Resolving Dual Photoreaction Channels of All-Trans-Retinal Using Femtosecond Stimulated Raman Spectroscopy

Le résumé fourni par la source

All- trans -retinal (ATR) plays a critical role in vision and light-sensing biological processes, serving as the retinyl chromophore in photoreceptor proteins. The excited-state dynamics of ATR include several singlet electronic states such as S 2 (1 B u + ), S 1 (2 A g – ), n π*, and the intramolecular charge transfer (ICT) state, which are pivotal in its isomerization and photoinduced processes. However, spectral overlaps in transient absorption spectra have rendered the differentiation of S 1 and ICT lifetimes challenging, resulting in two competing hypotheses regarding the ICT state: strongly coupled to S 1 or existing as a distinct electronic state. This study employs femtosecond stimulated Raman spectroscopy to investigate ATR’s structural dynamics across solvents with varying polarities and viscosities. Two separate photochemical pathways are identified: Channel 1: S 2 (1 B u + ) → S 1 (2 A g – )→ n π* → T → all- trans S 0 and Channel 2: S 2 (1 B u + ) → ICT → ICT′ → cis S 0 . Results show that solvent viscosity strongly influences isomerization in Channel 2, while Channel 1 remains unaffected. Furthermore, the ATR’s isomerization in Channel 2 involves large-scale one-bond flip torsional motions, distinct from the space-conserving bicycle-pedal isomerization observed in protein environments.

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