Two pathways to understanding electron transfer in reaction centers from photosynthetic bacteria: A comparison of...

The rates, yields, mechanisms and directionality of electron transfer (ET) are explored in twelve pairs of Rhodobacter (R.) sphaeroides and R. capsulatus mutant RCs designed to defeat ET from the excited primary donor (P*) to the A-side cofactors and re-direct ET to the normally inactive mirror-image B-side cofactors. In general, the R. sphaeroides variants have larger P⁺H_B^- yields (up to ∼90%) than their R. capsulatus analogs (up to ∼60%), where H_B is the B-side bacteriopheophytin. Substitution of Tyr for Phe at L-polypeptide position L181 near B_B primarily increases the contribution of fast P* → P⁺B_B^- → P⁺H_B^- two-step ET, where B_B is the "bridging" B-side bacteriochlorophyll. The second step (∼6-8 ps) is slower than the first (∼3-4 ps), unlike A-side two-step ET (P* → P⁺B_A^- → P⁺H_A^-) where the second step (∼1 ps) is faster than the first (∼3-4 ps) in the native RC. Substitutions near H_B, at L185 (Leu, Trp or Arg) and at M-polypeptide site M133/131 (Thr, Val or Glu), strongly affect the contribution of slower (20-50 ps) P* → P⁺H_B^- one-step superexchange ET. Both ET mechanisms are effective in directing electrons "the wrong way" to H_B and both compete with internal conversion of P* to the ground state (∼200 ps) and ET to the A-side cofactors. Collectively, the work demonstrates cooperative amino-acid control of rates, yields and mechanisms of ET in bacterial RCs and how A- vs. B-side charge separation can be tuned in both species.