Chlorophyll a and carotenoid triplet states in light-harvesting complex II of higher plants

Laser-flash-induced transient absorption measurements were performed on trimeric light-harvesting complex II to study carotenoid (Car) and chlorophyll (Chl) tripler states as a function of temperature. In these complexes efficient transfer of triplets from Chi to Car occurs as a protection mechanism against singlet oxygen formation. It appears that at room temperature all triplets are being transferred from Chl to Car; at lower temperatures (77 K and below) the transfer is less efficient and chlorophyll triplets can be observed. In the presence of oxygen at room temperature the Car triplets are partly quenched by oxygen and two different Car triplet spectral species can be distinguished because of a difference in quenching rate. One of these spectral species is replaced by another one upon cooling to 4 K, demonstrating that at least three carotenoids are in close contact with chlorophylls. The triplet minus singlet absorption (T-S) spectra show maxima at 504-506 nm and 517-523 nm, respectively. In the Chi Q(y) region absorption changes can be observed that are caused by Car triplets. The T-S spectra in the Chi region show an interesting temperature dependence which indicates that various Car's are in contact with different Chl a molecules. The results are discussed in terms of the crystal structure of light-harvesting complex II.