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Ultrafast terahertz Stark spectroscopy reveals the excited-state dipole moments of retinal in bacteriorhodopsin.
- Source :
- Proceedings of the National Academy of Sciences of the United States of America; 6/25/2024, Vol. 121 Issue 26, p1-7, 15p
- Publication Year :
- 2024
-
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<subscript>1</subscript> state that intersects with the ground state, or in a 3-state picture involving the S<subscript>1</subscript> and S<subscript>2</subscript> 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<subscript>1</subscript> and S<subscript>2</subscript> 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<subscript>1</subscript>-character of the excited state. In contrast, S<subscript>2</subscript>-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<subscript>1</subscript> and S<subscript>2</subscript> states in a 3-state model. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 00278424
- Volume :
- 121
- Issue :
- 26
- Database :
- Complementary Index
- Journal :
- Proceedings of the National Academy of Sciences of the United States of America
- Publication Type :
- Academic Journal
- Accession number :
- 178256763
- Full Text :
- https://doi.org/10.1073/pnas.2319676121