Activation and deactivation of vibronic channels in intact phycocyanin rods

Abstract

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 C5,10,15-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.

Linker proteins enable ultrafast excitation energy transfer in the phycobilisome antenna system of Thermosynechococcus vulcanus

“…Our observations fit well with the activation and deactivation of HOOP wagging, CvC stretching, and the N-H rocking in-plane vibration. 21 We also observed strong localized vibronic coupling between β 84 in adjacent PC trimers in the PC rod of A. marina. 9 In the hexameric PC assembly, Womick et al recently showed that the fastest EET component is 970 fs, 18 while, in a fragment of PC bound to APC, Sandström et al found 17 ps and 50 ps kinetic components.…”

“…There are two different states not located in the last heterohexamer, which some have suggested is the terminal emitter of the PBS in A. marina. 21 Since the APC trimer already forms an excitonic absorption state at 645 nm, there is no more evidence of further localized APC trimer excitonic state. Therefore, the origin of the terminal emitter, or its pigment composition, remains an open issue.…”

“…The femtosecond transient absorption kinetic trace (ΔA spectrum) is different from the kinetic components, because the kinetic components is derived from fitting transient absorption kinetic trace into the kinetic model, eqn (1). 21,26 In the section immediately below, we present the result of transient absorption traces (Fig. 2), while the section that follows after is devoted to the analysis of kinetic component.…”

“…This component could serve as the real low energy state for PC rods. 21 The bleaching observed in PC at 620 nm is the electronic transition of the PC620 inside the PC rod. This is the high energy level of PC Rod, which is induced by the direct excitation at 620 nm.…”

Evidence of additional excitation energy transfer pathways in the phycobiliprotein antenna system of Acaryochloris marina

“…The translation stage of 1 ns and the Lock-In amplifier were controlled by a computer, which was also used for data acquisition. The details and illustration of the set-up has been outlined by Nganou et al 33…”