Energy transfer within the bacteriochlorophyll antenna of purple bacteria at 77 K, studied by picosecond absorption recovery

Abstract

The dynamics of energy transfer within the bacteriochlorophyll antenna of Rhodobacter sphaeroides and Rhodospirillum rubrum, with closed reaction centers, has been studied at 77 K using low-intensity infrared picosecond absorption recovery. Measurements of isotropic decay as well as decay of induced anisotropy yielded a detailed picture of the energy transfer pathways in the two antenna systems. The BChl antenna of Rb. sphaeroides seems to consist of at least four different BChl a species: BChl 800, BChl 850, BChl 875, and BChl 896. Upon excitation of the highest-energy pigment, a transfer sequence towards lower energy is initiated. The transfer steps between the different pigment pools are characterized by the following time constants: BChl 800 → BChl 850, т = 2 ± 1 ps ; BChl 850 → BChl 875, т = 40 ± 5 ps ; BChl 875 → BChl 896, т = 20 ± 5 ps . Once the excitations are localized on B896 a slower quenching, т = 190 ± 10 ps , by closed reaction centers (P +) occurs. From measurements of decay of induced anisotropy it is further concluded that efficient transfer between BChl 800 molecules takes place on a time-scale comparable to the BChl 800 → BChl 850 transfer. A marked increase in anisotropy in the red wing of the absorption spectrum offers a clear evidence of the presence of the long-wavelength antenna component B896. The BChl antenna of R. rubrum is composed of two BChl a species, BChl 880, and BChl 896, and the energy transfer kinetics are observed to be very similar to the corresponding part of Rb. sphaeroides. Some evidence of further spectral inhomogeneity (apart from B896) or spectral shifts induced by excitation of the B880 antenna pigment was also obtained. Several possible models are discussed for the origin and organization of the B896 pigment.