Spin-density distribution of the carotenoid triplet state in the peridinin-chlorophyll-protein antenna. A Q-band pulse electron-nuclear double resonance and density functional theory study

The triplet state of the carotenoid peridinin in the refolded N-domain peridinin-chlorophyll-protein (PCP) antenna complex from Amphidinium carterae is investigated by orientation-selected pulse Q-band ENDOR spectroscopy (34 GHz). The peridinin triplet is created by triplet-triplet transfer from (3)Chl a, generated by illumination at 630 nm. The peridinin triplet lifetimes are close to the minimum duration of the pulse ENDOR experiment (similar to 10 mu s). Thirteen proton hyperfine coupling (hfc) tensors are deduced for the peridinin triplet state. Additionally, density functional theory (DFT) calculations are presented which aided in the assignment of proton hfcs. The number and magnitude of the resolved hfcs indicate that only one specific peridinin in PCP carries the triplet exciton. The experiments enable us to derive for the first time information about the wavefunction of the triplet electrons (S = 1) in a carotenoid molecule, which is a sensitive probe for the electronic and geometric structure of this short-lived excited state in the protein matrix.