Ultrafast terahertz Stark spectroscopy reveals the excited-state dipole moments of retinal in bacteriorhodopsin.
Proc Natl Acad Sci U S A
; 121(26): e2319676121, 2024 Jun 25.
Article
in En
| MEDLINE
| ID: mdl-38900801
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.
Key words
Full text:
1
Collection:
01-internacional
Database:
MEDLINE
Main subject:
Retinaldehyde
/
Bacteriorhodopsins
Language:
En
Journal:
Proc Natl Acad Sci U S A
Year:
2024
Document type:
Article
Affiliation country:
Germany
Publication country:
EEUU
/
ESTADOS UNIDOS
/
ESTADOS UNIDOS DA AMERICA
/
EUA
/
UNITED STATES
/
UNITED STATES OF AMERICA
/
US
/
USA