Fourier transform (FT) preresonance Raman spectra of Mg-containing bacteriochlorophyll a (BChla) and the metal-substituted Zn-, Ni-, Cu- and Co-bactleriopheophytins a (BPhea) allowed the characterization of those vibrational modes sensitive to the nature of the central metal, through its effect on the core size of these molecules. Six of the Raman modes active in the excitation conditions used (1064 nm, in preresonance with the Q(y) transition of the metal-bacteriopheophytin molecules) are shown to be sensitive to the conformation of the dihydrophorbin ring. Comparison of the FT-Raman spectra of BChla under conditions where the central Mg ion is either five-or six-coordinate show that these modes may he used to distinguish modifications of the dihydrophorbin core size as small as 1 pm, and thus that they may be used for characterizing conformational strains on BChla molecules both in vitro and in vivo.