Ordered Water Structure and Enhanced Reactivity

Reactions in Micellar Systems

The notion of "green chemistry" has encouraged even synthetic organic chemists to include water as a solvent. Incredible selectivities and activities can be achieved through the addition of amphiphiles with a defined structure. The morphology of supramolecular assemblies or associates formed by surfactants vary according to the temperature and concentration. As a rule, reactions are typically conducted using simple spherical aggregates, that is, micelles in the nanometer range. The strong polarity gradient present between the hydrophilic surface and the hydrophobic core of the micelle means that both nonpolar and polar reagents can be solubilized. This solubilization results in reactants becoming more concentrated within the micelle than in the surrounding water phase and leads to an acceleration of the reaction and causes selective effects. The kinetic treatment of reactions in micellar systems can be accomplished by considering them as microheterogeneous two-phase systems.

Mechanism of adaptation of an atypical alkaline p-nitrophenyl phosphatase from the archaeon Halobacterium salinarum at low-water environments

Enzymes suspended in organic solvents represent a versatile system for studying the involvement of water in catalytic properties and their flexibility in adapting to different environmental conditions. The extremely halophilic alkaline p-nitrophenylphosphate phosphatase from the archaeon Halobacterium salinarum was solubilized in an organic medium consisting of reversed micelles of hexadecyltrimethylammoniumbromide in cyclohexane, with 1-butanol as cosurfactant. Hydrolysis of p-nitrophenylphosphate was nonlinear with time when the enzyme was microinjected into reversed micelles that contained substrate. These data are consistent with a kinetic model in which the enzyme is irreversibly converted from an initial form to a final stable form during the first seconds of the encapsulation process. The model features a rate constant (k) for that transition and separate hydrolysis rates, v(1) and v(2), for the two forms of the enzyme. The enzyme conversion may be governed by the encapsulation process.

Reverse micelles and protein biotechnology

Reverse micelles are nanometer-sized (1-10 nm) water droplets dispersed in organic media obtained by the action of surfactants. Surfactant molecules organize with the polar part to the inner side able to solubilize water and the apolar part in contact with the organic solvent. Proteins can be solubilized in the water pool of reverse micelles. Studies on the structure-function relationships of proteins in reverse micelles are very important since the microenvironment in which the protein is solubilized has physico-chemical properties distinct from a bulk aqueous solution. Some of the unique characteristics of reverse micelles make them very useful for biotechnological applications. Charge and hydrophilic/hydrophobic characteristics of the protein and the selection of surfactant can be used to achieve selective solubilization of proteins. This has been used to extend the classical liquid-liquid extraction with solvents to protein bioseparation. For biocatalysis the presence of a bulk organic solvent allow synthetic reactions to be performed via the control of water content and the solubilization of hydrophobic substrates. This is accomplished with a higher interfacial area (about 100 m2/mL) than the conventional biphasic systems, minimizing mass transfer problems.

Kinetic regulation of an alkaline p-nitrophenylphosphate phosphatase from Halobacterium salinarum in low water system by Mn2+ and monovalent cations

Reversed micelles were used as a cytoplasmic model to study the effect of the multi-ionic equilibria on kinetics of extreme halophilic enzymes. The enzymatic system used was an alkaline p-nitrophenylphosphate phosphatase from the halophilic archaeon Halobacterium salinarum (earlier halobium). This enzyme was solubilised in reversed micelles of hexadecyltrimethylammonium bromide in cyclohexane, with 1-butanol as co-surfactant. The p-nitrophenylphosphate phosphatase is a good system to study the regulation of the enzymatic activity, because it utilises manganese, water and potassium or sodium as cofactors and reacts with p-nitrophenylphosphate. Kinetic behaviour was determined by the ratio between [Mn2+] and [Na+] or [K+]. When the [Mn2+] increased and [Na+] or [K+] decreased, the kinetics showed cooperative behaviour. Rabin's model describes the kinetic behaviour of the p-nitrophenylphosphate phosphatase in reversed micelles.

Structure and reactivity of water at biomaterial surfaces

Molecular self association in liquids is a physical process that can dominate cohesion (interfacial tension) and miscibility. In water, self association is a powerful organizational force leading to a three-dimensional hydrogen-bonded network (water structure). Localized perturbations in the chemical potential of water as by, for example, contact with a solid surface, induces compensating changes in water structure that can be sensed tens of nanometers from the point of origin using the surface force apparatus (SFA) and ancillary techniques. These instruments reveal attractive or repulsive forces between opposing surfaces immersed in water, over and above that anticipated by continuum theory (DLVO), that are attributed to a variable density (partial molar volume) of a more-or-less ordered water structure, depending on the water wettability (surface energy) of the water-contacting surfaces. Water structure at surfaces is thus found to be a manifestation of hydrophobicity and, while mechanistic/theoretical interpretation of experimental results remain the subject of some debate in the literature, convergence of experimental observations permit, for the first time, quantitative definition of the relative terms 'hydrophobic' and 'hydrophilic'. In particular, long-range attractive forces are detected only between surfaces exhibiting a water contact angle theta > 65 degrees (herein defined as hydrophobic surfaces with pure water adhesion tension tau O = gamma O cos theta < 30 dyn/cm where gamma O is water interfacial tension = 72.8 dyn/cm). Repulsive forces are detected between surfaces exhibiting theta < 65 degrees (hydrophilic surfaces, tau O > 30 dyn/cm). These findings suggest at least two distinct kinds of water structure and reactivity: a relatively less-dense water region against hydrophobic surfaces with an open hydrogen-bonded network and a relatively more-dense water region against hydrophilic surfaces with a collapsed hydrogen-bonded network. Importantly, membrane and SFA studies reveal a discrimination between biologically-important ions that preferentially solubilizes divalent ions in more-dense water regions relative to less-dense water regions in which monovalent ions are enriched. Thus, the compelling conclusion to be drawn from the collective scientific evidence gleaned from over a century of experimental and theoretical investigation is that solvent properties of water within the interphase separating a solid surface from bulk water solution vary with contacting surface chemistry. This interphase can extend tens of nanometers from a water-contacting surface due to a propagation of differences in self association between vicinal water and bulk-phase water. Physicochemical properties of interfacial water profoundly influence the biological response to materials in a surprisingly straightforward manner when key measures of biological activity sensitive to interfacial phenomena are scaled against water adhesion tension tau O of contacting surfaces. As examples, hydrophobic surfaces (tau O < 30 dyn/cm) support adsorption of various surfactants and proteins from water because expulsion of solute from solution into the interphase between bulk solid and solution phases is energetically favorable. Adsorption to hydrophobic surfaces is driven by the reduction of interfacial energetics concomitant with replacement of water molecules at the surface by adsorbed solute (surface dehydration). Hydrophilic surfaces (tau O > 30 dyn/cm) do not support adsorption because this mechanism is energetically unfavorable. Protein-adsorbing hydrophobic surfaces are inefficient contact activators of the blood coagulation cascade whereas protein-repellent hydrophilic surfaces are efficient activators of blood coagulation. Mammalian cell attachment is a process distinct from protein adsorption that occurs efficiently to hydrophilic surfaces but inefficiently to hydrophobic surfaces. (ABSTRACT TRUNCATED)

Role of water in some biological processes

The state of intracellular water has been a matter of controversy for a long time for two reasons. First, experiments have often given conflicting results. Second, hitherto, there have been no plausible grounds for assuming that intracellular water should be significantly different from bulk water. A collective behavior of water molecules is suggested here as a thermodynamically inevitable mechanism for generation of appreciable zones of abnormal water. At a highly charged surface, water molecules move together, generating a zone of water perhaps 6 nm thick, which is weakly hydrogen bonded, fluid, and reactive and selectively accumulates small cations, multivalent anions, and hydrophobic solutes. At a hydrophobic surface, molecules move apart and local water becomes strongly bonded, inert, and viscous and accumulates large cations, univalent anions, and compatible solutes. Proteins and many other biopolymers have patchy surfaces which therefore induce, by the two mechanisms described, patchy interfacial water structures, which extended appreciable distances from the surface. The reason for many conflicting experimental results now becomes apparent. Average values of properties of water measured in gels, cells, or solutions of proteins are often not very different from the same properties of normal water, giving no indication that they are averages of extreme values. To detect the operation of this phenomenon, it is necessary to probe selectively a single abnormal population. Examples of such experiments are given. It is shown that this collective behavior of water molecules amounts to a considerable biological force, which can be equivalent to a pressure of 1,000 atm (1.013 x 10(5) kPa). It is suggested that cells selectively accumulate K+ ions and compatible solutes to avoid extremes of water structure in their aqueous compartments, but that cation pumps and other enzymes exploit the different solvent properties and reactivities of water to perform work of transport or synthesis.