Conformation of bacteriochlorophyll molecules in photosynthetic proteins from purple bacteria

Fourier transform near-infrared resonance Raman spectroscopy can be used to obtain information on the bacteriochlorophyll a (BChl a) molecules respons ible for the redmost absorption band in photosynthetic complexes from purpl e bacteria. This technique is able to distinguish distortions of the bacter iochlorin macrocycle as small as 0.02 Angstrom, and a systematic analysis o f those vibrational modes sensitive to BChl a macrocycle conformational cha nges was recently published [Naveke et al. (1997) J. Raman Spectrosc. 28, 5 99-604]. The conformation of the two BChl a molecules constituting the prim ary electron donor in bacterial reaction centers, and of the 850 and 880 nm -absorbing BChl a molecules in the light-harvesting LH2 and LH1 proteins, h as been investigated using this technique. From this study it can be conclu ded that both BChl a molecules of the primary electron donor in the photoch emical reaction center are in a conformation close to the relaxed conformat ion observed for pentacoordinate BChl a in diethyl ether. In contrast, the BChl a molecules responsible for the long-wavelength absorption transition in both LH1 and LH2 antenna complexes are considerably distorted, and furth ermore there are noticeable differences between the conformations of the BC hl molecules bound to the alpha- and beta-apoproteins. The molecular confor mations of the pigments are very similar in all the antenna complexes inves tigated.