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

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

Free volume and dynamics in a lipid bilayer

The lateral diffusion of lipids and of small molecules inside a membrane is strictly related to the arrangement of acyl chains and to their mobility. In this study, we use FTIR and time resolved 2D-IR spectroscopic techniques to characterize the structure and dynamics of the hydrophobic region of palmitoyl-oleylphosphatidylcholine/cholesterol vesicles dispersed in water/dimethylsulfoxide solutions. By means of a non-polar probe, hexacarbonyl tungsten, we monitor the distribution of free volumes inside the bilayer and the conformational dynamics of hydrophobic tails in relation to the different compositions of the membrane or the different compositions of the solvent. Despite the important structural changes induced by the presence of DMSO in the solvating medium, the picosecond dynamics of the membrane is preserved under the different conditions