Location of the carotenoid 2Ag-state and its role in photosynthesis
1993
The location of the carotenoid 2Ag-state (S1) was studied by fluorescence spectroscopy in two series of carotenoids. One consisted of natural polyenes like phytoene, phytofluene, xi-carotene, and neurosporene, and the other of minicarotenes, i.e., compounds similar to beta-carotene, but with a smaller number of double bonds (n = 3-9). A decrease of the S1-Ps energy gap with n was observed in both series, and extrapolation to n = 11 gave the energies 14 500 and 13 200 cm-1 for beta-carotene and lycopene, respectively. The neurosporene S1 state was located at ca 16 000 cm-1. A good relationship between the nonradiative relaxation rate and the number of conjugation was observed. The rate increased by a factor of 2.5 per double bond. The possible role of the S1 state in reversible carotenoid in equilibrium with chlorophyll electron-exchange energy transfer is discussed.
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