The primary electron transfer (ET) processes at 295 and
77 K are
compared for the Rhodobacter sphaeroides reaction
center (RC) pigment–protein complex from 13 mutants including
a wild-type control. The engineered RCs bear mutations in the L and
M polypeptides that largely inhibit ET from the excited state P* of
the primary electron donor (P, a bacteriochlorophyll dimer)
to the normally photoactive A-side cofactors and enhance ET to the
C2-symmetry related, and normally photoinactive, B-side
cofactors. P* decay is multiexponential at both temperatures and modeled
as arising from subpopulations that differ in contributions of two-step
ET (e.g., P* → P+BB
– → P+HB
–), one-step superexchange ET (e.g., P* →
P+HB
–), and P* → ground
state. [HB and BB are monomeric bacteriopheophytin
and bacteriochlorophyll, respectively.] The relative abundances
of the subpopulations and the inherent rate constants of the P* decay
routes vary with temperature. Regardless, ET to produce P+HB
– is generally faster at 77 K than
at 295 K by about a factor of 2. A key finding is that the yield of
P+HB
–, which ranges from ∼5%
to ∼90% among the mutant RCs, is essentially the same at 77
K as at 295 K in each case. Overall, the results show that ET from
P* to the B-side cofactors in these mutants does not require thermal
activation and involves combinations of ET mechanisms analogous to
those operative on the A side in the native RC.