Efficient light-harvesting using non-carbonyl carotenoids: Energy transfer dynamics in the VCP complex from Nannochloropsis oceanica

Gürkan Kesana, Radek Litvína,b, David Bínaa,b, Milan Durchana,b, Václav Šloufa, Tomáš Polívkaa,b
aFaculty of Science, University of South Bohemia, Ceské Budejovice, Czech Republic; bInstitute of Plant Molecular Biology, Biological Centre, Czech Academy of Sciences, Ceské Budejovice, Czech Republic

Energy transfer pathways between carotenoids and chlorophylls in the violaxanthin-chlorophyll-a protein (VCP) from Nannochloropsis oceanica have been studied using steady-state and ultrafast pump-probe spectroscopy. VCP binds two carotenoids, violaxanthin (Vio) and vaucheriaxanthin (Vau), Absorption spectrum of VCP reveals two different carotenoid absorption bands at ~480 and ~500 nm, which are denoted as blue and red carotenoids. While the blue carotenoids transfer energy efficiently only from their S2 states (~70% efficiency), both S2 (~60%) and S1 states of red carotenoids are involved in energy transfer pathways in VCP.

Furthermore, energy transfer from the S1 state of red carotenoids proceed via two routes having 0.33 and 2.4 ps time constants. The relaxed S1 state of non-transferring blue carotenoids decays to the ground state within 21 ps while this time constant is 13 ps for the red carotenoids. This is significantly shorter than the S1 lifetime of Vio (26 ps) or Vau (29 ps) in methanol. This shortening indicates that there is a specific interaction between red carotenoids and protein. The total carotenoid-Chl energy transfer efficiency in VCP exceeds 90%, making VCP the first Chl-based light-harvesting system with such a high efficiency utilizing non-carbonyl carotenoids.