Water – the Most Anomalous Liquid

Water and Protein Activity (Nuclear Magnetic Resonance and Infrared Spectroscopy Studies from the Protein Dynamical Crossover to the Irreversible Unfolding)

24 Oct 2014, 13:30

1h

FB52 (Nordita, Stockholm)

Speaker

Prof. Francesco Mallamace (Universita di Messina and CNR-IPCF, Massachusetts Institute of Technology, Boston University)

Description

The effect of water on lysozyme is studied in a very large temperature range from 180 to 370 K. By using in a comparative way the Nuclear Magnetic Resonance and the FTIR spectroscopy (the vibrational modes) we explore this protein system at different hydration level h (h=0.3, 0.37, 0.42). The hydration level h=0.3 is equivalent to a single monolayer of water around the globular protein. Our interest is focused to study the water role in the protein dynamical transition (glass transition or the transition from an harmonic solid like behavior to an anharmonic and liquid like motion) and the irreversible unfolding. We demonstrate also by considering previous neutron scattering experiments that the protein dynamical transition belongs to the universal class of dynamical crossover characterizing supercooled liquids and materials. By means of a detailed study of the bending vibrational mode of water and of the amide’s peptide (amide I, II and III) we were able to follow the dynamics of the complex hydrogen bond (HB) network formed between water and hydrophilic moieties of the protein. In particular the amide II Infrared region (1450 – 1580 cm-1) contains structural information about the protein conformation reflected in the bending mode of NH groups and in the stretching mode of CN groups. Both these groups are involved in the formation of hydrogen bonds, the NH in a direct way whereas the CN indirectly throughout the carbonyl oxygen, determining the water accessible regions. More precisely these bonds have different character, whereas one is proton donor the other is proton acceptor by linking hydrophilic groups of the same and/or different peptides. The thermal evolution of the spectral features regarding these two contributions allows identifying that the dynamical crossover observed for water coincides with that of the protein dynamical transition. We stress that we are able to demonstrate at a molecular level the interaction of water with the protein peptides and how via the HB it drives the protein activity. Furthermore, the combination of FTIR, Neutron Scattering and NMR data (under a novel interpretation) allows us to clarify some of the underlying mechanisms that govern the reversibility of the folding-unfolding and irreversible denaturation processes of the protein. In particular, new NMR observations at the temperature above and below the protein irreversible unfolding (TD) show that folding-unfolding process takes place as a function of the temperature; we observe that T acts as a control parameter of the measured nuclear magnetization M(T). Whereas far from this singular temperature, in the protein native state, the M(T) behavior is Arrhenius, approaching TD (in a large T-interval) the system changes dramatically it energetic configurations by means a power law behavior. Hence, by following the thermal behavior of different protein-peptide metabolites we are able to explore the funneled energy landscape. On these bases, by taking advantage of the polymer physics we propose this complex process (protein folding/unfolding) as a sort of sol- gel transition driven by water as the cross-linker between different protein peptides, an with TD as the percolation threshold temperature.