¹H and ¹³C NMR spectra of the methanolic allomerization products of 13²(R)‐chlorophyll a

The spontaneous autoxidation (allomerization) of 13 2 ( R )‐chlorophyll a in methanol produces seven main products containing three epimer pairs. The 1 H and 13 C NMR spectra of these products in acetone‐ d 6 were recorded on a 500 MHz spectrometer and fully assigned using two‐dimensional HMQC and HMBC techniques. The absolute configurations of the oxidized carbons, originally C‐13 2 , were determined using the ROESY technique. In comparison with 13 2 ( R )‐chlorophyll a , the steric repulsion between the C‐17 side‐chain and the bulky substituents of the oxidized C‐13 2 causes steric strain in the chiral part of an allomer, relieved by conformational changes in rings D and E and to a lesser extent also in the whole macrocycle. These changes were estimated for each allomer from the Δδ values of the carbon and proton resonances and from proton‐proton coupling constants. Information about the orientation of the front part of the phytyl chain in 13 2 ( R )‐chlorophyll a and its methanolic allomers was obtained by analysing the variation in the form of the P1‐C H 2 signal.