Seminars Archive

Hydrogen bond rearrangements in formamide-water mixtures probed by extended depolarized light scattering (EDLS)

Abstract
Liquid formamide (FA) is a structured system characterized by the presence of an extended hydrogen bonding network which show similarities with that of liquid water [1]. The molecular dynamics in water has been recently subjected to numerous theoretical and experimental investigations leading to a consistent description of the molecular mechanism of hydrogen bond reorganization [2,3]. In this context, the study of FA aqueous solutions may provide further fundamental insights on the molecular restructuring taking place in associating liquid mixtures. Moreover, beside its relevance as common solvent, FA (HCONH2) contains a peptide linkage and FA-water solutions are suitable model for studying complex biorelevant processes (i.e. protein hydration, folding and aggregation), strongly influenced by the formation and competition of H-bonds involving H2O molecules, C=O and N-H groups [4]. In the present work the molecular dynamics of FA-water mixtures is investigated at different time scales by means of depolarized light scattering experiments, which probe the relaxation of the total anisotropic polarizability of the system. The dynamical susceptibility is measured over a broad frequency range (0.01-1000 cm^-1) trough the combined use of dispersive and interferometric instruments. This approach, referred to as extended frequency range depolarized light scattering (EDLS), was proven suitable for the study of the molecular mobility in aqueous solutions of biorelevant systems at time scales ranging from fractions to hundreds of picoseconds (10^-12 s) [5-7]. The EDLS technique will be introduced and the results obtained for FA-water solutions in the whole concentration range and at different temperatures will be presented and discussed in comparison with findings derived by molecular dynamics simulations [4] and other spectroscopic techniques. In particular, both the relaxation dynamics (tens of picosecond) and the intermolecular resonant modes (fractions of picosecond) are analyzed and interpreted in connection with the organization of the H-bonding network formed by the two species. Finally, the possibility of using complementary approaches based on line-shape analysis of intramolecular bands in the Raman spectrum, successfully employed for the study of related H-bonding systems [8], will be also considered. REFERENCES [1] Jadzyn,J.,Swiergiel, J., Phys. Chem. Chem. Phys. 14, 3170-3175 (2012). [2] Laage, D., Hynes, J., Science 311, 832-835 (2010). [3] Nicodemus, R. A., Ramasesha, K., Roberts, S. T., Tokmakoff, A., J. Phys. Chem. Lett. 1, 1068-1072 (2010). [4] Elola, M. D., Ladanyi, B. M., J. Chem. Phys. 126, 084504 (2007). [5] Perticaroli, S., Comez, L., Paolantoni, M., Sassi, P., Morresi, A., Fioretto, D. J. Am. Chem. Soc. 133, 12063-12068 (2011). [6] Lupi, L., Comez, L., Paolantoni, M., Fioretto, D., Ladanyi, B. M. J. Phys. Chem. B 116, 7499-7508 (2012). [7] Comez, L., Lupi, L., Morresi, A., Paolantoni, M., Sassi, P., Fioretto, D. J. Phys. Chem. Lett. 4, 1188-1192 (2013). [8] D’amico, F., Bencivenga, F., Gessini, A., Principi, E., Cucini, R., Masciovecchio, C. J. Phys. Chem. B 116, 13219-13227 (2013).