Abstract
Sub-picosecond photo-isomerization is the major primary process of energy conversion in retinal proteins and has as such been in the focus of extensive theoretical and experimental work over the past decades. In this review article, we revisit the long-standing question as to how the protein tunes the isomerization speed and quantum yield. We focus on our recent contributions to this field, which underscore the concept of a delicate mixing of reactive and non-reactive excited states, as a result of steric properties and electrostatic interactions with the protein environment. Further avenues and new approaches are outlined which hold promise for advancing our understanding of these intimately coupled chromophore-protein systems.
Highlights
While it has been shown that there is no correlation between excited state lifetime (ESL) and its photoisomerization quantum yield (IQY) [18,19,20,21], many theoretical predictions agree, that the potential energy surfaces (PES) topography and the nuclear motions at the conical intersection (CInt) decide on the branching between reactive and non-reactive pathways, on the overall IQY [17, 22, 23]
The concept, which emerges from these theoretical results, is that both steric and electrostatic effects act through the modifications of the S1/S2 energy landscape in determining the ESL of rPSB in different protein environments, and of rhodopsin-mimicking photo-switches in solution (Section 4)
We found that the fluorescence quantum yield (FQY) was lowest for wtASR and increased by a factor of 2.2 for L83Q, and by 7.9 and 6.9 for orange- and green-adapted W76S/Y179F, respectively
Summary
The protonation of the retinal Schiff base in VIS-absorbing retinal proteins makes these systems distinctively different from their UV-absorbing sisters, which function with a deprotonated chromophore [33].
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