Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo, 113-0033, Japan.
Angew Chem Int Ed Engl. 2022 Jan 10;61(2):e202111930. doi: 10.1002/anie.202111930. Epub 2021 Nov 29.
All-trans to 13-cis photoisomerization of the protonated retinal Schiff base (PRSB) chromophore is the primary step that triggers various biological functions of microbial rhodopsins. While this ultrafast primary process has been extensively studied, it has been recognized that the relevant excited-state relaxation dynamics differ significantly from one rhodopsin to another. To elucidate the origin of the complicated ultrafast dynamics of the primary process in microbial rhodopsins, we studied the excited-state dynamics of proteorhodopsin, its D97N mutant, and bacteriorhodopsin by femtosecond time-resolved absorption (TA) spectroscopy in a wide pH range. The TA data showed that their excited-state relaxation dynamics drastically change when pH approaches the pK of the counterion residue of the PRSB chromophore in the ground state. This result reveals that the varied excited-state relaxation dynamics in different rhodopsins mainly originate from the difference of the ground-state heterogeneity (i.e., protonation/deprotonation of the PRSB counterion).
全反式到质子化视黄醛 Schiff 碱（PRSB）发色团的 13-顺式光异构体是触发微生物视蛋白各种生物功能的主要步骤。虽然这个超快的初始过程已经被广泛研究，但人们已经认识到，相关的激发态弛豫动力学在不同的视蛋白之间有很大的不同。为了阐明微生物视蛋白中初始过程复杂的超快动力学的起源，我们通过飞秒时间分辨吸收（TA）光谱法在很宽的 pH 范围内研究了蛋白视紫红质、其 D97N 突变体和菌紫质的激发态动力学。TA 数据表明，当 pH 接近 PRSB 发色团的抗衡离子残基在基态下的 pK 时，它们的激发态弛豫动力学会发生剧烈变化。这一结果表明，不同视蛋白中不同的激发态弛豫动力学主要源于基态异质性的差异（即 PRSB 抗衡离子的质子化/去质子化）。
