Comparison of 1H NMR chemical shifts of bovine and human insulins

Two-dimensional NMR spectra have been recorded for bovine insulin at low pH in acetonitrile/water mixtures. In this solvent, insulin exists predominantly as the monomeric form. The spectra have been assigned and the 1H NMR chemical shifts compared with those for human insulin recorded in the same solvent system, with bovine insulin measured in trifluoroethanol/water, and with two human despentapeptide insulins. These comparisons were made to determine whether sequence or solvent differences have a significant effect on the solution structure of the monomer. It was found that the chemical shifts of the backbone protons for the bovine and human insulins were very similar in acetonitrile/water, suggesting that in this solvent mixture there is little difference in the solution structures of the two sequence variants. Similarly, for bovine insulin the chemical shifts in a trifluoroethanol/water solvent mixture are comparable to those observed in acetonitrile/water. This suggests that solvent induces only minor changes in the local conformation in solution. Most of the differences in chemical shifts in the various samples occurred away from the three helical regions seen in the x-ray crystallographic studies of the insulin structure. The observation of slowly exchanging amide resonances in both trifluoroethanol/water and acetonitrile/water solvent mixtures confirmed the existence of helical regions in solution. Exchange of amide resonances was slower in trifluoroethanol/water, suggesting greater stability for the B chain helix in this solvent