Stimulated photon-echo and transient-grating studies of protein-matrix solvation dynamics and interexciton-state radiationless decay in α phycocyanin and allophycocyanin

We have employed two third-order femtosecond spectroscopic methods, stimulated-photon-echo peak-shift (3PEPS) and transient-grating (TG) spectroscopy, to characterize solvation dynamics and interexciton-state radiationless decay in the a subunit of C-phycocyanin and in allophycocyanin. The alpha subunit contains a single phycocyanobilin chromophore in an isolated protein-matrix environment. Allophycocyanin contains exciton-coupled pairs of phycocyanobilins in the same type of binding site found in the alpha subunit. The results show that both systems exhibit a biphasic solvation response: the inertial phase, arising from librational motions of the amino acids or included water molecules in the phycocyanobilin-binding site, contributes a 80-100-fs component to the 3PEPS profile and appears as a rapidly damped 72-cm(-1) modulation of the TG signal; the diffusive phase, arising from collective protein-matrix motions, contributes a component in the TG signal and 3PEPS profile on the 5-20-ps time scale. Both systems exhibit nearly instantaneous (16-fs) components in the 3PEPS profiles that arise from intrachromophore vibrational modes. The 3PEPS profile observed with allophycocyanin exhibits additional fast decay components, with time constants of 56 and 220 fs, that apparently report the contributions to electronic dephasing arising from radiationless decay between imperfectly correlated exciton states. The TG signal evidences vibrational relaxation in the lower exciton state and incoherent energy transfer between the chromophores in a given pair. The results present complementary details on solvation and interexciton-state radiationless decay dynamics that were first observed in this laboratory using time-resolved pump-probe and anisotropy methods.