The phosphorescence of singlet oxygen ((1)X( *)) photosensitized by the carotenoidless reaction center (RC) of Rhodobacter sphaeroides R26.1 has been investigated, using H(2)O and D(2)O as the suspending media. To enhance (under neutral conditions) the triplet quantum yield of the special pair P(870) (P) by the radical pair mechanism, the quinone acceptor Q(A) was removed by means of a chemical treatment. The phosphorescence signal fits the functional form P(0)[exp (-t/tau)-exp(-t/zeta)], regardless of whether (1)X( *) is sensitized by P(dagger) or M(dagger) (where the dagger denotes triplet excitation and M is a water-soluble molecule). The time constant zeta was identified with the decay time of (1)X( *); when P(dagger) is the sensitizer, one finds zeta(P)((1))=3.3+/-0.3 micros, and zeta(P)((2))=34+/-3 micros, where the superscripts 1 and 2 refer to H(2)O and D(2)O, respectively; the corresponding values for sensitization by M(dagger) (in the absence of RC) are zeta(M)((1))=3.7+/-0.4 micros, and zeta(M)((2))=75+/-5 micros. The addition of RC's to the solution of M in D(2)O reveals that the RC is a quencher of (1)X( *); however, for equal concentrations of the RC, zeta(P)((2))

Multi-channel, flash kinetic spectroscopy with microsecond time resolution has been used for investigating the interactions between carotenoids and the following photoproducts of alpha-tocopherol (EH) in hexane, methanol, acetonitrile, and dimethyl sulfoxide: (a) the lowest triplet, (b) the tocopherol radical cation, which could be seen only in the polar aprotic solvents acetonitrile and dimethyl sulfoxide, and (c) the neutral tocopheroxyl radical. The first two species reconvert to EH by transferring triplet excitation and positive charge (respectively) to the carotenoid; the third is unreactive. The relevance of these observations to photoprotection and the photoionisation of sterically hindered phenols is pointed out.

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