CHARACTERIZATION OF THE LIGHT-HARVESTING ANTENNAS OF PHOTOSYNTHETIC PURPLE BACTERIA BY STARK SPECTROSCOPY .2. LH2 COMPLEXES - INFLUENCE OF THE PROTEIN ENVIRONMENT

We have performed low-temperature Stark spectroscopy on a variety of d ifferent LH2 complexes from four photosynthetic bacteria, with the aim of characterizing the electric field response of the B800 and B850 ab sorption properties as a function of the protein environment. The foll owing LH2 complexes were investigated: B800-850 and B800-820 of Rhodop seudomonas (Rps) acidophila; B800-850, B800-840 (alpha Tyr(+13)-->Phe) , and B800-826 (alpha Tyr(+13)-->Phe, alpha Tyr(+14)-->Leu) of Rhodoba cter (Rb.) sphaeroides; B800-850 and B800-830 (obtained at high LDAO) of Ectothiorhodospira sp.; and B800-850 of Rhodospirillum (Rsp.) molis chianum. For all these cases the spectral blue shift of B850 has been assigned to the loss hydrogen-bonding interaction with the acetyl carb onyl of bacteriochlorophyll a. \Delta mu\ values for the 850 nm bands as well as for the blue-shifted bands are all on the order of 3-4.5 D/ f. The loss of hydrogen-bonding interactions has only small effects on \Delta mu\ in these complexes. The values of the difference polarizab ility, Tr(Delta alpha), are large (600-1400 Angstrom(3)/f(2)). The res ults are discussed in terms of crystal-structure-based models for LH2, in which pigment-pigment and pigment-protein interactions are conside red; strong pigment-pigment interactions were found to be especially i mportant. The values of \Delta mu\ for the 800 nm band are small, 1.0- 1.5 D/f for LH2 complexes from Rb. sphaeroides and Rps. acidophila. Ho wever, in Rsp. molischianum and Ectothiorhodospira sp. \Delta mu\ valu es are much larger, of the order of 3 D/f. The difference in the B800 band is assigned to the difference in orientation of the B800 pigments in Rsp. molischianum and Ectothiorhodospira sp., as compared to the R ps. acidophila and Rb. sphaeroides. Due to the difference in orientati on, the interactions of the Bchl a with the surrounding protein and ne ighboring carotenoid pigments are also not identical.