ABSTRACT'5N nuclear magnetic resonance investigations of some amino acids were carried out in order to check the applicability of this method to biological problems. Because the natural abundance of the '5N isotope is not sufficient to get readable spectra in a reasonable time, 95% 15N isotope-enriched samples were used for the measurements. Besides ifie chemical shift values, the line widths, and the nuclear Overhauser enhancement factors, spin lattice relaxation times of the correspondent 15N resonances were measured as functions of pH and temperature.The functional groups that bear the 15N nucleus reveal characteristic differences in the measured uantities. The exchange behavior of protons attached to the AN with protons of the solvent water permits certain insights into the mechanism of these processes. The various contributions to spin lattice relaxation of the investigated '5N resonances are discussed. Although 15N nuclear magnetic resonance (NMR) spectroscopy appears to be very useful for investigating structures of nitrogen-containing molecules, the low natural abundance and unfavorable sensitivity of the 15N nucleus have limited its application (1). This is especially true with biologically relevant molecules, inasmuch as these substances are usually available only in fairly small quantities. Some progress may, however, be obtained by the enrichment of the 15N isotope to increase the signal-to-noise ratio of 15N NMR. Several authors have shown that 15N NMR in general and in particular of amino acid residues or other biologically important molecules may reveal data not only of molecular structure, i.e., values of the chemical shift or coupling constants, but also data of the dynamic behavior of the investigated molecule (2-6). The purpose of this paper is to report new results of 15N NMR work on various amino acids. Effects of chemical exchange, chemical shifts, coupling constants, line shapes, nuclear Overhauser enhancements, and nuclear relaxation rates of 15N-enriched glycine, lysine, glutamine, arginine, and proline have been measured as a function of pH and temperature.EXPERIMENTAL SECTION 15N-enriched (95%) glycine, glutamine (-CO15NH2), lysine (-a-15NH2), proline, and arginine [-NH-C-(15NH2)2] were purchased from Rohstoff Einfuhr GmbH., Dusseldorf, Germany.For all measurements the amino acids were dissolved in doubly distilled water; the pH value was adjusted with HCI and NaOH (reagent grade). In order to avoid paramagnetic impurities all solutions were shaken after the pH adjustment with Chelex 100 (Bio-Rad) according to the procedure of Irving and Lapidot (7). After removal of paramagnetic impurities the solution was transferred to an NMR sample tube (Wilmad, N.J.
