SOLUTION STRUCTURES OF 13(2)-METHOXYCHLOROPHYLL A EPIMERS

The absolute configurations at C-13(2) of 13(2)-methoxychlorophyll a ( 13(2)-MeO-Chl a) epimers, synthetic derivatives of Chl a, were determi ned by the 2D ROESY NMR technique. The ROESY spectra and the vicinal c oupling of the C-17 and C-18 protons also provided information on the conformation of ring D which was found to be more distorted in both Me O-Chl a epimers compared with 13(2)(R)-Chl a due to increased steric c rowding between the C-13(2) and C-17 substituents. This conclusion was supported by the conformational analysis of the C-17 propionic ester side-chain. The populations of the side-chain rotamers were calculated from vicinal proton coupling constants, obtained from spectral simula tions of the 17(1)- and 17(2)-CH2 proton multiplets. For the R and S e pimers of 13(2)-MeO-Chl a, the most populated conformers were found to be: ap 1a (populations 0.7 (R), 0.5 (S)) and g(+) 1b (0.3 (R), 0.5 (S )) of the C-171-C-17 bond rotamers, and ap 2a (0.88 (R), 0.82 (S)) of the C-172-C-171 bond rotamers (Fig. 5). In comparison, for 13(2)(R)-ph eophytin a, the previously found most populated conformers are: ap 1a (0.7) and g(+) 1b (0.3) of the C-171-C-17 bond rotamers, and ap 2a (0. 63) of the C-172-C-171 bond rotamers. The conformers of the ester frag ment and the front part of the phytyl chain in the Chl derivatives are discussed.