LOCATION OF THE CAROTENOID 2A(G)-STATE AND ITS ROLE IN PHOTOSYNTHESIS

The location of the carotenoid 2A(g)-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-S0 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 <-> chlorophyll electron-exchange energy transfer is discussed.