β-Carotene to Chlorophyll Singlet Energy Transfer in the Photosystem I Core ofSynechococcuselongatusProceeds via the β-Carotene S2and S1States

In this work, we describe the ultrafast excitation transfer dynamics of β-carotene in trimeric photosystem I preparations from Synechococcus elongatu s . β-Carotene was excited with a femtosecond laser, and the changes in absorption were followed in the spectral region 490−740 nm. The lifetime of the β-carotene S 2 (1B u + ) states is ∼60 fs, significantly shorter than that in nonpolar solution (150 fs), demonstrating efficient energy transfer to the chlorophylls from these states. About 70% of the remaining population of β-carotene S 1 (2A g - ) states is not in contact with the chlorophylls (lifetime 10 ps), whereas ∼30% transfers its energy efficiently to chlorophylls (lifetime 3 ps). We ascribe the S 1 transfer to a fraction of the β-carotenes (possibly the ones adopting a cis configuration) that exhibit favorable π−π stacking with nearby chlorophylls. We estimate that ∼70% of initial β-carotene S 2 excitations is transferred to the chlorophylls, namely, ∼60% from S 2 on a 60 fs time scale and ∼10% from S 1 on a 3 ps time scale, making the S 2 state to a major extent responsible for the larger yield of β-carotene to chlorophyll singlet energy transfer in comparison with the photosystem II core proteins.