Femtosecond heterodyne-detected four-wave-mixing studies of deterministic protein motions. 2. Protein response

The initial structural evolution of carboxymyoglobin (MbCO) following photo dissociation of CO is studied using optically heterodyne-detected (OHD) tra nsient grating (TG) spectroscopy. This method provides derailed dynamical i nformation on the electronic and structural states of the heme protein foll owing photoexcitation. The phase anisotropy of MbCO is found to develop on subpicosecond to picosecond time scales and is much greater than can be att ributed to the symmetry of the heme dipole transition. Control studies of c arboxyprotoheme and deoxymyoglobin were used to identify the components due to protein structural relaxation and thermal relaxation, respectively. A g eometric decomposition of the MbCO grating signals into contributions relat ive to the molecular axes provides evidence that the protein effectively ch anges its shape within 500 fs following ligand dissociation. These anisotro pic mass displacements are a signature of functionally important motions si nce they imply a certain degree of directionality or mode selective couplin g to the response. The anisotropic relaxation and observed dynamics provide further evidence that the low-frequency collective modes of proteins play an important role in transducing reaction forces into functions.