| Popis: |
Pentapeptides containing either l-methionine, l-proline, l-arginine, or l-lysine as central residue in the pattern glycylglycyl-X-glycylglycine have been studied at natural abundance by proton-decoupled 13C nuclear magnetic resonance spectroscopy. Resonances were assigned in most cases by comparison with the free amino acids and by detailed observation of change in chemical shift with pH over the range 1.09 to 12.47. The ratio of trans to cis isomers with respect to the -Gly-Pro- bond was approximately 9:1 as judged by the presence of resonance pairs for Cα, Cβ, Cγ, and Cδ of the pyrrolidine ring. Chemical shift assignments for the lysine peptide were made by comparison with the free amino acids l-lysine and l-δ-hydroxylysine and were confirmed by pH effects and spin-lattice relaxation times (T1). The effect of varying the nature of the central residue on the chemical shifts of the flanking glycine residues was small except for l-proline. Each pentapeptide resonance was categorized according to the degree of sensitivity to the state of protonation of terminal and side chain groups, and pK values at 26° for those groups were determined. The pK value for the e-amino group of the lysine peptide was 10.47. The pattern of grading of T1 values within each peptide backbone and the side chains of methionine, arginine, and lysine reflects substantial rotational freedom for the peripheral protonated carbon nuclei. The T1 values for the pyrrolidine ring reflect elements of rotational anisotropy in the trans form of the proline pentapeptide. This evidence suggests that the conformational flexibility of the pyrrolidine ring is sterically hindered by the adjacent glycine residue on the NH2-terminal side. |
