Single molecule spectroscopy on photosynthetic light-harvesting complexes

Abstract

Single molecule spectroscopy (SMS) is a powerful approach to study subtle, fundamental properties of biological systems generally obscured by the ensemble average. SMS allows for a detailed understanding of the molecular mechanisms underlying the biological function of many systems. In this thesis, SMS was used to investigate the photophysical properties of photosynthetic light-harvesting complexes (LHCs) under different environments. The two LHCs studied are LHCII, the major lightharvesting complex of higher plants (specifically Spinacia oleracea), and LH2, one of the major light-harvesting complexes of purple bacteria (specifically Rhodopseudomonas acidophila). In the first part, the photodynamics of LHCII in two different oxygen-depleted environments, i.e., in the presence of enzymatic oxygen scavengers and under nitrogen gas purging, were investigated. In the presence of oxygen scavengers, we observed at least two distinct states, which are characterized as unquenched and quenched, where quenching refers to energy dissipation in the form of heat. Under the nitrogen gas atmosphere, the majority of LHCII complexes exhibited only an unquenched state, with a negligible probability of switching to the quenched stated. Moreover, we found that the rate at which LHCII switches between the unquenched and quenched states was two orders of magnitude lower compared to that in the presence of oxygen scavengers. We speculate that the quenched state in LHCII could be activated by molecular oxygen, which, in turn, might play a key role in regulating light harvesting in oxygenic photosynthesis. Surprisingly, LH2, a pigment-protein from an anoxygenic organism, was also found to be incredibly stable under nitrogen gas purging. Overall, these results will help to increase our understanding of the photophysical mechanisms underlying the regulation of light harvesting, with a view of developing robust bio-solar devices as well as improving biomass yields. In the second part, the effects of plasmonic coupling on the fluorescence dynamics of LHCII were explored. We demonstrated that the brightness (fluorescence intensity) of a single LHCII can be significantly enhanced when coupled to a gold nanorod (AuNR). The increase in brightness is due to the enhanced rate of excitation and increased decay rate of LHCII placed near the nanorod. The AuNRs utilized in this study were chemically synthesized, and the LHCII/AuNR hybrid system was constructed using a simple and economical spin-assisted layer-by-layer technique. A fluorescence brightness increment of up to 240-fold was obtained, accompanied by a two orders of magnitude decrease in the average (amplitude-weighted) fluorescence lifetime down to a few picoseconds. This large fluorescence enhancement is explained by the strong spectral overlap of the longitudinal localized surface plasmon resonance of the utilized AuNRs and the absorption or emission bands of LHCII. In principle, these results provide an effective strategy to study the fluorescence dynamics of weakly emitting photosynthetic LHCs, especially at the single-molecule level where the fluorescence signal is usually overwhelmed by the background noise.