β-carotene to chlorophyll singlet energy transfer in the photosystem I core of Synechococcus elongatus proceeds via the β-carotene S2 and S1 states

In this work, we describe the ultrafast excitation transfer dynamics of beta-carotene in trimeric photosystem I preparations from Synechococcus elongatus. beta-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 beta-carotene S-2 (1B(u)(+)) states is similar to60 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 beta-carotene S-1 (2A(g)(-)) states is not in contact with the chlorophylls (lifetime 10 ps), whereas similar to30% transfers its energy efficiently to chlorophylls (lifetime 3 ps). We ascribe the S-1 transfer to a fraction of the beta-carotenes (possibly the ones adopting a cis configuration) that exhibit favorable pi-pi stacking with nearby chlorophylls. We estimate that similar to70% of initial beta-carotene S-2 excitations is transferred to the chlorophylls, namely, similar to60% from S-2 on a 60 fs time scale and similar to10% from S-1 on a 3 ps time scale, making the S-2 state to a major extent responsible for the larger yield of beta-carotene to chlorophyll singlet energy transfer in comparison with the photosystem II core proteins.