Phosphorescence from the primary electron donor in Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers

Phosphorescence has been detected from the dimeric primary electron donor in Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers. For Rb. sphaeroides the emission maximum occurs at 1318 + 2 nm at 20 K and is nearly independent of temperature. The emission linewidth (full width at half maximum) is 240 + 20 cm-! at 20 K and increases to 580 + 40 cm-i at 280 K. The phosphorescence lifetime is identical to that of the triplet state of the primary electron donor measured by ground state recovery or triplet-triplet absorption kinetics; likewise, the same magnetic-field effect is observed for the triplet state quantum yield by phosphorescence and absorption measurements. A comparison of the difference between the fluorescence and phosphorescence maxima of the special pair (3424 cm -1) and of pure monomeric bacteriochlurophyU a (4600 cm t ) and the observation that the phosphorescence linewidth for the special pair in the reaction center is smaller than the fluorescence iinewidth suggest that the triplet state has weak charge-transfer character. For Rps. vb'idis the emission maximum occurs at 1497 42 um and the energy difference between the maxima of the fluorescence and phosphorescence is 3130 cmt, compared to 4210 cm-t for pure monomeric bacteriochlorophyii b.