Optically Detected Magnetic Resonance of Chlorophyll Triplet States in Water-Soluble Chlorophyll Proteins from Lepidium virginicum: Evidence for Excitonic Interaction among the Four Pigments.

Optically detected magnetic resonance of triplet states populated by photoexcitation in water-soluble chlorophyll proteins (WSCPs) from Lepidium virginicum has been performed using both absorption and fluorescence detection. Well resolved triplet-singlet (T-S) spectra have been obtained and interpreted in terms of electronic interactions among the four chlorophylls (Chls), forming two dimers in the WSCP tetramer. Localization of the triplet state on a single Chl leads to a redistribution of the oscillator strength in the remaining three Chls of the complex. By comparing the spectra with those obtained on a substoichiometric WSCP complex containing only 2 Chls per protein tetramer, we proved that, to interpret the optical spectra of the WSCP fully loaded with 4 Chls, the interactions between the two dimers have to be taken into account and cannot be considered negligible. The results show that the WSCP may well be considered as an ideal model system to study Chl-Chl interactions, also in view of the possibility to modify the number and molecular structure of the bound porphyrin chromophores.