Evidence for an O2-barrier in the light-harvesting chlorophyll-a/b-protein complex LHC II

An O2-barrier in the intact light-harvesting complex LHC II protects chlorophylls (Chl) and xanthophylls (Car) from photooxidation. Direct evidence for the limited access of O2 to pigment sites comes from the decay kinetics of the first excited triplet state of Car (3Car-). The LHC-bound 3Car- in air-saturated solution decays mono-exponentially with a lifetime of 6.7-7.1 µs as compared to the approx. 1.2 µs of the β-carotene triplet in hexane and the 8.8-9 µs observed for both systems under anaerobiosis. Further properties of the photostable complex are the limited access of protons to pigment sites and the efficient energy transfer from 1Car- to Chl-a and from 3Chl- to Car. Fatty acids with increasing chain length increasingly lower both, the efficiency of the O2 barrier and the photo- and acid stability of the LHC-bound pigments while singlet and triplet energy transfer between the pigments is maintained. Therefore, the close proximity of Chl and Car is not sufficient to protect the pigments from photooxidation; in addition, an O2-barrier limiting the access of O2 to pigment sites is required for efficient photoprotection. Structural properties of the photostable LHC II possibly underlying its O2-barrier function are discussed.