Abstract
Understanding how the interactions between a chromophore and its surrounding protein control the function of a photoactive protein remains a challenge. Here, we present the results of photoelectron spectroscopy measurements and quantum chemistry calculations aimed at investigating how substitution at the coumaryl tail of the photoactive yellow protein chromophore controls competing relaxation pathways following photoexcitation of isolated chromophores in the gas phase with ultraviolet light in the range 350-315 nm. The photoelectron spectra are dominated by electrons resulting from direct detachment and fast detachment from the 2(1)ππ* state but also have a low electron kinetic energy component arising from autodetachment from lower lying electronically excited states or thermionic emission from the electronic ground state. We find that substituting the hydrogen atom of the carboxylic acid group with a methyl group lowers the threshold for electron detachment but has very little effect on the competition between the different relaxation pathways, whereas substituting with a thioester group raises the threshold for electron detachment and appears to 'turn off' the competing electron emission processes from lower lying electronically excited states. This has potential implications in terms of tuning the light-induced electron donor properties of photoactive yellow protein.
Highlights
Photoactive proteins are exploited widely in nature and technology to make systems responsive to light
In a previous anion photoelectron spectroscopy study of gasphase pCAÀ and its ortho- and meta-isomers, we found that moving the position of the OÀ group on the phenoxide group of the chromophore controlled the competition between electron emission and internal conversion.[27]
We have used photoelectron spectroscopy and quantum chemistry calculations to investigate the role of the thioester group in controlling the competition between internal conversion and electron emission in isolated photoactive yellow protein (PYP) chromophores in the gas phase
Summary
Photoactive proteins are exploited widely in nature and technology to make systems responsive to light. There have been numerous studies of PYP and the PYP chromophore, in the gas-phase and in solution, aimed at investigating the role of the environment on its electronic structure and dynamics.[29,30,31,32,33,34,35,36,37] Various strategies have been employed, ranging from studies of a few solvent molecules or the closest protein residues to quantum-mechanical/ molecular mechanics and averaged solvent electrostatic potential/ molecular dynamics methods Such studies have highlighted the importance of torsions around the C–C single bond between the phenoxide ring and the CQC double bond in controlling the trans–cis isomerisation process in the protein.[29] They have rationalised the blue solvatochromic shift of pCAÀ and its derivatives in terms of a transfer of electron density from the phenolate end of the chromophore to the rest of the chromophore, resulting in a decrease in dipole moment between the ground and first electronically excited states.[34] Garcıa-Prieto and coworkers have recently reported a detailed theoretical study of the absorption spectra of different chromophore analogues in the gas-phase and solution and found that the presence of a sulfur atom on the coumaryl tail modulates the solvent effect for the first few excited electronic states[37] which could explain the differences observed in the excited state dynamics in aqueous solution discussed above.[9,11,12,18]. Improving our understanding of the role of the thioester link in controlling electron emission in PYP may provide a basis for practical applications such as the rational design of photoactive materials with specific redox properties
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