Femtosecond Fluorescence Upconversion Studies of Light Harvesting by β-Carotene in Oxygenic Photosynthetic Core Proteins

Abstract

Energy transfer from β-carotene to chlorophyll in the photosystem (PS) I core complex and the CP43, CP47, and reaction center (RC) proteins of PSII was studied by the femtosecond fluorescence upconversion technique. The carotenoid S2 lifetimes/transfer efficiencies, the latter obtained by comparison with β-carotene in solution, are ∼97 fs/33% (CP43), ∼100 fs/31% (CP47), ∼106 fs/26% (PSII−RC), and ∼62 fs/57% (PSI). By combining previous steady-state fluorescence excitation measurements with our ultrafast results on the carotenoid S2 lifetime and chlorophyll rise kinetics, we conclude that the β-carotene S1 state is insignificant for light harvesting in the PSII core proteins. Measurement of the steady-state fluorescence excitation spectrum of PSI at 77 K in this work yielded a β-carotene to chlorophyll energy transfer efficiency of 62 ± 3%, suggesting dominant transfer from the carotenoid S2 state. Interestingly, while both PSI and PSII almost exclusively utilize the carotenoid S2 state for light harvestin...