Vibronic effects in the spectroscopy and dynamics of C-phycocyanin

Femtosecond laser spectroscopies are used to investigate the influence of intramolecular nuclear modes on electronic relaxation in the cyanobacterial light harvesting protein, C-phycocyanin (CPC). Of particular interest are sub-ps dynamics localized on pairs of closely spaced phycocyanobilin pigments (i.e. dimers). Experiments conducted under different polarization conditions are used to distinguish isotropic and anisotropic vibrational modes within the dimers. Two isotropic nuclear modes are detected near 185 and 260 cm−1 using two-dimensional photon echo spectroscopy. In addition, a transient absorption anisotropy measurement reveals vibrational resonances associated with (out-of-plane) anisotropic nuclear modes near 640 and 815 cm−1. We investigate two possible origins for the recurrences in the anisotropy. A mechanism involving ground state nuclear coherences in the Condon approximation is ruled out by comparing the potential energy surfaces of the excitons to the direction of wavepacket motion. Electronic structure calculations suggest that non-Condon effects are the most likely explanation for the beats observed in the anisotropy. Such non-Condon effects also hold interesting implications for the vibronic exciton electronic structure of CPC. We calculate non-Condon intermolecular couplings in the dimer as large as 10 cm−1, which suggests that these effects are not negligible and deserve further consideration. Our findings provide additional insights into the sub-100 fs vibronic relaxation channel found in the closely related protein, allophycocyanin, whose pigment dimers possess nearly the same geometry and intermolecular Coulombic interactions as CPC. This study underscores the complex interplay of intramolecular vibronic coupling and site energy tuning in photosynthetic light harvesting. (Some figures may appear in colour only in the online journal)