Fourier transform near-infrared resonance Raman spectroscopy can be used to obtain information on the bacteriochlorophyll a (BChl a) molecules responsible for the redmost absorption band in photosynthetic complexes from purple bacteria. This technique is able to distinguish distortions of the bacteriochlorin macrocycle as small as 0.02 A, and a systematic analysis of those vibrational modes sensitive to BChl a macrocycle conformational changes was recently published [Näveke et al. (1997) J. Raman Spectrosc. 28, 599-604]. The conformation of the two BChl a molecules constituting the primary 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, has been investigated using this technique. From this study it can be concluded that both BChl a molecules of the primary electron donor in the photochemical reaction center are in a conformation close to the relaxed conformation 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 furthermore there are noticeable differences between the conformations of the BChl molecules bound to the alpha- and beta-apoproteins. The molecular conformations of the pigments are very similar in all the antenna complexes investigated.