Activation and deactivation of vibronic channels in intact phycocyanin rods

We investigated the excitation modes of the light-harvesting protein phycocyanin (PC) from Thermosynechococcus vulcanus in the crystalline state using UV and near-infrared Raman spectroscopy. The spectra revealed the absence of a hydrogen out-of-plane wagging (HOOP) mode in the PC trimer, which suggests that the HOOP mode is activated in the intact PC rod, while it is not active in the PC trimer. Furthermore, in the PC trimer an intense mode at 984 cm ^-1 is assigned to the C-C stretching vibration while the mode at 454 cm^-1 is likely due to ethyl group torsion. In contrast, in the similar chromophore phytochromobilin the C_5,10,15-D wag mode at 622 cm^-1 does not come from a downshift of the HOOP. Additionally, the absence of modes between 1200 and 1300 cm^-1 rules out functional monomerization. A correlation between phycocyanobilin (PCB) and phycoerythrobilin (PEB) suggests that the PCB cofactors of the PC trimer appear in a conformation similar to that of PEB. The conformation of the PC rod is consistent with that of the allophycocyanin (APC) trimer, and thus excitonic flow is facilitated between these two independent light-harvesting compounds. This excitonic flow from the PC rod to APC appears to be modulated by the vibration channels during HOOP wagging, C = C stretching, and the N-H rocking in-plan vibration.