Microwave and Optical Spectroscopy of Carotenoid Triplets in Light-Harvesting Complex LHC II of Spinach by Absorbance-Detected Magnetic Resonance

Absorbance-detected magnetic resonance (ADMR) of the light-harvesting complex LHC II of spinach revealed two triplet contributions, having different D values, but equal E value (vertical bar E vertical bar = 0.00379 cm(-1)). The two triplets are assigned to two of the three carotenoids present in LHC II: lutein (vertical bar D vertical bar = 0.03853 cm(-1)) and neoxanthin (vertical bar D vertical bar = 0.04003 cm(-1)). The ADMR-detected Triplet-minus-Singlet (T-S) optical difference spectrum of the carotenoid (Car) triplet transition of LHC II showed, apart from bands in the Car absorption region, a contribution. the chlorophyll (Chl) absorption region due to a change in interaction between lutein and Chl a at 670 nm, and neoxanthin and Chl a at 670 and 677 nm. From Linear Dichroic (LD-)ADMR-detected LD-(T-S) spectra we have determined that the triplet z-axis (which corresponds roughly to the polyenal axis) of lutein and neoxanthin makes angle of 47 degrees and 38 degrees with the Q(y) transition moment of their adjacent Chl a molecules, for the Chls absorbing at 670 and 677 nm, respectively. The T(z) triplet magnetic transition moment of lutein is parallel to the lutein singlet and triplet absorptions, whereas the T(z) axis of neoxanthin makes angle of about 20 degrees with the optical transition moments of the carotenoid molecule. The major Chl a absorption bands of the optic. absorption spectrum and the ADMR-detected T-S spectrum is best explained by assuming that all Chl a is present in dimers. It is proposed that a free Chl dimer absorbs at 664 and 670 nm, whereas a Chl dimer bound to a carotenoid absorbs at 670 and 677 nm