N.m.r. Spectra of porphyrins. Part 35. An examination of the proposed models of the chlorophyll a dimer

Abstract

The n.m.r. complexation shifts at 500 MHz of the protons of chlorophyll a have been obtained from titration experiments with [²H₄]methanol and chlorophyll a dissolved in CDCl₃ at such concentrations (3mM) that only the chlorophyll dimer is present. This more accurate and more complete set of complexation shifts than obtained previously has been used to examine critically proposed models of the chlorophyll a dimer, using the double-dipole model of the chlorophyll ring current to calculate the dimer shifts. Proposals based on the crystal structure of ethyl chlorophyllide-a dihydrate (the Strouse model), on exciton-theoretical and infrared data (the Shipman model), and the various skew structures, do not lead to any agreement with the observed complexation shifts. The face-to-face proposal (the Fong model) gives the correct overall pattern for the complexation shifts, but not quantitative agreement. The unsymmetrical head-to-tail (‘piggy-back’) structure does give quantitative agreement with the observed shifts for all the protons measured. These last two models appear to involve co-ordination of the C-10 methoxycarbonyl CO with the adjoining magnesium atom, which is not consistent with observations on the pyrochlorophyllide series, in which similar complexation shifts are observed. A new structure which both quantitatively fits the observed complexation shifts and which appears to satisfy these other observations is proposed. This is a back-to-back structure in which both the C-10 methoxycarbonyl groups in the dimer are exo to the dimer structure. The arrangement is not symmetric and the two chlorophyll molecules are in very different environments. In one molecule the C-9 CO is near to the neighbouring magnesium atom and could be co-ordinated to the magnesium via a bridging water molecule. In the other molecule the C-7 propionic CO is situated near to the neighbouring magnesium atom and could also be co-ordinated to it. The C-10 methoxycarbonyl groups are not involved in the dimer binding interaction.