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

► Native H-bonding network of the PYP chromophore has been studied theoretically. ► H-bonds donated by Tyr42/Glu46/Cys69 amino acids have been explicitly addressed. ► Tyr42/Glu46 H-bonds suppress single-bond torsion allowing for double-bond torsion. ► Cys69 H-bond counteracts Tyr42/Glu46 H-bonds, favoring single-bond torsion. ► Suppression of single-bond torsion can be enhanced by shorter Tyr42/Glu46 H-bonds.