Nature of excited states and relaxation mechanisms in C-phycocyanin.

The electronic structure and photoinduced relaxation dynamics of the cyanobacterial light harvesting protein, C-Phycocyanin (CPC), are examined using transient grating and two-dimensional (2D) photon echo spectroscopies possessing sub-20 fs time resolution. In combination with linear absorption and fluorescence measurements, these time-resolved experiments are used to constrain the parameters of a Frenkel exciton Hamiltonian. Particular emphasis is placed on elucidating the nature of excited states involving the alpha84 and beta84 phycocyanobilin pigment dimers of CPC. This paper obtains new experimental evidence suggesting that electronic relaxation proceeds by way of incoherent energy transfer between the alpha84 and beta84 pigment sites (i.e., the weak coupling limit of energy transfer). Transient absorption anisotropies simulated in the weak coupling limit agree well with measurements, whereas signals computed in an exciton basis possess short-lived (electronic) coherent components not present in the experimental data. In addition, 2D photon echo spectra for CPC show no sign of the interfering nonlinearities predicted by a theoretical model to be characteristic of exciton formation. Another important new observation is that the sub-100 fs dynamics in the transient absorption anisotropy are dominated by an impulsively excited hydrogen out-of-plane wagging mode similar to those observed in phytochrome and retinal. Detection of this 795 cm(-1) coherence is of particular interest because our recent study of a closely related protein, Allophycocyanin (APC), assigns a similar coordinate as a promoting mode enabling ultrafast internal conversion. Together, the experiments conducted for APC and CPC suggest that interactions between the pigments and environment are the key to understanding why electronic relaxation in CPC is more than three times slower than APC despite the nearly identical geometries of the pigment dimers. Most important in reaching this conclusion is the present finding that relaxation of the 2D photon echo line shapes of CPC is approximately two times faster than that measured for APC. Overall, the present results underscore the ability of phycobiliproteins to control light harvesting dynamics through solvation and variation in the conformations of open-chain tetrapyrrole chromophores.