The water association band as a marker of hydrogen bonds in trehalose amorphous matrices

The relevant role played by residual water in modulating the dynamics and structure of a protein, a matrix and their coupling has been thoroughly studied in bioprotective amorphous saccharide matrices via experiments and simulations. In order to better characterize this residual water and the hydrogen bond structures in which it is involved, in this work infrared spectroscopy experiments are conducted on trehalose-water systems. The properties of water are inferred from the study of a peculiar infrared band, the water association band, which we exploited as a marker of the hydrogen bonds in which water is involved. Our aim was the identification of populations of water molecules, which give rise to the different components to which the water association band can be easily decomposed. The attribution of these components to families of water molecules is accomplished by studying the band behaviour with a suitable use of Hofmeister salts, known to have a structure-making or structure-breaking activity, and therefore able to modify the hydrogen bond network by enhancing or depressing the local order. The results allow ascribing, in almost all samples, five band components to either a chaotropic or kosmotropic environment, and further define two of them as bulk-like or ice-like water. The characterization of different components enables the use of this band as a tool to deepen the knowledge of other low-water hydrated matrices with a new approach. A differential analysis of peak frequencies and populations of the components in a bulky system, containing or not embedded components or interfaces (e.g. proteins, polymers, surfaces or even massive cosolutes), makes it possible to draw information on the properties of hydrogen bonds which are formed in the investigated systems.