The environment of the four nitrogen atoms of bacteriochlorophyll a in solutions as revealed by 15N NMR spectroscopy

The 50 MHz 15N NMR spectra of 15N-enriched bacteriochlorophyll a were recorded in twelve different solvents. The 15N signals of the four nitrogens in the tetrapyrrole rings I, II, III and IV (signals I, II, III and IV) were assigned by the use of correlations with those of the α, β and δ methine 1H. Signals I and III appeared in the higher field, while signals II and IV appeared in the lower field; the difference in the chemical shifts within each pair of signals was small. The results indicate an approximate D2h symmetry of the nitrogen environment, with higher electronic density on the ring I and ring III nitrogens.