Electric-field jumps trigger a protein response preceeding the ultrafast photoisomerization

The reaction cascade of photoreceptors is initiated by light absorption. The photoreaction of the chromophore is generally assumed to determine the subsequent cascade. Here, we demonstrate for the bacteriorhodopsin from Halobacterium salinarum HsBR that retinal chromophore-induced electric-field jumps in the excited state (ES) modify protein conformation within 200 fs prior isomerization. Protein response is reflected by appearance of a transient continuum absorption band (CB), and frequency shifts at Asp96, proton-release group, and amide I band. The CB decays in the ES, while the other signals are long-lived. Combination of femtosecond Vis-pump IR-probe experiments and multiscale simulations reveal strengthening of hydrogen bonds induced by electric-field jump. This occurs at the counterion and leads to the CB, conformational changes about 10 Å away from the retinal chromophore due to negligible screening in this time-scale and energy transfer to the amide I band. These long-lasting changes are supposed to govern the protein reaction cascade in concert with the isomerized chromophore. This challenges our current understanding of photoreceptor dynamics.