Dynamics of β-Carotene-to-Chlorophyll Singlet Energy Transfer in the Core of Photosystem II

The core of photosystem II comprises three major subcomplexes: the photochemical reaction center (RC) and the core antennas CP43 and CP47. All contain β-carotene as the only carotenoid, but the quantum efficiency of β-carotene-to-chlorophyll singlet energy transfer in the CP43, CP47, and RC complexes is poor. From a femtosecond transient absorption study of the isolated subcomplexes following β-carotene excitation, we show that mainly the β-carotene S 2 (1B u + ) state is involved in the energy-transfer process. At most, a minor amount of energy transfer from “hot” β-carotene S 1 (2A g - ) states is present in CP43 and CP47, but this S 1 contribution is absent in the RC. In all complexes, the relaxed S 1 state is not participating in energy transfer. This absence of energy transfer from S 1 states explains the low overall yield of β-carotene-to-chlorophyll singlet energy transfer in the PSII core.