Oxygen-17 n.m.r. of peptides

Peptide-17O chemical shifts of linear dipeptides with and without protecting groups in H2O, CH3OH, CH2Cl2, CHCl3, CCl4, CH3CN and DMSO were between 256-350 ppm downfield from external water. Increasing solvent H-bond donating ability correlated with shifts to higher field. The 17O resonance of several cyclic dipeptides appeared at higher field relative to comparable linear dipeptides (303-317 p.p.m. vs. 327-337 p.p.m.). Separate signals were simultaneously observed by 13C and 17O n.m.r. for cis and trans N-tert.-butyl-formamide in binary mixtures with H2O, (CH3)2CO, and CCl4. The differences in the 17O nuclear screening of the amide isomers and most probably for cis and trans peptides were independent of contributions from H-bonding at the amide or peptide linkage, apparently reflecting differences between geometric isomers in electron distribution and through space effects. Peptide-17O of Gly-Ala, Gly-Leu and Gly-Glu in aqueous solution experienced upfield shifts of 6-12 p.p.m. and 12-16 p.p.m. upon deprotonation of the C-terminal COOH and of the N-terminal NH3+ groups respectively. These observations were rationalized in terms of the attendant changes in substituent effects, especially on the pi electron donating ability of the N atom at the peptide linkage and increased partial negative charge on the peptide oxygen. Temperature studies of peptide-17O of Gly-Ala between pH 1.5-9.0 revealed a chemical shift coefficient of 0.08 p.p.m./degree K and similar behavior of T1 and T2 relaxation times. Ea for molecular rotation was 5 kcal/mol between 301-331 degrees K. Rotational correlation times, tau c, were within the range expected from the Stokes-Einstein relation.