17O NMR chemical shifts of the twenty protein amino acids in aqueous solution

Publisher Website
Google Scholar
Abstract
The 17O chemical shifts of the α‐carboxyl groups of the protein amino acids, including 4‐hydroxyproline, sarcosine and N,N‐dimethylglycine, were measured at 40°C in aqueous solution at variable pH (17O enrichment 10 atom‐%). The chemical shifts of the amino acids were found to be between 250 and 257 ppm at low pH and between 266 and 273 ppm at neutral pH. For the purpose of comparison, several aliphatic and halogen‐containing carboxylic acids have also been measured under identical conditions. The high‐frequency titration shifts observed on deprotonation of the α‐carboxyl oxygens of the amino acids (15‐17 ppm) are similar to those of α‐haloalkanoic acids. A linear correlation of the titration shifts and the pKa values was found for the carboxylic acids, including the amino acids. The differences in the chemical shifts of the amino acids in either the cationic or zwitterionic state can be explained by the substituent effects of the side‐chains, introducing γ shielding and δ deshielding values of approximately half the size of those for aldehydes and ketones. Deprotonation of the primary α‐amino group of the amino acids induced low‐frequency shifts between ‐2.3 and ‐3.9 ppm, with the exception of serine and threonine (‐1.3 and ‐1.7 ppm, respectively). On deprotonation of the secondary α‐amino group of proline, 4‐hydroxyproline and sarcosine, a decrease in the titration shifts was observed (‐0.9, ‐0.2 and ‐1.1 ppm, respectively) and on deprotonation of the tertiary α‐amino group of N,N‐dimethylglycine the sign was reversed (+3.9 ppm). The titration shifts can be explained by an inductive shift to high frequency, which is counterbalanced by an electric field shift of variable magnitude to low frequency.