Chloroplast biogenesis-47. Spectroscopic study of net spectral shifts induced by axial ligand coordination in metalated tetrapyrroles

Abstract

Net absorption and fluorescence spectral shifts, directly induced by coordination of metalated tetrapyrroles to axial ligands, were calculated for the Soret and visible regions of the electromagnetic spectrum. An examination of the calculated net spectral shifts confirmed the conclusions of several other investigators and revealed that the axial coordination potential of a metalated tetrapyrrole is strongly influenced (a) by functional group distribution around the periphery of the tetrapyrrole macrocycle; (b) by the temperature of the sample; (c) by the availability of adventitious ligands in the immediate environment of the metalated tetrapyrroles and (d) by the nature of the central metal atom of the metallotetrapyrrole. In general, electron withdrawing peripheral groups, low temperatures and the availability of unhindered Lewis bases all enhanced the formation of hexacoordinated complexes in Mg-tetrapyrroles. For example, in ether at room temperature, all Mg-tetrapyrroles coordinated to one axial ligand thus forming pentacoordinated complexes. At 77 K, all Mg-porphyrins with electron withdrawing side chains occurred mainly in the pentacoordinated state and to a much lesser extent in the hexacoordinated state. On the other hand in ether at 77 K, Mg-chlorins, such as monovinyl and divinyl chl(ide) a, coordinated to two axial ligands and occured predominantly in the hexacoordinated state. The relevance of these observations to the positive charge density on the central metal atom of metallotetrapyrroles and to the orientation and organization of Mg-tetrapyrroles in biological membranes is discussed. © 1984.