Water, biological interfaces

As has been noted, much of the interest in hlms of proteins, steroids, lipids, and so on, has a biological background. While studies at the air-water interface have been instructive, the natural systems approximate more closely to a water-oil interface. A fair amount of work has therefore been reported for such interfaces in spite of the greater experimental difhculties. 

The molecular structure and dynamics of the ice/water interface are of interest, for example, in understanding phenomena like frost heaving, freezing (and the inhibition of freezing) in biological systems, and the growth mechanisms of ice crystals. In a series of simulations, Haymet and coworkers (see Refs. 193-196) studied the density variation, the orientational order and the layer-dependence of the mobilitity of water molecules. The ice/water basal interface is found to be a relatively broad interface of about... 

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

A BLM can even be prepared from phospholipid monolayers at the water-air interface (Fig. 6.10B) and often does not then contain unfavourable organic solvent impurities. An asymmetric BLM can even be prepared containing different phospholipids on the two sides of the membrane. A method used for preparation of tiny segments of biological membranes (patch-clamp) is also applied to BLM preparation (Fig. 6.10C). 

Microbial transformations and generally not chemical transformations characterize the sewer environment in terms of quality transformations of the wastewater. On the other hand, the physicochemical characteristics, e.g., diffusion in the biofilm and exchange of substances across the water-air interface, play an important role and must be integrated with the microbial transformations. The hydraulics and the sewer solids transport processes have a pronounced impact on the sewer performance. These physical processes, however, are typically dealt with in hydraulics and are, therefore, only included in the text when directly and closely related to the chemical and biological processes. 

The three fundamental lyotropic liquid crystal structures are depicted in Figure 1. The lamellar structure with bimolecular lipid layers separated by water layers (Figure 1, center) is a relevant model for many biological interfaces. Despite the disorder in the polar region and in the hydrocarbon chain layers, which spectroscopy reveals are close to the liquid states, there is a perfect repetition in the direction perpendicular to the layers. Because of this one-dimensional periodicity, the thicknesses of the lipid and water layers and the cross-section area per lipid molecule can be derived directly from x-ray diffraction data. 

One type of lipid that is dominant in biological interfaces is lecithin, and lecithin-water systems have therefore been examined extensively by different physical techniques. Small s binary system (3) for egg lecithin-water is presented in Figure 2. The lamellar phase is formed over a large composition range, and, at very low water content, the phase behavior is quite complex. Their structures as proposed by Luzzati and co-workers (4) are either lamellar with different hydrocarbon chain packings or based on rods both types are discussed below. 

The interest in lipid bilayers is due to their relevance to biological membranes [1], They exhibit a richness of structures due to the interplay between many different inter- and intrabilayer forces. Among all the multilamellar bilayer structures, probably the most pertinent to biological membranes are the lamellar ones. Their equilibrium spacing is considered to be the result of a balance between attractive and repulsive forces. While the former forces are just the usual van der Waals interactions, the latter are composed of double layer forces (for charged bilayers) [2], hydration forces (due to the structuring of water near interfaces) [3] and repulsive forces generated by the thermal undulation of the membranes [4]. 

The bioactivity of silica is attributed to the nanodimensionality of its primary particles, to the presence of surface coordination compounds of water molecules, hydroxylated silicon atoms and strongly sorbed/highly structured bioactive water. Biologically active water (BAW) is a special form of water whose molecules are strongly bound to the BAS surface but at the same time are weakly associated. BAW is formed within gaps among biologic objects and particles of amorphous nanodimensional silica at a certain water content, spatial structure, and hydrophobic-hydrophilic balance at interfaces.10... 

There has been considerable interest in the characterization of the conformation and orientation of amphiphilic molecules in Langmuir monolayers in the past few decades [1-5]. Monolayers at the air-water interface are widely employed as convenient experimental paradigms that mimic many vital biological processes. The advantages realized when using monolayers as models for biological interfaces arise primarily from the ease with which experimental variables may be manipulated. These include parameters that are not readily controlled in bulk phases or in films prepared on solid substrates, such as lateral pressure, surface area, and domain size and shape [6]. 

The above account has provided sufficient background for analysis of the properties of aqueous solutions. The analysis has been restricted to bulk water the properties of water near interfaces, including biological surfaces, is very interesting but outside the scope of this review. It should be noted, however, that the properties of vicinal water differ from those of bulk water, these differences being important in biological systems (Drost-Hansen, 1972 1973). Thermal anomalies in the properties of water also seem explicable in terms of interfacial phenomena (Drost-Hansen, 1968). 

Membrane reactors using biological catalysts can be used in enantioselective processes. Methodologies for the preparation of emulsions (sub-micron) of oil in water have been developed and such emulsions have been used for kinetic resolutions in heterogeneous reactions catalyzed by enantioselective enzyme (Figure 43.4). A catalytic reactor containing membrane immobilized lipase has been realized. In this reactor, the substrate has been fed as emulsion [18]. The distribution of the water organic interface at the level of the immobUized enzyme has remarkably improved the property of transport, kinetic, and selectivity of the immobilized biocatalyst. 

The results on the solid phases of Ross Sea, an area where trace metals were poorly investigated in the past, allowed the distribution and the role of these elements in Antarctiea to be better assessed. Further expeditions will consider in more detail some peeuliar aspeets, such as the phenomena at the water-ice interface, and their eorrelations with biological events. 

All cellular processes take place in aqueous solution, and it is essential to understand the properties of water in order to understand biological processes. This statement comes from one of the most recent textbooks on molecular cell biology. Sadly, but common to such texts, what follows is but a brief description of the ordinary liquid. Its physical properties and peculiarities are described, but the reader is given no information about the nature and role of water at interfaces such as in the environment of the cell. In short, from such descriptions one must infer that water in cells is just the same as water in a beaker or a cup of tea—that water is water. 

Drost-Hansen, W. (1971). Structure and properties of water at biological interfaces. In Chemistry of the Cell Interface, (Brown, H. D ed.), Vol. 2, pp. 1-184, Academic Press, New York. 

Drost-Hansen, W. (1978). Water at biological interfaces—structural and functional aspects. Phys. Chem. Liq. 7, 243-348. 

Drost-Hansen, W. (1971). Structure and properties of water at biological interfaces. In Chemistry of the Cell Interface, Part B (Brown, H. D., ed.), pp. 1-184, Academic Press, New York. Drost-Hansen, W. Clegg, J. S. (eds). (1979). Cell-Associated Water, Academic Press, New York. Franks, F. Mathias, S. (eds.) (1982). Biophysics of Water, John Wiley Sons, New York. 

---