Photoisomerization in proteorhodopsin mutant D97N

The first steps of the photocycle of the D97N mutant of proteorhodopsin (PR) have been investigated by means of ultrafast transient absorption spectroscopy. A comparison with the primary dynamics of native PR and D85N mutant of bacteriorhodopsin is given. Upon photoexcitation of the covalently bound all-trans retinal the excited state decays biexponentially with time constants of 1.4 and 20 ps via a conical intersection, resulting in a 13-cis isomerized retinal. Neither of the two-deactivation channels is significantly preferred. The dynamics is slowed down in comparison with native PR at pH 9 and reaction rates are even lower than for native PR at pH 6, where the primary proton acceptor (Asp97) is protonated. Therefore, the ultrafast isomerization is not only controlled by the charge distribution within the retinal binding pocket. This study shows that in addition to direct electrostatics other effects have to be taken into account to explain the catalytic function of Asp97 in PR on the ultrafast isomerization reaction. This may include sterical interactions and/or bound water molecules within the retinal binding pocket.