Abstract
Photoinduced double-bond isomerisation of the chromophore of photoactive yellow protein (PYP) is highly sensitive to chromophore-protein interactions. On the basis of high-level ab initio calculations, we scrutinise the effect of hydrogen bonds on the photophysical and photochemical properties of the chromophore. We identify four resonance structures - two closed-shell and two biradicaloid - that elucidate the electronic structure of the ground and first excited states involved in the isomerisation process. Changing the relative energies of the resonance structures by hydrogen-bonding interactions tunes all photochemical properties of the chromophore in an interdependent manner. Our study sheds new light on the role of the chromophore electronic structure in tuning in photosensors and fluorescent proteins.
Highlights
Photoactive yellow protein (PYP)[1] is a remarkable model system for studying double-bond isomerisation in photoreceptor proteins, as this photoreceptor has amply been characterised using a wide variety of advanced methods such as time-resolved X-ray crystallography,[2,3,4,5] neutron crystallography,[6] NMR,[7] and ultrafast spectroscopy.[8,9,10,11,12] These studies provided important insights into photoinduced double-bond isomerisation of the PYP chromophore that triggers the PYP photoresponse
The cross-section is composed of several stationary points on the S0 and S1 potential energy surfaces (PESs) and points of S1/S0 minimum-energy conical intersections (CoIns)
The geometries of the models at the stationary points are depicted in Fig. S1 in Electronic supplementary information (ESI).† The energies given in Fig. 2 are not corrected for the basis set superposition error (BSSE), which according to our estimation introduces only marginal changes in the cross-section energies (Table S3 in ESI†)
Summary
Photoactive yellow protein (PYP)[1] is a remarkable model system for studying double-bond isomerisation in photoreceptor proteins, as this photoreceptor has amply been characterised using a wide variety of advanced methods such as time-resolved X-ray crystallography,[2,3,4,5] neutron crystallography,[6] NMR,[7] and ultrafast spectroscopy.[8,9,10,11,12] These studies provided important insights into photoinduced double-bond isomerisation of the PYP chromophore that triggers the PYP photoresponse. The moderate molecular size and high sensitivity to intermolecular interactions render the pCTMÀ chromophore (model 1 in Fig. 1b) suitable for accurate quantumchemistry calculations addressing the chromophore’s tuning by intermolecular interactions.[16,17,18,19,20,21,22] to other ionic chromophores of photoresponsive proteins, pCTMÀ undergoes a pronounced charge-transfer, from the phenolic moiety to the. The accurate description of the models is attained by using a high-level ab initio method, the extended multi-state multi-configurational quasi-degenerate perturbation theory of second order (XMCQDPT2),[29] which accounts for both static and dynamical electron correlation. In this context, the present work critically extends the previous studies performed using the complete-active-space self-consistent field. We compare our findings to previously published results on PYP, fluorescent proteins and rhodopsin photoreceptors noting multiple parallels among the properties of ionic chromophore in these proteins
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