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

Violaxanthin-chlorophyll a protein (VCP) from Nannochloropsis oceanica is a Chl a-only member of the LHC family of light-harvesting proteins. VCP binds carotenoids violaxanthin (Vio), vaucheriaxanthin (Vau), and vaucheriaxanthin-ester (Vau-ester). Here we report on energy transfer pathways in the VCP complex. The overall carotenoid-to-Chla energy transfer has efficiency over 90%. Based on their energy transfer properties, the carotenoids in VCP can be divided into two groups; blue carotenoids with the lowest energy absorption band around 480 nm and red carotenoids with absorption extended up to 530 nm. Both carotenoid groups transfer energy efficiently from their S-2 states, reaching efficiencies of similar to 70% (blue) and similar to 60% (red). The S-1 pathway, however, is efficient only for the red carotenoid pool for which two S-1 routes characterized by 0.33 and 2.4 ps time constants were identified. For the blue carotenoids the S-1-mediated pathway is represented only by a minor route likely involving a hot S-1 state. The relaxed S-1 state of blue carotenoids decays to the ground state within 21 ps. Presence of a fraction of non-transferring red carotenoids with the S-1 lifetime of 13 ps indicates some specific carotenoid-protein interaction that must shorten the intrinsic S-1 lifetime of Vio and/or Vau whose S-1 lifetimes in methanol are 26 and 29 ps, respectively. The VCP complex from N. oceanica is the first example of a light-harvesting complex binding only non-carbonyl carotenoids with carotenoid-to-chlorophyll energy transfer efficiency over 90%. (C) 2015 Elsevier B.V. All rights reserved.