Natural abundance nitrogen-15 nuclear magnetic resonance spectroscopy. Medium effects on the nitrogen-15 chemical shifts of small peptides

Abstract

Medium effects upon the ¹⁵N chemical shifts of some small peptides are reported, to allow the relative importance of solvent, pH, and sequence effects to be delineated. A change in the pH of a solution of carnosine (β-alanylhistidine) from 0.4 to 11.0 causes the His ¹⁵N resonance to shift downfield by 8.5 p.p.m. Changing the solvent from dimethyl sulphoxide (DMSO) to trifluoroacetic acid (TFA) causes downfield shifts of ca. 4 p.p.m. for the peptide nitrogen resonances of some N-acetyldipeptides, and explains why the ¹⁵N shift observed for polyglycine in TFA solution is to low field of that predicted earlier for a glycine residue of a peptide in DMSO solution. The magnitude of these pH and solvent effects is such that they may mask any sequence effects upon peptide ¹⁵N shifts, which are reported to be 1–4 p.p.m. for dipeptides in aqueous solution.