Natural-Abundance Oxygen-17 Magnetic Relaxation in Aqueous Solutions of Apolar Amino Acids and Glycine Peptides

The physicochemical properties of amino acids and oligo peptides in aqueous solutions have drawn much attention for several decades. These properties, closely related to the solute-water interaction, have been chiefly studied by thermodynamic methods. The importance of the hydration of amino acids is generally recognized in connection with the structure and function of proteins. It has been pointed out that there is a dynamic correspondence between the enzyme function and the dynamic structure of water. 1' The dynamic characteristics of the hydration of amino acid as a model compound of protein, therefore, is very important. Not enough such studies, however, have yet been carried out; moreover, there have been few attempts to relate the thermodynamic properties to the dynamic properties. In the investigation of such thermodynamic pro perties as the volumes,2-® heat capacities,4'6) and compressibilities® of aqueous solutions of amino acids or peptides, frequent attempts have been made to examine the linear correlation between such properties and the number of carbon atoms or peptide groups in the backbone.2-4' It is very difficult to compare the properties of amino acids with those of peptides, since these properties are plotted as a function of a variable with a different base. It is, therefore, necessary to use the same variable for the purpose of investigating consistently the hydration of amino acids and pep tides. Recently, it was shown that the hydration properties of sugars in water can be systematicall y explained by the dynamic hydration number (DHN), obtained by measuring the 17O relaxation rates of water in aqueous sugar solutions.7' In this paper we report the spinlattice relaxation times, Ti, of natural-abund ance 17O