1. Ultrafast terahertz Stark spectroscopy reveals the excited-state dipole moments of retinal in bacteriorhodopsin.
- Author
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Jia Zhang, Singh, Poonam, Engel, Dieter, Fingerhut, Benjamin P., Broser, Matthias, Hegemann, Peter, and Elsaesser, Thomas
- Subjects
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TERAHERTZ spectroscopy , *BACTERIORHODOPSIN , *DIPOLE moments , *EXCITED state energies , *ELECTRIC dipole moments - 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 S1 state that intersects with the ground state, or in a 3-state picture involving the S1 and S2 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 S1 and S2 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 S1-character of the excited state. In contrast, S2-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 S1 and S2 states in a 3-state model. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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