Electron Spin Density Distribution of the Carotenoid Triplet State in the Peridinin-Chlorophyll-Protein Antenna of Dinoflagellates Determined by Pulse ENDOR Spectroscopy and Density Functional Theory
The triplet state of the peridinin carotenoid 3Per in the refolded N-domain peridinin-chlorophyll-protein antenna complex from Amphidinium carterae is investigated by orientationselected pulse ENDOR spectroscopy at 34 GHz. The peridinin triplet is created by triplet-triplet transfer from 3Chl α, which is generated by illumination at 630 nm. The 3Per lifetimes fall in the range of 10–50 μs, which is close to the minimum duration of the pulse ENDOR experiment (̃10 μs). Additionally, density functional theory (DFT) calculations of the magnetic resonance parameters of the carotenoid triplet state are presented, using the ORCA program package. An assignment of the 1H hyperfine coupling constants (hfcs) to protons of the peridinin is achieved based on a comparison of the experimental and theoretical magnitudes, anisotropies and signs of the hfcs. This enables us to derive for the first time 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.
KeywordsPeridinin-chlorophyll-protein triplet state dinoflagellates EPR ENDOR DFT
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