Subpicosecond characterization of the optical properties of the primary electron donor and the mechanism of the initial electron transfer in Rhodobacter capsulatus reaction centers

The optical difference spectrum of the excited primary electron donor, P*, in Rb. capsulatus reaction centers has been measured 600 fs after a 350-fs flash at 870 nm. The spectrum is characterized by bleaching in the ground state absorption bands at 855 and 600 nm, and a weak featureless transient absorption in between. The lack of significant (if any) bleaching at 800 nm indicates that P does not contribute appreciable oscillator strength to the 800-nm ground state absorption band. The conversion of P* to P⁺BPH^-_L is accompanied by isosbestic points in the transient difference spectra at 765 and 798 nm. The existence of the longer-wavelength isosbestic point, occurring essentially at the near-infrared absorbance maximum of the accessory BChls, provides compelling evidence that if state P*BChl^-_L forms, its transient concentration is exceedingly small. At certain wavelengths in the near-infrared the absorption changes develop somewhat more slowly than the rate at which P* decays, a finding that may reflect a contribution from readjustments in the pigment-protein complex in response to electron transfer.