Solution structures of 13 2-methoxychlorophyll a epimers

Abstract

The absolute configurations at C-13 2 of 13 2-methoxychlorophyll a (13 2-MeO-Chl a) epimers, synthetic derivatives of Chl a, were determined by the 2D ROESY NMR technique. The ROESY spectra and the vicinal coupling 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 MeO-Chl a epimers compared with 13 2( R)-Chl a due to increased steric crowding 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 simulations of the 17 1- and 17 2-CH 2 proton multiplets. For the R and S epimers 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 17 1 C 17 bond rotamers, and ap 2a (0.88 ( R), 0.82 ( S)) of the C 17 2 C 17 1 bond rotamers (Fig. 5). In comparison, for 13 2( R)-pheophytin a, the previously found most populated conformers are: ap 1a (0.7) and g + 1b (0.3) of the C 17 1 C 17 bond rotamers, and ap 2a (0.63) of the C 17 2 C 17 1 bond rotamers. The conformers of the ester fragment and the front part of the phytyl chain in the Chl derivatives are discussed.