CHARACTERIZATION OF THE STRONGLY COUPLED, LOW-FREQUENCY VIBRATIONAL-MODES OF THE SPECIAL PAIR OF PHOTOSYNTHETIC REACTION CENTERS VIA ISOTOPIC LABELING OF THE COFACTORS

Low-frequency (50-425-cm(-1)), near-infrared-excitation resonance Rama n (RR) spectra are reported for bacterial photosynthetic reaction cent ers (RCs) from Rhodobacter sphaeroides in which the bacteriochlorophyl l (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 sep aration process in RCs. In order to gain a complete picture of the vib rational characteristics, the low-frequency RR spectra of the accessor y BChls and the BPhs were examined in addition to those of P. The RR s pectra 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 compari sons and the predictions of semiempirical normal coordinate calculatio ns, a self-consistent set of assignments has been developed for all th e RR active modes of the different BChl and BPh cofactors in the RC wh ich are observed in the very low-frequency regime (50-250 cm(-1)). The assignments indicate that the strongly coupled, low-frequency modes o f 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-plan e 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 a ll four pyrrole rings (143 cm(-1)). The calculations further suggest t hat 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 coupl ing of these modes is consistent with the structure of the dimer in wh ich 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 macrocyc le of the other. The unique interplay between the structural, electron ic, and vibronic characteristics of the primary electron donor suggest s that the strong coupling of certain vibrations is an intrinsic conse quence of the structure of the dimer and may have important functional ramifications.