THE INTERNAL-CONVERSION RATE OF THE PRIMARY DONOR IN REACTION CENTERSOF RHODOBACTER-SPHAEROIDES

The most prominent cornerstone of reaction center function is the high quantum yield of transmembrane charge separation approaching unity in the systems studied so far. This feature necessarily implies that the deactivation rate of the excited primary donor P-1 not involving ele ctron transfer is not accessible in native systems. Therefore, reactio n centers of Rhodobacter sphaeroides have been modified via thermal ex change of the relevant primary electron acceptor B-A. Substitution of B-A with 3-vinyl-13(2)-OH-bacteriochlorophyll increases the lifetime o f P-1 from about 2 to 240 ps at 90 K. Transient absorption studies re veal that this increase is due to a decrease of the rate of charge sep aration. At 90 K this decrease causes a concomitant drop in the quantu m yield of charge separation Y to 78% leading to a fast ground state P recovery of 22%. This directly translates into an internal conversion rate from P-1 to the ground state of k(IC) = (1.1 ns)(-1) at 90 K. T he corresponding data at 270 K (tau(F) (P-1) = 66 ps; Y = 91%) result in k(IC) = (750 ps)(-1) suggesting a weak thermal activation as usual ly observed for internal conversion processes. These internal conversi on data constitute an important parameter for theoretical modelling of the primary charge separation along both, the active and inactive pig ment branches.