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. Author manuscript; available in PMC: 2009 Jul 1.
Published in final edited form as: Biochim Biophys Acta. 2008 May 16;1777(7-8):684–688. doi: 10.1016/j.bbabio.2008.05.005

Fig. 4.

Fig. 4

Simplified scheme of excited-state energy transfer from carotenoid to retinal. Absorption of a photon by the carotenoid produces the shortlived S2 state, as S1 is forbidden. Energy migration to the S1 state of the retinal occurs only from S2, because the carotenoid S1 level is lower than the retinal S1. It competes with internal conversion to S1 of carotenoid (dashed vertical arrow). Decay of the all trans retinal S1 state produces 13-cis,15-anti configuration.