Native hydrogen bonding network of the photoactive yellow protein (PYP) chromophore: Impact on the electronic structure and photoinduced isomerization

The local hydrogen bonding network of the p-coumaric thio-ester chromophore in the photoactive yellow protein (PYP) has been studied in some detail from a theoretical point of view, using the ab initio coupled-cluster CC2 method. The main focus is put on the impact of the native H-bonds, donated by the Tyr42, Glu46 and Cys69 amino acids, on (i) the molecular and electronic structure of the chromophore in the ground and first excited state and (ii) the photoinduced torsion of the chromophore around its single and double carboncarbon bonds. The H-bonds of both the Tyr42/Glu46 pair and the Cys69 amino acid are shown to substantially stabilize the negative charge of the chromophore in the ground state but have a different influence on the excited state. The Tyr42/Glu46 H-bonds tend to suppress the single-bond torsion in the excited state, thereby stabilizing the double-bond torsion, whereas the Cys69 H-bond has an opposite effect. The stabilization of the double-bond torsion is shown to depend on the length of the Tyr42/Glu46 H-bonds. Shorter H-bonds can significantly enhance the stabilization, which is proposed to be an important factor for the successful trans–cis isomerization of the chromophore in the native protein.