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 beta-carotene as the only carotenoid, but the quantum efficiency of beta-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 beta-carotene excitation, we show that mainly the beta-carotene S-2 (1B(u)(+)) state is involved in the energy-transfer process. At most, a minor amount of energy transfer from "hot" beta-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 beta-carotene-to-chlorophyll singlet energy transfer in the PSII core.