METAL-SUBSTITUTED BACTERIOCHLOROPHYLLS - 2 - CHANGES IN REDOX POTENTIALS AND ELECTRONIC-TRANSITION ENERGIES ARE DOMINATED BY INTRAMOLECULARELECTROSTATIC INTERACTIONS

Changes in the electronic transition energies and redox potentials bec ause of metal substitution in bacteriochlorophyll a justify the recent ly suggested correlation between electronegativity chi(M), covalent ra dius, and an effective charge, Q(M), at the metal atom center. A simpl e electrostatic theory in which Q(M) modifies the energies of the fron tier molecular orbitals by Coulombic interactions with the charge dens ities at the atomic pi centers is suggested. The relative change in el ectrostatic potential at a distance r(a) from the metal center is Delt a Q(M)/r(a), where Delta Q(M), the change in the metal effective posit ive charge because of Mg being substituted by another metal, varies wi th the change in metal electronegativity (Mulliken's values) Delta chi (M) and covalent radius Delta r(M)(c). Delta Q(M) consists of two comp onents: the major component, Delta Q(M)(0), characteristic of the cent ral metal, is independent of the molecular environment and proportiona l to the electronegativity of the metal at a typical valence state. Th e second component, Delta q(M,N) reflects those perturbations induced by the molecular frame. It depends on the overlap between the metal an d ligand orbitals hence changes both with the metal covalent radius (i .e., its typical ''size'') and the particular orbital environment. For the series of metals that we examined, we determined that Delta QM(0) = (0.12 +/- 0.02) Delta chi(M). Significant contributions of Delta q( M,N) to Delta Q(M,N) were found for the changes in the energies of the y-polarized electronic transitions B-y and Q(y) and to a lesser exten t the first oxidation potential E-Ox(1). Minor contributions were foun d for the changes in the energies of the x-polarized electronic transi tions B-x and Q(x) and the first reduction potential E-Red(1). The mod el agrees well with target testing factor analysis performed on the en tire data space. Simulations of the experimental redox potentials and the four electronic transitions required mixing of single-electron pro motions; however, the coefficients for the configuration interactions were assumed to be metal-independent within the examined series becaus e the relative oscillator strengths of the various transitions did not show significant changes upon metal substitution. The reported observ ations and the accompanying calculations provide experimental support to the modern concepts of electronegativity and may help in better und erstanding biological redox centers consisting of porphyrins or chloro phylls.