Cryospectroscopy Studies of Intact Light-Harvesting Antennas Reveal Empirical Electronic Energy Transitions in Two Cyanobacteria Species

Understanding of electronic energy transition (EET) mechanisms from the light-harvesting unit to the reaction center in a natural system has been limited by (a) the use of conventional transient time-resolved spectroscopy at room temperature, which result in high signal-to-noise ratio and (b) examining extracts instead of intact light-harvesting units. Here, we report previously unknown differences and new insight in EET of two cyanobacteria species, Acaryochloris marina and Thermosynechoccocus vulcanus, which have been found only after using UV-vis, hole-burning, and fluorescence spectroscopy at ultralow temperature and examining their intact light-harvesting unit, phycobilisomes (PBS). Although the exciton formation is similarly induced by photoexcitation of chromophore assemblies in phycocyanin (PC) and allophycocyanin (APC) in PBSs of both species, the EET mechanisms are totally different, being adiabatic in A. marina and nonadiabatic in T. vulcanus. The PBS of A. marina has only one APC trimer and energy transfer is through coupling of α₈₄ in APC with β₈₄ in adjacent PC. In T. vulcanus, the PBS has three components: coupling between APC core and the entire PC rod and couplings of β-β₁₈ and of L_CM to β in the adjacent APC-like trimer. A total of 80% of the excitation energy is trapped in the coupling β-β₁₈ and regulates the flow of energy from the high- to low-level terminal electronic transition emitter β-L_CM. All these details cannot be observed at room temperature and in extracted units.