Editorial of virtual special issue Frontiers in Water Biophysics 2017

The present virtual special issue (VSI) of the Journal of Molecular Liquids contains the proceedings of the 4th Conference on Frontiers in Water Biophysics (FWB2017) held in Erice, Sicily (Italy) from 23 to 27 May 2017 at the Ettore Majorana Foundation and Centre for Scientific Culture, in the frame of the V Course of the International School of Sta- tistical Physics (Directors: P. Hanggi, F. Marchesoni)

Local order in protic ionic liquid/methanol mixtures

Protic ionic liquids (PILs), such as ethylammonium nitrate (EAN), are ionic compounds with low melting point that are prepared by combination of a Brønsted acid and base. As such they are characterised by the presence of protondonor and -acceptor sites, which contribute to develop a hydrogen-bonded network. Moreover, in the case of EAN, distinct mesoscopic order exists as a consequence of its inherent amphiphilicity. Such a structural complexity is deemed to increase upon mixing with another H-bonding active compound, such as methanol. In this presentation we will show the results from a study conducted taking advantage of the high complementarity of x-ray and neutron scattering, to probe the mesoscopic structure. The local order of these mixtures will also be investigated using Raman and IR spectroscopy data, to monitor the structural changes as a function of both concentration and temperatureProtic ionic liquids (PILs), such as ethylammonium nitrate (EAN), are ionic compounds with low melting point that are prepared by combination of a Brønsted acid and base. As such they are characterised by the presence of protondonor and -acceptor sites, which contribute to develop a hydrogen-bonded network. Moreover, in the case of EAN, distinct mesoscopic order exists as a consequence of its inherent amphiphilicity. Such a structural complexity is deemed to increase upon mixing with another H-bonding active compound, such as methanol. In this presentation we will show the results from a study conducted taking advantage of the high complementarity of x-ray and neutron scattering, to probe the mesoscopic structure. The local order of these mixtures will also be investigated using Raman and IR spectroscopy data, to monitor the structural changes as a function of both concentration and temperatur

Unfolding and aggregation of lysozyme: A thermodynamic and kinetic study by FTIR spectroscopy

International audienceThe unfolding of hen egg-white lysozyme dissolved both in DO and CHCHOD/DO was studied by Fourier Transform Infrared (FTIR) absorption spectroscopy at different protein concentrations. A detailed description of the local and global rearrangement of the secondary structure upon a temperature increase, in the range 295 to 365K, was obtained through the analysis of the amide I band. Thermodynamic parameters for the melting, and the effect of the co-solvent in determining a change in thermal stability of the protein were evaluated. The protein-protein interactions were also followed as a function of temperature: a strong dependence of the cluster stability and aggregation yield on the solvent composition was observed. Finally, FTIR spectra taken at successive time steps of the aggregation enabled intermolecular contacts to be monitored as a function of time, and kinetic information to be obtained showing that both unfolded and folded states of lysozyme act as reactants for the clustering event

More Is Different: Experimental Results on the Effect of Biomolecules on the Dynamics of Hydration Water

Biological interfaces characterized by a complex mixture of hydrophobic, hydrophilic, or charged moieties interfere with the cooperative rearrangement of the hydrogen-bond network of water. In the present study, this solute-induced dynamical perturbation is investigated by extended frequency range depolarized light scattering experiments on an aqueous solution of a variety of systems of different nature and complexity such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins. Our results suggest that a reductionist approach is not adequate to describe the rearrangement of hydration water because a significant increase of the dynamical retardation and extension of the perturbation occurs when increasing the chemical complexity of the solute

Dynamics of biological water: insights from molecular modeling of light scattering in aqueous trehalose solutions

Extended depolarized light scattering (EDLS) measurements have been recently employed to investigate the dynamics of water solvating biological molecules, giving evidence of the presence of two different dynamical regimes among water molecules. An interpretation of EDLS has been proposed that provides an independent estimate of the retardation factor of slowdown with respect to fast water molecules and of the number of solvent molecules affected by this slowing down. Nevertheless this measure is an inherently complex one, due to the collective nature of the physical property probed. In the present work a molecular dynamics (MD) approach has been used to more deeply understand experimental results. Time correlation functions of the collective polarizability anisotropy have been calculated for the prototype disaccharide trehalose in aqueous solutions as a function of concentration. The unique capability of MD to disentangle the contributions to the dynamics arising from solute, solvent, and cross terms between the two allowed us to check the reliability of an interpretation that assumes a spectral separation of water and sugar dynamics, as well as to highlight the very presence of two distinct relaxation processes in water. The two processes have been attributed to the dynamics of bulk and hydration water, respectively. A retardation factor of ~5 and concentration dependent hydration numbers have been observed, in good agreement with experimental results [Paolantoni, M.; et al. J. Phys. Chem. B 2009, 113, 7874-7878]

Interfacial Water and Micro-heterogeneity in Aqueous Solutions of Ionic Liquids

In this work, aqueous solutions of two prototypical ionic liquids (ILs), [BMIM][BF4] and [BMIM][TfO], were investigated by UV Raman spectroscopy and small-angle neutron scattering (SANS) in the water-rich domain, where strong heterogeneities at mesoscopic length scales (microheterogeneity) were expected. Analyzing Raman data by a differential method, the solute-correlated (SC) spectrum was extracted from the OH stretching profiles, emphasizing specific hydration features of the anions. SC-UV Raman spectra pointed out the molecular structuring of the interfacial water in these microheterogeneous IL/water mixtures, in which IL aggregates coexist with bulk water domains. The organization of the interfacial water differs for the [BMIM][BF4] and [BMIM][TfO] solutions, being affected by specific anion−water interactions. In particular, in the case of [BMIM][BF4], which forms weaker H-bonds with water, the aggregation properties clearly depend on concentration, as reflected by local changes in the interfacial water. On the other hand, stronger water−anion hydrogen bonds and more persistent hydration layers were observed for [BMIM][TfO], which likely prevent changes in IL aggregates. The modeling of SANS profiles, extended to [BPy][BF4] and [BPy][TfO], evidences the occurrence of significant concentration fluctuations for all of the systems: this appears as a rather general phenomenon that can be ascribed to the presence of IL aggregation, mainly induced by (cation-driven) hydrophobic interactions. Nevertheless, larger concentration fluctuations were observed for [BMIM][BF4], suggesting that anion−water interactions are relevant in modulating the microheterogeneity of the mixture

Denaturation and Preservation of Globular Proteins: The Role of DMSO

The thermal denaturation of hen egg white lysozyme (HEWL) in D₂O was followed by IR absorption after addition of dimethyl sulfoxide (DMSO) at different molar fractions. Amide I intensity and position revealed that DMSO reduces the thermal stability of the native protein and favors the formation of ordered aggregates. The comparison with ethanol/water solutions evidenced that ethanol (partially deuterated ethanol EtOD) has a stronger effect on the thermal stability of HEWL: the same down-shift of melting temperature was measured at 0.18 and 0.30 molar fraction of ethanol and DMSO, respectively. This is probably due to lower polarity of EtOD/D₂O with respect to DMSO/D₂O solutions. A kinetic study of protein assembling at 0.30 DMSO molar fraction, was also performed at different temperatures. The high viscosity of the solvent was observed to cause a sensitive slowing down of aggregation rate in comparison to that of water/alcohol solutions. The evidence of a retarded self-assembling put forward a possible explanation for the use of dimethyl sulfoxide as a protectant of protein structure. In fact, for both organic solvents a nonspecific interaction with the protein and a water-mediated action is deduced, but the addition of DMSO reduces the irreversible denaturation due to kinetic effects and this can be exploited for lessening one of the main degradation routes of globular proteins during freezing-thawing cycles