Spectroscopic Investigation on the Primary Photoreaction of Bathy Phytochrome Agp2-Pr of Agrobacterium fabrum: Isomerization in a pH-dependent H-bond Network

Bathy phytochrome Agp2 from Agrobacterium fabrum exhibits an unusually low pK(a)=7.6 in the Pr state in contrast to a pK(a)>11 in the Pfr state, indicating a pH-dependent charge distribution and H-bond network in the Pr chromophore binding pocket around neutral pH. Here, we report on ultrafast UV/Vis absorption spectroscopy of the primary Pr photoisomerization of Agp2 at pH6 and pH9 and upon H2O/D2O buffer exchange. The triexponential Pr kinetics slows down at increased pH and pronounced pH-dependent kinetic isotope effects are observed. The results on the Pr photoreaction suggest: 1)component-wise hindered dynamics on the chromophore excited-state potential energy surface at high pH and 2)proton translocation processes either via single-proton transfer or via significant reorganization of H-bond networks. Both effects reflect the interplay between the pH-dependent charge distribution in the Pr chromophore binding pocket on the one hand and chromophore excitation and its ZE isomerization on the other hand.