NMR Spectra of Porphyrins. 21. Applications of the Ring-Current Model to Porphyrin and Chlorophyll Aggregation

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A program incorporating the previously described double dipole model of the macrocyclic ring current in the porphyrin and chlorin ring is presented which allows the geometry of the aggregated species in solution to be obtained from observed NMR aggregation shifts. The model is capable of handling dimeric or oligomeric aggregates with either head-to-head or head-to-tail structures; also, one porphyrin may be rotated with respect to the other. Side chains may be rotated separately. The application of the model to a variety of previously recorded aggregates is presented. In the porphyrin series, the refined model allows a more rigorous search procedure than hitherto, and this shows that the structure of the dimers of zinc(II) meso-nitroocta-ethylporphyrin (1) and zinc(II) α,β-meso-dinitrooctaethylporphyrin (2) are more similar than was previously recognized. In both structures the coordination is between the zinc atom and the most electron-rich pyrrole subunit of the adjacent molecule. The aggregation shifts of zinc(II) 2-vinylphylloerythrin methyl ester (3) were analyzed and shown to agree with both a centrosymmetric and a twofold symmetric dimer structure. The latter allows maximum interactions between the electron-rich and electron-deficient rings, and is preferred. In neither case is there any evidence of interaction between the metal and side-chain donor groups. Application of the model to the published aggregation shifts of chlorophyll a (Chi a) (4) and chlorophyll b (Chi b) (5) provides the first quantitative explanation of these aggregation shifts and defines new structures for the aggregates. Although the aggregation shifts of Chi a agree reasonably well with the previously proposed face-to-face head-to-tail structure, they fit better for an unsymmetrical “piggy-back” structure in which the Chi molecules are rotated 180° to each other. In this structure, both the C-10 carbomethoxy group and the C-9 keto group are in proximity to the magnesium atoms of the adjacent molecules. The aggregation shifts of CH1 b are interpreted on the basis of an essentially head-to-head “piggy-back” structure in which the C-3 formyl group and the C-9/C-10 keto system are in proximity to the magnesium atoms of adjacent molecules. In both Chl a and Chl b, the lateral separation of the parallel Chi molecules is ca. 6 Å, which is too large for direct C=0 to magnesium bonding, and implies the possible presence of linking water molecules. Also, in both molecules, but more particularly in Chl b, the analysis provides evidence for the formation of higher aggregates than the dimer, which can be easily visualized in terms of the proposed structures. © 1983, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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