H-1 AND C-13 NMR-SPECTRA OF THE METHANOLIC ALLOMERIZATION PRODUCTS OF13(2)(R)-CHLOROPHYLL-ALPHA

The spontaneous autoxidation (allomerization) of 13(2)(R)-chlorophyll a in methanol produces seven main products containing three epimer pai rs. The H-1 and C-13 NMR spectra of these products in acetone-d(6) wer e recorded on a 500 MHz spectrometer and fully assigned using two-dime nsional HMQC and HMBC techniques. The absolute configurations of the o xidized carbons, originally C-13(2), were determined using the ROESY t echnique, In comparison with 13(2)(R)-chlorophyll a, the steric repuls ion between the C-17 side-chain and the bulky substituents of the oxid ized C-13(2) causes steric strain in the chiral part of an allomer, re lieved by conformational changes in rings D and E and to a lesser exte nt also in the whole macrocycle. These changes were estimated for each allomer from the Delta delta values of the carbon and proton resonanc es and from proton-proton coupling constants. Information about the or ientation of the front part of the phytyl chain in 13(2)(R)-chlorophyl l a and its methanolic allomers was obtained by analysing the variatio n in the form of the P1-CH2 signal.