Dynamics of amino acid water solutions as studied by dielectric spectroscopy

The role of water as the biological solvent has been subject of intense research over the last few decades. In particular, the coupling between water and protein dynamics, and its influence on protein structure stabilization, function and catalytic processes has been investigated by means of a variety of experimental and theoretical techniques. Dielectric Relaxation Spectroscopy (DRS) is a powerful tool for studying protein dynamics in aqueous media, due to its sensitivity for detecting fluctuations of the dipole moments, and to the large frequency window it can span. However, the intricate relation between dielectric magnitudes and microscopic properties, and the complexity of typical proteinsolution spectra, may obscure the analysis of the results. Therefore, we have studied at first the dielectric response of water solutions of single amino acids (AA), the constituents of proteins, and that of small peptides, for progressively consider more complex systems. For this Master Thesis, we have focused on ten different AA, and a dipeptide (L-carnosine), composed of two of the former molecules. In a first part, we have analyzed the dielectric spectra obtained in the GHz – region at room temperature, where we have found that dipole-dipole correlations among AA molecules are negligible, even in concentrated solutions. Moreover, we have shown that DRS is well suited for studying molecular solvation, an issue of great importance in order to better understand interactions present in biological systems. We have also compared the relaxation times of the different AA, finding interesting conclusions with regard to their structure and hydrodynamic coupling. Secondly, the case of the highly soluble AA Lproline in the GHz – region has been analyzed. Here, the effect of the hydration water (at the interface of the AA) is noticeable: a strong increase in the water relaxation time and a progressively more heterogeneous environment for water molecules is observed for low water content solutions (below 75 wt %). Finally, some preliminary results on the dynamics of concentrated L-lysine solutions at low temperatures (below the glasstransition Tg), and lower frequencies, are given. This last study was possible since these solutions do not crystallize on cooling.I would like to acknowledge the support of the University of the Basque Country, giving me the opportunity of carrying out this projectPeer reviewe