THE PHYSICAL ORIGIN OF PROTEIN DYNAMICAL TRANSITION: A LIQUID-LIQUID TRANSITION IN HYDRATION WATER?

In this thesis I study, by means of neutron scattering, calorimetry, and dielectric spectroscopy, the physical origin of protein dynamical transition (PDT) which is usually observed at ~230 K in protein hydrated powders and is deemed necessary for protein function. Measurements reported in this thesis have been performed on hydrated powders of Myoglobin. The combined use of different experimental techniques gives a coherent description of the PDT and reveals a connection with a liquid-liquid crossover occurring in the protein hydration water at about the same temperature. In order to deepen our understanding of this connection and to obtain a direct experimental evidence of the existence of a liquid-liquid transition (LLT) in supercooled water at low temperatures, we investigated a second system, i.e. deeply cooled water confined within the pores of a 3-dimensional disordered SiO2 xerogel. In this system the hydrophilic surface of the matrix pores mimics the protein surface, while water confined within the pores mimics the protein hydration water. Using the same experimental techniques, we obtained evidence for the presence of a LLT, occurring at about 230 K, between a low density liquid (LDL) predominant at lower temperatures and a high density liquid predominant at higher temperatures. In conclusion, we suggest that the LLT in protein hydration shell is the physical origin of the biologically relevant protein dynamical transition.