Title

NMR Spectra of Porphyrins. 28. Detailed Solution Structure of a Bacteriochlorophyllide d Dimer

Document Type

Article

Publication Date

1-1-1986

Abstract

Methyl bacteriochlorophyllide d [Et, Et] in chloroform solution forms a stable dimer which has a sufficiently long lifetime that separate NMR signals are observed from the spectroscopically nonequivalent components of the dimer. The stable dimeric struture is broken down by addition of competitive ligands such as pyridine or methanol. Extensive titration experiments (with methanol-d4), saturation transfer and nuclear Overhauser enhancement difference experiments at 500 MHz, and specific deuterium labeling of the C-10 and -5a methylenes and of the C-7 propionic side chain enable a complete assignment of the dimer spectrum. The resulting large set of complexation shifts (δdimer - δmonomer) allows a precise characterization of the dimer structure with a previously described ring-current model. Two possible structures are shown to be computationally compatible with the observed shifts. One model is a face-to-face structure, whereas the other has a “piggy-back” arrangement of the two molecules. In both structures the separation of the chlorin molecules is ca. 3.5 Å, and the hydroxyl of the C-2 1-hydroxyethyl substituent is coordinated with the magnesium atom of the adjacent molecule in the dimeric pair. Detailed analysis of the observed complexation shifts of the side-chain protons permits a possible differentiation in favor of the piggy-back structure in which an internal hydrogen bond may exist between one of the coordinating hydroxyl groups and the carbonyl of the companion C-7 propionate. Both of the proposed structures bear similarities to the X-ray defined reaction center special pair in Rhodopseudomonas viridis, but significant differences are also apparent and are discussed. Finally, a similar dimer is also shown to exist in the bacteriochlorophyll c series. © 1986, American Chemical Society. All rights reserved.

Publication Source (Journal or Book title)

Journal of the American Chemical Society

First Page

1111

Last Page

1120

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