N.m.r. spectra of porphyrins. Part 37. The structure of the methyl pyrochlorophyllide a dimer

Abstract

The n.m.r. complexation shifts of methyl pyrochlorophyllide a(MeP; 1) in CDCl₃ solution have been investigated and analysed. Titration experiments with C₅D₅N and CD₃OD at high dilution enabled both the complete assignment of the proton spectrum and the proton complexation shifts to be obtained. Considerable line broadening was observed in rigorously dried solutions of MeP in CDCl₃, which was removed by addition of water or other ligands. Water, however, did not fully dissociate the complex, in contrast with pyridine or methanol, probably due to the limited solubility of water in CDCl₃. The complexation shifts observed for MeP are almost identical with those of chlorophyll a in the same solvent, confirming the similar complexation behaviour of the two molecules used, and the unimportance of the C₁₀-CO₂Me group of chlorophyll a in chlorophyll complexation. The only significant differences in the complexation shifts are in ring ‘D’(8-H, 8-Me), perhaps due to conformational differences in the two molecules in this very flexible part of the molecule.

Detailed analysis of the observed complexation shifts with the ring-current model previously described was performed. Only two of the various proposed models of the chlorophyll dimer gave reasonable agreement with the observed shifts; these were the ‘piggy-back’ dimer and the ‘back-to-back’ model. The Fong model was in qualitative agreement with the observed shifts but none of the other proposed models gave even qualitative agreement. The geometries of the ‘piggy-back’ and ‘back-to-back’ models were refined using this set of complexation shifts which were more complete than any previous data. In both models the interplane separation is ca. 5.5–6.0 Å and also the C₉-keto groups of both molecules in the dimer are in the vicinity of the magnesium atom of the adjacent molecule. The proposed structures are consistent with a bonding mechanism in the dimer which involves a water molecule co-ordinated to the magnesium atom, and also hydrogen bonded to both the C₉-keto and C_7d-ester carbonyl functions of the adjacent molecule. In the ‘piggy-back’ structure only one of these hydrogen-bonding chains is possible, but in the ‘back-to-back’ model both C₇-propionate groups are endo to the dimer structure and capable, in principle, of forming these hydrogen-bonded chains. The larger aggregates could be formed by analogous layered structures in the ‘piggy-back’ model, but in the ‘back-to-back’ model the dimer structure differs from that of the aggregate in the position of the C₇-propionate groups. It was not possible to decide unambiguously between the two proposed structures with the experimental evidence available. Both provide a reasonable explanation for many of the previous results on chlorophyll aggregation.