Oxygen-17 NMR relaxation times of the protein amino acids in aqueous solution: estimation of the relative hydration numbers in the cationic, anionic, and zwitterionic forms

Oxygen-17 NMR spectroscopy: Basic principles and applications (part II)

“…The values, therefore, of v for the nitrogen and oxygen nuclei in the amino acids change little from one compound to another [773]. Furthermore, the v( 17 O) of the amino acid is independent of both the ionization and the degree of hydration of the carboxyl group [772]. The increase in the 17 O linewidths at acidic pH ($100 ± 31 Hz), relative to those at neutral pH, was interpreted by a change in the rotational correlation time of the amino acids, Eq.…”

On the Hydration State of Amino Acids and Their Derivatives at Different Ionization States: A Comparative Multinuclear NMR and Crystallographic Investigation

“…The 17 O [ 10 , 11 ] and 14 N NMR [ 11 ] line widths of several protein amino acids were measured in aqueous solution to investigate the effect of molecular weight on the line widths ( Table 1 ). The 14 N and 17 O line widths, under composite proton decoupling, increase with the bulk of the amino acid, and increase at low pH.…”

“…The linear correlation between Δ ν 1/2 and MW at pH 6 for both 14 N and 17 O nuclei ( Figure 4 ) is in agreement with the hydrodynamic model of ( 3 ) [ 11 ]. Furthermore, the χ ( 17 O) of the amino acid is independent of both the ionization and the degree of hydration of the carboxyl group [ 10 ]. The increase in the 17 O line widths at acidic pH (~100 ± 31 Hz), relative to those at neutral pH, was interpreted by a change in the rotational correlation time and, thus, effective MW of the amino acids, ( 3 ).…”

“…The increase in the 17 O line widths at acidic pH (~100 ± 31 Hz), relative to those at neutral pH, was interpreted by a change in the rotational correlation time and, thus, effective MW of the amino acids, ( 3 ). This implies that the cationic form of the amino acids is more hydrated by an access of 1.3 to 2.5 molecules of water relative to that in the zwitterionic form [ 11 ] with lifetimes that are longer than the overall molecular rotational correlation time, presumably 2–10 ps [ 10 ].…”

Synthetic methodologies in organic chemistry involving incorporation of [17O] and [18O] isotopes

“…[ 17 O] enrichment of the amino acids was also obtained by the saponification of their esters with labeled hydroxide, derived from sodium metal or sodium methoxide and labeled water. While the carboxyl group does not exchange with water in alkaline solution, the corresponding ester, especially the O ‐methyl ester, is highly reactive toward alkaline hydrolysis.…”