Spectroscopic Investigations of Excitation Energy Dissipation in the CP43 Preassembly Complexes of Photosystem II Embedded in Clear Native Gel

The importance of the spectrochemical properties of the pigment-bound CP43 protein lies in its role as the inner antenna complex of photosystem II. Multiple populations of the CP43-containing protein complexes are known to exist in vivo. These complexes have minor differences in molecular masses, and the conventional separation approach using glycerol-gradient ultracentrifugation lacks the resolution to separate them clearly. This hinders the assessment of the spectrochemical properties of different CP43 protein complexes in solution. Using high-resolution clear native PAGE, we successfully separated two known subclasses of the CP43 preassembly complex, ranging between 70 and 90 kDa, and performed spectroscopic analysis of the gel-embedded pigment proteins. These gel-embedded complexes, named CP43p-H and CP43p-L, were investigated by applying various static and time-resolved optical spectroscopies to elucidate their differences. The studies were performed at room temperature and 77 K. The application of time-resolved transient absorption followed by target analysis of the data sets demonstrated fine-tuning in pigment-pigment interactions within the complex. Time-resolved fluorescence demonstrated a truncated fluorescence emission decay for CP43p-L with respect to CP43p-H at both temperatures. The process contributing to such an accelerated quenching in CP43p-L is intriguing. We suggest that in CP43p-L, the pigments are arranged to facilitate a Chl <i>a</i>-to-carotenoid (β-carotene) singlet-singlet excitation energy transfer.