient broadening and spectral shift of electronic absorption is induced by a picosecond THz field of several megavolts/cm and mapped by a 120-fs optical probe pulse. For both BR variants, we derive a moderate electric dipole change of 5 [Formula: see text] 1 Debye, which is markedly smaller than predicted for a neat S[Formula: see text]-character of the excited state. In contrast, S[Formula: see text]-admixture and temporal averaging of excited-state dynamics over the probe pulse duration gives a dipole change in line with experiment. Our results support a picture of electronic and nuclear dynamics governed by the interaction of S[Formula: see text] and S[Formula: see text] states in a 3-state model.
摘要：
视网膜希夫碱的光诱导全反式至13顺式异构化代表了细菌视紫红质（BR）反应循环中的超快第一步。广泛的实验和理论工作已经通过与基态的锥形相交解决了激发态动力学和异构化。在相互矛盾的分子图片中，激发态势能表面已被建模为与基态相交的纯S[公式：见文本]状态，或在涉及S[公式：见文本]和S[公式：见文本]状态的3状态图片中。这里,光激发系统通过两个交叉区域返回到基态。希夫碱在S[公式：见正文]和S[公式：见正文]状态下的电偶极矩差异很大，并且，因此，它的测量允许评估激发态电势的特征。我们应用超快太赫兹（THz）Stark光谱的方法来测量野生型BR和BRD85T突变体在电子激发下的电偶极子变化。电子吸收的完全可逆的瞬态展宽和光谱偏移是由几兆伏/厘米的皮秒THz场引起的，并由120fs光学探测脉冲映射。对于两种BR变体，我们推导出5的中等电偶极子变化[公式：见正文]1德拜，明显小于激发态的纯S[公式：见文字]-字符的预测值。相比之下,S[公式：参见正文]-在探测脉冲持续时间内激发态动力学的混合和时间平均给出了与实验一致的偶极变化。我们的结果支持3状态模型中由S[公式：见文本]和S[公式：见文本]状态相互作用控制的电子和核动力学图。
