Fluorescence property of photosystem II protein complexes bound to a gold nanoparticle
Development of an efficient photo-anode system for water oxidation is key to the success of artificial photosynthesis. We previously assembled photosystem II (PSII) proteins, which are an efficient natural photocatalyst for water oxidation, on a gold nanoparticle (GNP) to prepare a PSII–GNP conjugate as an anode system in a light-driven water-splitting nano-device (Noji et al., J. Phys. Chem. Lett., 2011, 2, 2448–2452). In the current study, we characterized the fluorescence property of the PSII–GNP conjugate by static and time-resolved fluorescence measurements, and compared with that of free PSII proteins. It was shown that in a static fluorescence spectrum measured at 77 K, the amplitude of a major peak at 683 nm was significantly reduced and a red shoulder at 693 nm disappeared in PSII–GNP. Time-resolved fluorescence measurements showed that picosecond components at 683 nm decayed faster by factors of 1.4–2.1 in PSII–GNP than in free PSII, explaining the observed quenching of the major fluorescence peak. In addition, a nanosecond-decay component arising from a ‘red chlorophyll’ at 693 nm was lost in time-resolved fluorescence of PSII–GNP, probably due to a structural perturbation of this chlorophyll by interaction with GNP. Consistently with these fluorescence properties, degradation of PSII during strong-light illumination was two times slower in PSII–GNP than in free PSII. The enhanced durability of PSII is an advantageous property of the PSII–GNP conjugate in the development of an artificial photosynthesis device.