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

An O-2-barrier in the intact light-harvesting complex LHC II protects chlorophylls (Chl) and xanthophylls (Car) from photooxidation. Direct evidence for the Limited access of O-2 to pigment sites comes from the decay kinetics of the first excited triplet state of Car ((3)Car*). The LHC-bound (3)Car* in air-saturated solution decays mono-exponentially with a lifetime of 6.7-7.1 mu s as compared to the approx. 1.2 mu s of the beta-carotene triplet in hexane and the 8.8-9 mu s 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 (1)Car* to Chl-a and from (3)Chl* to Car. Fatty acids with increasing chain length increasingly lower both, the efficiency of the O-2 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 O-2-barrier limiting the access of O-2 to pigment sites is required for efficient photoprotection. Structural properties of the photostable LHC LI possibly underlying its O-2-barrier function are discussed.