%0 Journal Article
%T On the Hydration State of Amino Acids and Their Derivatives at Different Ionization States: A Comparative Multinuclear NMR and Crystallographic Investigation
%A Charalampos G. Pappas
%A Andreas G. Tzakos
%A Ioannis P. Gerothanassis
%J Journal of Amino Acids
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/565404
%X 2D, 13C, 14N, and 17O NMR and crystallographic data from the literature were critically evaluated in order to provide a coherent hydration model of amino acids and selected derivatives at different ionization states. 17O shielding variations, longitudinal relaxation times (T1) of 2D and 13C and line widths (¦¤¦Í1/2) of 14N and 17O, may be interpreted with the hypothesis that the cationic form of amino acids is more hydrated by 1 to 3 molecules of water than the zwitterionic form. Similar behaviour was also observed for N-acetylated derivatives of amino acids. An exhaustive search in crystal structure databases demonstrates the importance of six-membered hydrogen-bonded conjugated rings of both oxygens of the ¦Á-carboxylate group with a molecule of water in the vicinity. This type of hydrogen bond mode is absent in the case of the carboxylic groups. Moreover, a considerable number of structures was identified with the propensity to form intramolecular hydrogen bond both in the carboxylic acid (NH£¿O=C) and in the carboxylate ( £¿) ionization state. In the presence of bound molecules of water this interaction is significantly reduced in the case of the carboxylate group whereas it is statistically negligible in the carboxylic group. 1. Introduction Water plays a fundamental role in the conformation and activity of every biological macromolecule [1]. Peptide- and protein- hydration is the dominant factor in the stabilization of spatial molecular structure, in the process of protein folding by gating hydrophobic residues, and in the mechanisms of peptide and protein mediated reactions [1¨C4]. Water molecules, therefore, can be considered as an integral component of biomolecular systems with dynamic, functional, and structural roles [4¨C7]. Investigation of the structural and functional role of water molecules, bound to proteins and peptides, requires a sufficient understanding of the hydration process of their building blocks [1, 2]. The hydration of amino acids and their derivatives at a molecular level, therefore, is of great importance and has been extensively studied with X-ray crystallography [1, 3] and a variety of spectroscopic techniques including multinuclear magnetic resonance spectroscopy [2, 8¨C13], IR and Raman spectroscopy [14¨C16], ICR mass spectrometry [17], and laser ablation in combination with microwave spectroscopy [18]. We present here, for the first time in the international literature, a comparative investigation of literature 2D, 13C, 14N, and 17O NMR and crystallographic data in order to provide a coherent hydration model of amino acids and
%U http://www.hindawi.com/journals/jaa/2012/565404/