CHLOROPHYLL-LIGAND INTERACTIONS FROM NUCLEAR MAGNETIC RESONANCE STUDIES

Chlorophyll-ligand coordination interactions, revealed by nmr spectra, have been studied by observing ring-current effects on proton chemical shifts of the ligands bound to chlorophyll. The use of deuteriochlorophyll simplifies interpretation of the nmr spectra. The equilibrium constant for the disaggregation of chlorophyll a dimers by methanol in CCl4 to form chlorophyll monosolvate is found to be K1 = 56 1. mol−1. Equilibria involving the disolvate are less important, K2 = 1.3 1. mol−1. Pentacoordinate magnesium(II) appears to dominate the equilibria. Data are given for the interaction of other aliphatic alcohols and deuteriochlorophylls a and b and deuteriobacteriochlorophyll. Solvent and temperature dependence were examined. The Mg-O bond distance deduced from ring-current considerations for alcohol-chlorophyll interactions in halocarbon solvents is 3.1 Å. © 1968, American Chemical Society. All rights reserved.