Co-ordination and aggregation of bacteriochlorophyll a : an n.m.r. and electronic absorption study

Abstract

A detailed analysis is presented of the aggregation and co-ordination equilibria available to bacteriochlorophyll a, and n.m.r. spectroscopic titration experiments are proposed for their investigation. The results show that bacteriochlorophyll a is dimeric in wet benzene, five-co-ordinate monomeric in acetone or ether solution with one bound solvent molecule, and six-co-ordinate monomeric in pyridine or methanol solution with two bound solvent molecules. The second derivatives of the electronic absorption spectra show separate resolved peaks for five- and six-co-ordinate species near 575 and 600 nm respectively.