Shallow Sink in an Antenna Pigment System of Photosystem I of a Marine Centric Diatom, Chaetoceros gracilis, Revealed by Ultrafast Fluorescence Spectroscopy at 17 K

Abstract

The ultrafast fluorescence dynamics of photosystem I (PS I) purified from a marine centric diatom, Chaetoceros gracilis, at 17 K was studied using fluorescence up-conversion and streak-camera setups. The experiments were done for two kinds of sample preparations containing different amounts of the peripheral antenna proteins, the fucoxanthin-chlorophyll (Chl) binding proteins associated with PS I (FCPI). Upon excitation at 430 nm, which selectively excites Chl a mainly contained in the core complex, the fluorescence dynamics of both samples was roughly expressed by four decay-associated spectra (DASs) with time constants of ca. 5, ca. 22, ca. 100, and ca. 400 ps. These DAS components have corresponding counterparts in the results of a previous study of Thermosynechococcus elongatus PS I (Shibata et al. J. Phys. Chem. B 2010, 114, 2954) except for that with a time constant of ca. 22 ps. The similar distribution of the time constants suggests a shared light-harvesting pathway by PS I of these two organisms. The DAS with a ca. 400 ps time constant has its peak wavelength at around 710 nm, suggesting the presence of antenna pigment states with slightly lower excitation energy than that of P700. This antenna state acts as a shallow sink in the core complex of the diatom PS I and causes a specific temperature dependence of its fluorescence spectrum below 77 K. Excitation energy funneling into the shallow-sink state seems to take place within 0.2 ps, suggesting an extremely efficient energy transfer. Upon the selective excitation of Chl c in FCPI by a 460 nm laser, three DAS components suggesting excitation energy transfers were obtained. The 0.2 ps DAS shows the energy transfer from Chl c to Chl a within FCPI, while the 0.7 and 40 ps DASs suggest the energy transfer from FCPI to the core complex. The excitation energy seems to be effectively transferred from FCPI to the core complex in diatom PS I because the selective excitation of Chl c in FCPI does not induce a severe retardation of the overall light-harvesting kinetics.