The internal conversion rate of the primary donor in reaction centers of 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 electron transfer is not accessible in native systems. Therefore, reaction centers of Rhodobacter sphaeroides have been modified via thermal exchange of the relevant primary electron acceptor B-A. Substitution of B-A with 3-vinyl-13(2)-OH-bacteriochlorophyll increases the lifetime of P-1* from about 2 to 240 ps at 90 K. Transient absorption studies reveal that this increase is due to a decrease of the rate of charge separation. At 90 K this decrease causes a concomitant drop in the quantum 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. The 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 usually observed for internal conversion processes. These internal conversion data constitute an important parameter for theoretical modelling of the primary charge separation along both, the active and inactive pigment branches.