Effects of Salts of the Hofmeister Series on the Hydrogen Bond Network of Water

Delayed luminescence: a novel technique to obtain new insights into water structure

Fully understanding the structure of water is a crucial point in biophysics because this liquid is essential in the operation of the engines of life. Many of its amazing anomalies seem to be tailored to support biological processes and, during about a century, several models have been developed to describe the water structuring. In particular, a theory assumes that water is a mixture of domains constituted by two distinct and inter-converting structural species, the low-density water (LDW) and the high-density water (HDW). According to this theory, by using some particular solutes or changing the water temperature, it should be possible to modify the equilibrium between the two species, changing in this way the water behavior in specific biological processes, as in governing the shape and stability of the structures of proteins. In this work, we assess the possibility of obtaining information on the structures induced in water by specific salts or by temperature by measuring the delayed luminescence (DL) of some salt solutions and of water in the super-cooled regime. Previous works have demonstrated that the delayed luminescence of a system is correlated with its dynamic ordered structures. The results show significant DL signals only when the formation of LDW domains is expected. The measurement reveals a similar activation energy for the domains both in aqueous salt solutions and super-cooled water. It is worth noting that the time trend of DL signals suggests the existence of structures unusually long-lasting in time, up to the microsecond range.

Probing the effect of water-water interactions on enzyme activity with salt gradients: a case-study using ribonuclease t1

Water molecules interact with one another via hydrogen bonds. Experimental and theoretical evidence indicates that these hydrogen bonds occur in two modalities--high- and low-angle hydrogen bonding--and that the addition of various solutes to water affects only the number of water molecules participating in a specific type of hydrogen bond interactions, not the nature of the water-water interactions. In this work, we have investigated the effect of each of these hydrogen bonding types upon the activity of the enzyme ribonuclease t1. This was done through perturbation of the water hydrogen bonding distribution by using various salts. Our results indicate that various salts differ in their ability to reduce the enzymatic activity of ribonuclease t1, and this ability is well correlated with the ability of each salt to promote high-angle hydrogen bonding in water. By applying the two-phase model of liquid water (i.e., liquid water being modeled as an equilibrium existing between two phases, LD and HD water), we demonstrate that our results are compatible with the assumption that increasing the population of high-angle hydrogen bonds among water molecules stabilizes the more compact, less active conformations of the enzyme. This indicates that the structures that proteins adopt in water solution depend upon the nature of interactions between water molecules.

Hosting anions. The energetic perspective

Hosting anions addresses the widely spread molecular recognition event of negatively charged species by dedicated organic compounds in condensed phases at equilibrium. The experimentally accessible energetic features comprise the entire system including the solvent, any buffers, background electrolytes or other components introduced for e.g. analysis. The deconvolution of all these interaction types and their dependence on subtle structural variation is required to arrive at a structure-energy correlation that may serve as a guide in receptor construction. The focus on direct host-guest interactions (lock-and-key complementarity) that have dominated the binding concepts of artificial receptors in the past must be widened in order to account for entropic contributions which constitute very significant fractions of the total free energy of interaction. Including entropy necessarily addresses the ambiguity and fuzziness of the host-guest structural ensemble and requires the appreciation of the fact that most liquid phases possess distinct structures of their own. Apparently, it is the perturbation of the intrinsic solvent structure occurring upon association that rules ion binding in polar media where ions are soluble and abundant. Rather than specifying peculiar structural elements useful in anion binding this critical review attempts an illumination of the concepts and individual energetic contributions resulting in the final observation of specific anion recognition (95 references).

Investigation of protein conformation and interactions with salts via X-ray absorption spectroscopy

Nitrogen K-edge spectra of aqueous triglycine were measured using liquid microjets, and the effects of Hofmeister-active salts on the spectra were observed. Spectra simulated using density functional theory, sampled from room temperature classical molecular dynamics trajectories, capture all major features in the measured spectra. The spectrum of triglycine in water is quite similar to that in the presence of chaotropic sodium bromide (and other halides), which raises the solubility of proteins. However, a new feature is found when kosmotropic Na(2)SO(3), which lowers solubility, is present; this feature results from excitations of the nitrogen atom in the terminal amino group of triglycine. Both direct interactions between this salt and the protonated amino terminus, as well as corresponding changes in the conformational dynamics of the system, contribute to this new feature. These molecular measurements support a different mechanism for the Hofmeister effect than has previously been suggested based on thermodynamic measurements. It is also shown that near edge X-ray absorption fine structure (NEXAFS) is sensitive to strong direct interaction between certain salts and charged peptides. However, by investigating the sensitivity of NEXAFS to the extreme structural differences between model beta-sheets and alpha-helices, we conclude that this technique is relatively insensitive to secondary structure of peptides and proteins.

Water in the half shell: structure of water, focusing on angular structure and solvation

Water is a highly polar molecule, consisting of a very electronegative atom, oxygen, bonded to two weakly electropositive hydrogen atoms with two lone pairs of electrons. These features give water remarkable physical properties, some of which are anomalous, such as its lower density in the solid phase compared with the liquid phase. Its ability to serve as both a hydrogen bond donor and hydrogen bond acceptor governs its role as a solvent, a role that is of central interest for biological chemists. In this Account, we focus on water's properties as a solvent. Water dissolves a vast range of solutes with solubilities that range over 10 orders of magnitude. Differences in solubility define the fundamental dichotomy between polar, or hydrophilic, solutes and apolar, or hydrophobic, solutes. This important distinction plays a large part in the structure, stability, and function of biological macromolecules. The strength of hydrogen bonding depends on the H-O...O H-bond angle, and the angular distribution is bimodal. Changes in the width and frequency of infrared spectral lines and in the heat capacity of the solution provide a measure of the changes in the strength and distribution of angles of the hydrogen bonds. Polar solutes and inorganic ions increase the population of bent hydrogen bonds at the expense of the more linear population, while apolar solutes or groups have the opposite effect. We examine how protein denaturants might alter the solvation behavior of water. Urea has very little effect on water's hydrogen bond network, while guanidinium ions promote more linear hydrogen bonds. These results point to fundamental differences in the protein denaturation mechanisms of these molecules. We also suggest a mechanism of action for antifreeze (or thermal hysteresis) proteins: ordering of water around the surface of these proteins prior to freezing appears to interfere with ice formation.

A theoretical study of the neutral and the double-charged cation of cyclo[8]pyrrole and its interaction with inorganic anions

A theoretical study of the complexation of cyclo[8]pyrrole dication, 2, and the corresponding system in neutral form, 3, with six anionic molecules has been carried out up to the B3LYP/6–311++G(2d,2p) computational level. The effect of the water solvation has been taken into account by means of the PCM method. The gas phase results correspond to the very large interaction energies expected for the interaction of molecules of opposite charge. In all the complexes, the analysis of the electron density by means of the Atoms In Molecules (AIM) methodology shows the presence of eight intermolecular interactions between the individual molecules. The results, using the water solvent model, indicate that the 2:SO42− complex is more stable than the 2:NO3−, in agreement with experimental results.

Infrared spectroscopy used to study ice formation: the effect of trehalose, maltose, and glucose on melting

We report the use of infrared (IR) spectroscopy to detect ice crystals in biological solutions. The method is based on the temperature dependence of the OH bending and stretch bands of water. By using mixtures of D(2)O and H(2)O, water's absorption bands can be made to be on-scale in transmission mode. Water's stretch band moves to lower frequency and sharpens with freezing, and the bending band goes to higher frequency and becomes less sharp. The technique is demonstrated for the study of the hysteresis of freezing in the presence of glucosyl sugars, namely glucose, maltose, and trehalose.

Influence of surface groups of proteins on water studied by freezing/thawing hysteresis and infrared spectroscopy

The influence of proteins and solutes on hysteresis of freezing and melting of water was measured by infrared (IR) spectroscopy. Of the solutes examined, poly-L-arginine and flounder antifreeze protein produced the largest freezing point depression of water, with little effect on the melting temperature. Poly-L-lysine, poly-L-glutamate, cytochrome c and bovine serum albumin had less effect on the freezing of water. Small compounds used to mimic non-polar (trimethylamine N-oxide, methanol), positively charged (guanidinium chloride, NH(4)Cl, urea) and negatively charged (Na acetate) groups on protein surfaces were also examined. These molecules and ions depress water's freezing point and the melting profiles became broad. Since infrared absorption measures both bulk solvent and solvent bound to the solutes, this result is consistent with solutes interacting with liquid water. The amide I absorption bands of antifreeze protein and poly-L-arginine do not detectably change with the phase transition of water. An interpretation is that the antifreeze protein and poly-L-arginine order liquid water such that the water around the group is ice-like.