DYNAMICS OF VARIOUS METAL-OCTAETHYLPORPHYRINS IN SOLUTION STUDIED BY RESONANCE RAMAN AND LOW-TEMPERATURE OPTICAL-ABSORPTION SPECTROSCOPIES - ROLE OF THE CENTRAL METAL

The temperature dependence of the Sorer band of various metal-octaethy lporphyrins [i.e., Ni(II), Cu(II), and Pd(II)] dissolved in the glass- forming mixture 50% (v/v) isopentane/ethyl ether in the temperature ra nge 300-40 K was studied. Co(II)-octaethylporphyrin dissolved in dichl oromethane was also investigated in the temperature range 300-180 K. T he aim of the work was to investigate the role of the central metal in the conformational flexibility of porphyrins and in communicating sol vent motions to their macrocycle. We used resonance Raman spectroscopy to determine the vibronic coupling of high-frequency modes to the ele ctronic transition into the porphyrin B state. The corresponding coupl ing of a ''bath'' of low-frequency porphyrin/solvent motions was taken into account by a temperature-dependent Gaussian width (sigma) of the Soret band. Following the approach of Melchers et al. (Biophys. J. 19 96, 70, 2092-2099), the mean square fluctuations (MSF) of the central metal atom with respect to the porphyrin plane were determined from th e sigma temperature dependence. MSF values exhibited a harmonic behavi or only at low temperatures, whereas an increase of the MSF well above the predictions of the harmonic model was observed above the glass tr ansition temperature of the solvent mixture. This result was rationali zed by invoking coupling of solvent motions to the vibrations of the c entral metal atom. The magnitude of this coupling is strongly metal de pendent in that it is large for Ni(II) and Pd(II), weak for Co(II), an d almost negligible for Cu(II). This dependence is discussed in terms of the ionic radii and electronic structure of the various metals. Mor eover, for Ni(II), we found the MSF to have an amplitude comparable wi th what has earlier been obtained for iron-heme complexes in proteins and in solution.