Nitrogen-15 nuclear magnetic resonance of aliphatic tripeptides.

The 15N chemical shifts of eight aliphatic tripeptides have been measured at the natural-abundance level. For a given tripeptide, the resonances of the COOH-terminal and NH2-terminal amino acids can be identified by measurements at low or high pH. The shifts of the NH2-terminal amino acid nitrogens are essentially independent of the amino acids in the rest of the peptide. The shifts of the other nitrogens are characteristic of the amino acids themselves and of the immediately preceding amino acid toward the NH2 terminus. Non-terminal amide nitrogens have shifts of about 6 ppm upfield of COOH-terminal amide nitrogens at the isoelectric point of measurement. 15N chemical shifts appear to have considerable potential value for peptide sequencing.