Femtosecond Phase of Charge Separation in Reaction Centers of Chloroflexus aurantiacus

Difference absorption spectroscopy with temporal resolution of ∼20 fsec was used to study the primary phase of charge separation in isolated reaction centers (RCs) of Chloroflexus aurantiacus at 90 K. An ensemble of difference (light-minus-dark) absorption spectra in the 730–795 nm region measured at −0.1 to 4 psec delays relative to the excitation pulse was analyzed. Comparison with analogous data for RCs of HM182L mutant of Rhodobacter sphaeroides having the same pigment composition identified the 785 nm absorption band as the band of bacteriopheophytin ΦB in the B-branch. By study the bleaching of this absorption band due to formation of Φ B − , it was found that a coherent electron transfer from P* to the B-branch occurs with a very small delay of 10–20 fsec after excitation of dimer bacteriochlorophyll P. Only at 120 fsec delay electron transfer from P* to the A-branch occurs with the formation of bacteriochlorophyll anion B A − absorption band at 1028 nm and the appearance of P* stimulated emission at 940 nm, as also occurs in native RCs of Rb. sphaeroides. It is concluded that a nuclear wave packet motion on the potential energy surface of P* after a 20-fsec light pulse excitation leads to the coherent formation of the P+Φ B − and P+B A − states.