Abstract
Photoactivation in the Photoactive Yellow Protein, a bacterial blue-light photoreceptor, proceeds via photoisomerization of the double C=C bond in the covalently attached chromophore. Quantum chemistry calculations, however, have suggested that in addition to double-bond photoisomerization, the isolated chromophore and many of its analogues can isomerize around a single C–C bond as well. Whereas double-bond photoisomerization has been observed with X-ray crystallography, experimental evidence of single-bond photoisomerization is currently lacking. Therefore, we have synthesized a chromophore analogue, in which the formal double bond is covalently locked in a cyclopentenone ring, and carried out transient absorption spectroscopy experiments in combination with nonadiabatic molecular dynamics simulations to reveal that the locked chromophore isomerizes around the single bond upon photoactivation. Our work thus provides experimental evidence of single-bond photoisomerization in a photoactive yellow protein chromophore analogue and suggests that photoisomerization is not restricted to the double bonds in conjugated systems. This insight may be useful for designing light-driven molecular switches or motors.
Highlights
Photoactivation in the Photoactive Yellow Protein, a bacterial blue-light photoreceptor, proceeds via photoisomerization of the double C C bond in the covalently attached chromophore
The photocycle of Photoactive yellow protein (PYP) is initiated by an ultrafast photoisomerization of the conjugated p-coumaric acid chromophore that is covalently bound to the protein via a thioester linkage
Chromophore analogues undergo ultrafast deactivation in solution, but for many analogues, no photoproduct is detected.[5−10] This paradox was addressed with nonadiabatic molecular dynamics simulations of the pCK analogue in water.[11,12]
Summary
We observe a distinct positive feature at 410 nm for the DB-locked chromophore and at 430 nm for pCK, which grows initially but decays, leaving no long-lived absorption signal. This transient signal is more prominent in pCK than in the DBlocked chromophore We attribute this band to the absorption of the vibrationally hot ground state that is formed immediately after excited-state decay via a conical intersection, as in previous work.[10]. To confirm that the ultrafast decay observed in our experiments is due to single-bond photoisomerization, we performed 100 nonadiabatic molecular dynamics simulations of the deprotonated DB-locked chromophore in water.
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