Observation of a bacteriochlorophyll anion radical during the primary charge separation in a reaction center

The primary light-induced charge separation in reaction centers of
Rhodobacter sphaeroides was investigated with femtosecond time
resolution. The absorption changes in the time range 100 fs to 1 ns
observed after direct excitation of the primary donor P at 860 nm
could only be explained by a kinetic model which uses three time
constants. This finding supports the following reaction scheme: (i)
the electronically excited primary donor P* decays with a time
constant of 3.5 ps and populates a very short-lived intermediate
involving a reduced accessory bacteriochlorophyll molecule; (ii)
with a time constant of 0.9 ps the electron is transferred to the
neighboring bacteriopheophytin molecule; and (iii) from there
within 200 ps to the quinone.