Confined water and protein dynamics
Water plays a major role in biosystems, greatly contributing to determine their structure, stability, and function. A “special” role is played by the hydration water shells (Fig. 1a). These water molecules display physico-chemical properties that are apparently different from molecules in pure bulk water. For instance, they are prevented from crystallising just below 0 °C because the competition of their mutual interaction and their hydrogen bonding to the protein makes it difficult for them to arrange into the typical tetrahedral ice structure. Remarkably, proteins’ denaturation restores water ability to crystallise (Fig. 1b). In this context, we have shown for the first time, by means of high-resolution elastic neutron scattering, a de-coupling between protein and its hydration water dynamics (Fig. 2). More in general, we are interest in the dynamical behaviour of water molecules confined in both biological and non biological matrixes. Neutron scattering and computer simulations are the two major approaches used in this research line.
Fig. 1 - Model protein lysozyme and its hydration water. (a) Crystal structure of human lysozyme (green) together with (∼280) hydration water molecules obtained by X-ray diffraction at 1.8 Å of spatial resolution. (b) Differential scan calorimetric heating profiles before (blue) and after (red) thermal denaturation (blue peak). The hydration water due to the interaction with the protein is not able to form ice, but after the thermal denaturation, due to a different state of the protein, part of the hydration water molecules recover their ability to form ice (red peak).
Fig. 2 - High-resolution elastic neutron data showing the decoupling between the dynamics of protein and of its hydration water. Total elastic neutron-scattered intensity versus temperature for D2O hydrated lysozyme (green), H2O hydrated lysozyme (blue), and dry lysozyme (red). The D2O signal from a protein hydrated in D2O is negligible, and relaxations in the elastic spectra can be related to the protein itself, which also contains a high number of hydrogen atoms. In this way it is possible to probe the relaxation dynamics of protein alone, yet hydrated. In the case of a protein hydrated in H2O, the contributions of water and protein have very similar weights in the signal (i.e., the densities of hydrogen atoms in the protein and in hydration water are nearly equivalent). As a result, by measuring the protein hydrated in H2O, relaxations in the elastic spectra arise from relaxations of either the protein or its hydration water. From this set of data it was possible to determine the that the protein starts to relax at T_p = 195 K, decoupled from the transition in the dynamics of hydration water, which only starts to relax 16 K lower, that is, T_hw = 179 K.
Selected publications:
Low-Temperature Decoupling of Water and Protein Dynamics Measured by Neutron Scattering
A. Benedetto, The Journal of Physical Chemistry Letters, 8, 4883, (2017)
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A. Benedetto, The Journal of Physical Chemistry Letters, 8, 4883, (2017)
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Hydrogen-Bond Dynamics at the Bio-Water Interface in Hydrated Proteins: a Molecular Dynamics Study
P. Nandi, N. English, Z. Futera, and A. Benedetto, Physical Chemistry Chemical Physics, 19, 318, (2017)
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P. Nandi, N. English, Z. Futera, and A. Benedetto, Physical Chemistry Chemical Physics, 19, 318, (2017)
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