High‐field proton NMR study of the liquid solution structure of monomeric bacteriochlorophyll a and chlorophyll a

The 500 MHz proton spectra of monomeric bacteriochlorophyll a and chlorophyll a were recorded and assigned. The proton chemical shifts and proton‐proton coupling constants were all determined except for the P‐5 to P‐15 phytyl side‐chain protons. The conformational structures of the reduced rings and associated side‐chains in these compounds were deduced from vicinal coupling constants. The reduced ring IV and the propionic ester side‐chain in bacteriochlorophyll a and chlorophyll a have identical conformations which, in addition, are very similar to those reported previously for methyl pheophorbide a and pheophytin a . The reduced ring IV of bacteriochlorophyll a , which is attached to the phytyl side‐chain, is more buckled than the other reduced ring (ring II).