ИСТИНА

Ionic chromophores bound to a photoreceptor protein trigger biological photoresponses such as animal vision or bacterial phototaxis by undergoing double-bond isomerization. Despite extensive studies dedicated to rationalizing the underlying molecular mechanism, one of the central questions how the photochemical yield of a specific photoproduct is maximized by the protein-chromophore interactions remains only partially answered. Photoactive yellow protein (PYP) is a well-characterized photoreceptor system that allows studying how hydrogen bonds (H-bonds) facilitate the double-bond isomerization. Upon light signaling, the anionic chromophore of PYP derived from the deprotonated p-coumaric acid (pCA) undergoes photochemical trans-cis isomerization that eventually alters hydrogen bonding at the protein active site. In addition to trans-cis isomerization, the chemical structure of pCA permits excited-state decay via single-bond rotation, which in combination with the double-bond isomezazation results in several photoproducts that may reduce photoactivation efficiency. Experimental and computational studies provided evidence that specific H-bonds between the chromophore and protein modulate the topology of the excited-state potential energy surface favoring the double-bond isomerization and at the same time disfavoring rotation about the single bond. We present a qualitative model explaining the role of the H-bonds in controlling photoisomerization of the pCA chromophore, obtained from accurate energy and molecular property calculations using the high-level ab initio XMCQDPT2 method. To qualitatively interpret the computational results, we derived four resonance structures - a pair of closed-shell (CS) structures and a pair of biradicaloid (BR) structures - that account for changes of the electronic structure of pCA upon the single- and double-bond photoisomerization, providing a clue to rationalize the effect of the H-bonds. It is the relative energy of the four resonance structures, determined by the chromophore’s chemical structure and intermolecular interactions that governs photoisomerization via single- and double-bond rotation.