Proton nuclear magnetic resonance relaxation rates in aqueous solutions of amino acids

Variations of water-proton NMR relaxation rates induced by amino acids are very different for longitudinal and transverse relaxations. The presence of amino acids increases the longitudinal relaxation rate only slightly, when compared with the increase induced by the oxygen content of the solution. The measured transverse relaxation rate is dependent on the pulse delay of a Carr-Purcell-Meiboom-Gill sequence. Transverse proton relaxation dispersions are shown to be consistent with rapid proton exchange between water and the NH + 3 groups of the amino acids. The mean exchange time is about 1 ms. The variation of the exchange time with temperature yields the enthalpy and the entropy of the apparent first-order reaction of this exchange. Changes of pH, temperature and oxygen content give rise to opposite effects on relaxation rates of amino acid solutions. The discussion here underlines the fact that, even in such a simple system, relaxation-rate measurements give valuable information only if the physico-chemical conditions are precisely defined.