INTERFACIAL AND CONFINED WATER Part 6 pdf

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6 Role of interfacial water in

biological function

Importance of water in biology is well known: life on the earth cannot

exist without water. There is a large amount of water in living organisms

(about 60% by weight in human body), both inside and outside the bio￾logical cells. Water is involved in various biochemical reactions and acts

as a solvent for biomolecules. Despite the relatively high water content

in living organisms, pure liquid water is practically absent in biosystems.

Both intracellular and extracellular liquids consist mainly of water, but

the concentration of organic compounds, including large biomolecules,

is very high (about 20 to 30%). The central role of water in biological

function is recognized [442, 443], but the numerous questions concern￾ing the physical mechanisms behind the importance of water for life

remain unanswered. There are several important physical phenomena,

which should be taken into account when considering water properties in

biosystems and the role of water in biological function.

First phenomenon is related to the bulk phase transitions in aqueous

mixtures. In biosystems, water is a component of a multicomponent fluid

mixture with various biomacromolecules, small organic molecules, ions,

etc. This complex mixture unavoidably possesses a rich phase diagram

with numerous phase transitions and respective critical points, which may

occur close to the thermodynamic conditions typical of living organisms

on the earth. The general features of these phase transitions are similar

to the ones of the liquid–liquid transitions of binary mixtures of small

organic molecules with water. However, there are several factors that

make the phase transitions in biological liquids much more complex.

Multiplicity of the transitions in a multicomponent mixture assumes mul￾tiplicity not only of the stable but also of the metastable states, which may

exist during a long period of time. Phases enriched with macromolecules

are usually not liquids but solid-like structures with some level of order￾ing at the mesoscopic or macroscopic scales (micelles, fibrils, etc.).

Biomolecules have variety of conformational states, which are strongly

coupled with the phase state of a system. Strictly speaking, conforma￾tional transition of a single biomolecule and the phase transition, which

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152 Interfacial and confined water

involves an ensemble of such molecules, cannot be considered separately.

Finally, situation is complicated by the possible chemical reactions in

complex biosystems.

The phase state of the aqueous mixture, in particular its location with

respect to the phase transitions, governs the clustering of both water and

organic molecules. For example, being inside the two-phase region, two

phases may appear as two macroscopic clusters of like molecules. In the

system being in the one-phase region, the clustering of like molecules

(water or biomolecules) is determined by the proximity to the phase

transition. When the phase transition is approached, clustering of the

minor component enhances. This approaching may be achieved by vary￾ing temperature, pressure, pH and by adding some cosolvents, ions, etc.

Majority of aqueous solutions of organic molecules show a closed-loop

phase diagram, which terminates by the lower critical solution tempera￾ture (LCST) and upper critical solution temperature from low and high

temperature sides, respectively. For example, the system in a one-phase

region below LCST separates into two phases upon heating. Accordingly,

the trend of the biomolecules to form clusters intensifies when the system

approaches solution temperature upon heating. In chemical literature,

clustering of solute molecules in water is often described as a manifes￾tation of “hydrophobic interactions.” Note that the phase transition and

related clustering of biomolecules inside the relatively small biological

cells may be affected by the finite size effect [332], which should suppress

aggregation of biomolecules [444].

Second phenomenon is related to the surface phase transitions. It is

natural to expect preferential adsorption of water or another component

of the biological liquids on the cell wall or other biosurfaces. Obviously,

this adsorption strongly affects the properties of biological liquids near

the walls. In particular, adsorption of biomolecules may facilitate forma￾tion of their ordered aggregates. If the effective attraction of biomolecules

to a surface is strong enough, we may expect a surface phase transition,

which results in the formation of a specific surface phase. Description

of the biological fluids based on the statistical theory of the bulk and

surface phase transitions should be very useful for understanding their

properties. Due to the extremely complex character of these systems, full

application of such approach seems to be possible in the long-term per￾spective only. However, the phase behavior and properties of water in