Carotenoid dark state to chlorophyll energy transfer in isolated light-harvesting complexes CP24 and CP29

Abstract

We present a comparison of the energy transfer between carotenoid dark states and chlorophylls for the minor complexes CP24 and CP29. To elucidate the potential involvement of certain carotenoid–chlorophyll coupling sites in fluorescence quenching of distinct complexes, varying carotenoid compositions and mutants lacking chlorophylls at specific binding sites were examined. Energy transfers between carotenoid dark states and chlorophylls were compared using the coupling parameter, $\varPhi_{\text{Coupling}}^{{{\text{Car S}}_{ 1} {-}{\text{Chl}}}}$, which is calculated from the chlorophyll fluorescence observed after preferential carotenoid two-photon excitation. In CP24, artificial reconstitution with zeaxanthin leads to a significant reduction in the chlorophyll fluorescence quantum yield, $\varPhi_{\text{F1}}$, and a considerable increase in $\varPhi_{\text{Coupling}}^{{{\text{Car S}}_{ 1} {-}{\text{Chl}}}}$. Similar effects of zeaxanthin were also observed in certain samples of CP29. In CP29, also the replacement of violaxanthin by the sole presence of lutein results in a significant quenching and increased $\varPhi_{\text{Coupling}}^{{{\text{Car S}}_{ 1} {-}{\text{Chl}}}}$. In contrast, the replacement of violaxanthin by lutein in CP24 is not significantly increasing $\varPhi_{\text{Coupling}}^{{{\text{Car S}}_{ 1} {-}{\text{Chl}}}}$. In general, these findings provide evidence that modification of the electronic coupling between carotenoid dark states and chlorophylls by changing carotenoids at distinct sites can significantly influence the quenching of these minor proteins, particularly when zeaxanthin or lutein is used. The absence of Chl612 in CP24 and of Chl612 or Chl603 in CP29 has a considerably smaller effect on $\varPhi_{{{\text{F}}1}}$ and $\varPhi_{\text{Coupling}}^{{{\text{Car S}}_{ 1} {-}{\text{Chl}}}}$ than the influence of some carotenoids reported above. However, in CP29 our results indicate slightly dequenching and decreased $\varPhi_{\text{Coupling}}^{{{\text{Car S}}_{ 1} {-}{\text{Chl}}}}$ when these chlorophylls are absent. This might indicate that both, Chl612 and Chl603 are involved in carotenoid-dependent quenching in isolated CP29.