Ultrafast terahertz Stark spectroscopy reveals the excited-state dipole moments of retinal in bacteriorhodopsin

• 成果类型: Article
• 署名作者: Zhang, Jia; Singh, Poonam; Engel, Dieter; Fingerhut, Benjamin P.; Broser, Matthias; Hegemann, Peter; Elsaesser, Thomas
• 署名单位: Leibniz Association; Max Born Institute for Nonlinear Optics & Short Term Spectroscopy; University of Munich; University of Munich; Humboldt University of Berlin
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-14419
• DOI: 10.1073/pnas.2319676121
• 发表日期: 2024-06-25
• 关键词: dynamics; isomerization; chromophores; chemistry; MODEL; bond

摘要

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 1 state that intersects with the ground state, or in a 3 -state picture involving the S 1 and S 2 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 1 and S 2 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 +/- 1 Debye, which is markedly smaller than predicted for a neat S 1 -character of the excited state. In contrast, S 2 -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 1 and S 2 states in a 3 -state model.