Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Chemical Shifts of Small Peptides as a Function of pH

Abstract

Carbon-13 spectra of 27 peptides have been obtained by high-resolution nmr spectroscopy with the aid of proton decoupling. The chemical shifts of the different amino acid moieties are found to have systematic deviations from those of the free zwitterionic amino acids. These effects depend very largely on the position of an amino acid in a peptide, i.e., on whether it is a C-terminal, an N-terminal, or a nonterminal unit, but not on the nature of its neighbors in the chain, unless one of these is proline. The results have substantial value for the determination of amino acid sequences in di- and tripeptides. Protonation and deprotonation of zwitterionic peptides have considerable effects on the chemical shifts which are, for the most part, limited to the amino acid units directly involved in the reaction. The one important exception to this rule that we have observed is interpreted as being the consequence of a conformational change which the peptides undergo when transformed from the zwitterion to the anion or cation.