Low-frequency (50-425-cm(-1)), near-infrared-excitation resonance Raman (RR) spectra are reported for bacterial photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides in which the bacteriochlorophyll (BChl) and bacteriopheophytin (BPh) cofactors are labeled with N-15 or Mg-26. The focus of the study is the identification of the very low-frequency modes of the dimer of BChls (P) which are strongly coupled to the P* electronic transition which initiates the primary charge separation process in RCs. In order to gain a complete picture of the vibrational characteristics, the low-frequency RR spectra of the accessory BChls and the BPhs were examined in addition to those of P. The RR spectra of the isotopically labeled cofactors in the RCs were compared with one another and with the spectra obtained for solid-film samples of isolated, isotopically labeled BChl and BPh. Based on these comparisons and the predictions of semiempirical normal coordinate calculations, a self-consistent set of assignments has been developed for all the RR active modes of the different BChl and BPh cofactors in the RC which are observed in the very low-frequency regime (50-250 cm(-1)). The assignments indicate that the strongly coupled, low-frequency modes of Pall involve either deformations localized on pyrrole ring I or the macrocycle core. The so-called "marker mode" of P, observed near 135 cm(-1), is due to a cluster of three modes, specifically, the in-plane deformation of the C-2-acetyl group (130 cm(-1)), the doming motion of the Mg(II) ion (137 cm(-1)), and a core deformation that involves all four pyrrole rings (143 cm(-1)). The calculations further suggest that the very strongly; coupled mode observed near 35 cm(-1) is due to the out-of-plane deformation of the C-2-acetyl group. The strong coupling of these modes is consistent with the structure of the dimer in which overlap occurs primarily in the region of ring I. This geometrical arrangement of the cofactors also places the C-2-acetyl substituents of one constituent of P in close proximity to the core of the macrocycle of the other. The unique interplay between the structural, electronic, and vibronic characteristics of the primary electron donor suggests that the strong coupling of certain vibrations is an intrinsic consequence of the structure of the dimer and may have important functional ramifications.