Density functional theory calculations of the structure and the 15N and 13C chemical shifts of methyl bacteriopheophorbide a and bacteriochlorophyll a

Ab initio (DFT) chemical shifts calculations and geometry optimizations were performed in methyl bacteriopheophorbide a (MBPheo-a) using the D95 and D95** basis sets and in bacteriochlorophyll a (BChl-a) using the LanL2DZ basis set. The results show that it is possible to calculate chemical shifts in large systems, such as MBPheo-a and BChl-a, and that the calculated chemical shifts are quite sensitive to the geometries used in their calculation. Calculations of the chemical shifts using molecular geometries in which at least the positions of the protons have been optimized produce results in much better agreement with the experimental values than those using the X-ray experimental structure. The results for C-13 shifts are of such quality that they can be used to verify the assignments of NMR resonances separated by more than 5 ppm. The calculations presented here strongly suggest that several assignments of the C-13 resonances of the pyrrolic ring carbons in MBPheo-a and BChl-a should be revised.