Formation and geminate quenching of singlet oxygen in purple bacterial reaction center

The phosphorescence of singlet oxygen (X-1*) photosensitized by the carotenoidless reaction center (RC) of Rhodobacter sphaeroides 826.1 has been investigated, using H2O and D2O 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 X-1* is sensitized by P-dagger or M-dagger (where the dagger denotes triplet excitation and M is a water-soluble molecule). The time constant was identified with the decay time of X-1*; when P-dagger is the sensitizer, one finds zeta((1))(P) = 3.3 +/- 0.3 ps, and ((2) = 34 3 Vs, where the superscripts 1 and 2 refer to H2O and D2O, respectively; the corresponding values for sensitization by Mt (in the absence of RC) are M l = 3.7 +/- 0.4 Vs, and M l = 75 5 Vs. The addition of RC's to the solution of M in D2O reveals that the RC is a quencher of X-1*; however, for equal concentrations of the RC, zeta((2))(P) < zeta((2))(M), showing that X-1* is deactivated, after its entry into the suspending medium, mainly by the solvent or the same RC which acts as the sensitizer. The values of tau(P) are similar in both solvents, ca. 2 Vs, but this time constant does not figure in the disappearance of Pt, which follows a bi-exponential course, alpha(1)exp(-t/tau(1)) + alpha(2)exp(-t/tau(2)). The time constants tau(1) and tau(2) (72 +/- 5 mu s and 12 +/- 1 mu s, respectively) as well as the factor alpha(2) are insensitive to the oxygen content, and quenching of Pt is manifested only through a threefold reduction in the magnitude of a,; these data imply the absence of dynamic quenching and heterogeneity of the RC. The mean lifetime of X-1* inside the protein matrix is identified with tau(P), and the absence of a prompt component in the phosphorescence signal rationalized by proposing that the radiative decay of X-1* within the RC is much slower than that in an aqueous environment. (c) 2007 Elsevier B.V. All rights reserved.