Inertial Protein-Matrix Solvation of a Light-Harvesting Chromophore

How the surrounding protein controls the reaction dynamics of active centers in enzymes is the subject of many current studies. In an initial effort to characterize the solvation response of the protein matrix, we have performed femtosecond transient hole-burning (THB) studies on a globular protein system, the a subunit of C-phycocyanin, which contains a single open-chain tetrapyrrole chromophore.1 Our results suggest that nearly all of the solvation response of the α subunit to the formation of the chromophore’s S1 state occurs in a manner consistent with an inertial response on the sub-100-fs time scale, with an exponential, diffusive response following on a much longer time scale (>10 ps). This character of the solvation response of the α subunit can be distinguished from that exhibited by small solvent molecules,2,3 allowing us to assign the observed response to the protein matrix that binds the open-chain tetrapyrrole chromophore.