Carbon 13 nuclear magnetic resonance of pentapeptides of glycine containing central residues of aliphatic amino acids.

Pentapeptides containing either glycine, L-alanine, Lvaline, L-leucine, or L-isoleucine as central residue in the pattern glycylglycyl-X-glycylglycine have been studied by proton-decoupled natural abundance W Fourier transform nuclear magnetic resonance spectroscopy. Resonances were assigned by comparison with the free amino acids and by detailed observation of changes in chemical shifts with pH over the range 1.32 to 10.43. The effect of varying the central residue on the chemical shifts of the flanking glycine residues was established. Small effects ascribable to incipient aggregation were noted, both in terms of chemical shifts and relaxation properties. Each resonance was categorized according to the degree of sensitivity to the state of protonation of the terminal groups, and the pK values at 26” for those groups were determined from the pH dependence of the most sensitive resonances. The sensitive terminal resonances undergo broadening which is maximum near the PK. Spin-lattice relaxation times, T1, of protonated carbon nuclei were measured at two or more pH values in all cases except for the glycine pentapeptide. For each peptide C” resonances showed gradients in increasing T1 to each side from the central residue, indicative of contributions from segmental motion and internal rotation along the backbone in addition to the over-all rotational motion of the molecule. Internal rotational modes are expressed as well in the aliphatic side chains, especially by the most peripheral methyl groups for which spinning around the attaching bond is clearly prominent. Comparison of the anionic and cationic forms of the alanine, valine, and leucine peptides indicates that the rate of internal rotation of C” of the NHS-terminal glycine residues increases with deprotonation of the ammonium group.