Evidence for monomeric bacteriochlorophyll in P800 of the photoreaction center from Rhodospirillum rubrum.

To find out whether weak or strong coupling exists between the bacteriochlorophyll molecules of the photoreaction center, the relative efficiency of energy transfer to P870 was measured at 795 nm and at 808 nm, at room temperature and at 77 degrees K. At room temperature, both relative efficiencies are close to 100%. However, at 77 degrees K, 795 nm light has a quantum efficiency of 76% and 808 nm light has an efficiency of 87%. These results confirm the fact that P800 is formed of at least one short wavelength component and one long wavelength component. Moreover, the short wavelength component is weakly coupled to both P870 and to the long wavelength component of P800. The conclusion is that the short wavelength component is due to monomeric bacteriochlorophyll. By comparison with other data, all four bacteriochlorophyll molecules of the photoreaction center are inferred to be monomeric.