BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 2. THE ROLE OF WATER, MACRO- AND MICROELEMENTS IN THE VITAL ACTIVITY OF ORGANISMS

2.2. Bound Water

As seen in Fig. 2.1, a Water molecule can attach two other water molecules via its two protons. However, other water molecules can also attach to it using their own protons; in other words, an H₂O molecule can simultaneously act as a donor and an acceptor of two H-bonds. The acceptor H-bonds are "anchored" to the orientation of the donor H-bonds, forming four bonds directed toward the vertices of a tetrahedron together with them. Electrical charges are localized at its vertices (Fig. 2.1) — two negative and two positive, with values of ±0.171 e — at a distance of 0.099 nm from the oxygen atom Nucleus. This means that each water molecule is coordinated by four other molecules at the tetrahedron vertices, with two bonds acting as Donors and two as acceptors for the centrally located molecule. Simultaneously, each molecule also serves as a vertex for two other tetrahedra formed by H₂O molecules. The main feature of structures built in this manner is that they represent a network rather than dense structures (resembling the work of a knitter rather than a stonemason). Hence the lightness and openness of water molecule structures: for the close packing of spheres with a radius of 0.14 nm (which is the molecular radius of an H₂O molecule with a molecular mass of 18), the density would be 1.92 g/cm³. The actual density of ice, in which water molecules "Touch" via opposite poles, is 0.92 g/cm³, indicating that Hydrogen Bonds support its "airy construction".

Thus, the openness and the presence of internal voids are Fundamental properties of H₂O molecular networks. The Structure of ice is built of interlinked rings (Fig. 2.5).

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Fig. 2.5. Water structures:

A - ice projected onto the basal plane; B - view of the repeating fragment; C - clathrate butylamine hydrate;

D - semiclathrate diethylamine hydrate (dashed lines in D represent hydrogen bonds between the guest molecule and the water molecule)

Three water molecules of the upper ring and three from the lower ring form a triangular prism into which a sphere with a radius of 0.146 nm can be inscribed, touching all six water molecules located at the prism vertices. Crucially, the space inside such prisms remains empty not only in ice but also to a large extent in liquid water. Since liquid water is only 10% denser than ice, this increase in density is achieved through the occupation of these cavities by other water molecules.

Ice is not the loosest structure featuring a framework of water molecules. Elucidation of gas hydrate structures has demonstrated that, while maintaining the same type of coordination between intermolecular distances as in ice, the network of water molecules forms more spacious cages known as clathrates (cage hydrates). Such a framework can only be genuinely stable when guest molecules or atoms are present in the cavities, referred to as "guests" (Fig. 2.5, C and D: the guests are butylamine and diethylamine). The number of such molecules reaches up to 8 moles per elementary unit Cell consisting of 46 moles of H₂O. The enhancement of structural stability due to the filling of voids is called the "help-gas effect". Van der Waals interactions between the guest molecules and water make a major contribution to the stability of such hydrates.

If a guest molecule is capable of more active interaction with water molecules (as a donor or acceptor of H-bonds), the Stability of the hydrate increases. This leads to certain deviations in polymer geometry compared to polyhedra with "hydrophobic guests". Such geometric changes can be significant, even resulting in the merger of several cavities into one, accompanied by the replacement of water molecules located at the junctions of adjacent cavities with guest particle groups. Naturally, the total amount of water in such structures decreases. Hydrates featuring the described H-bonds are semiclathrates.

A clear regularity is observed: The amount of bound water is maximized in hydrates of hydrophobic substances (noble gases, methane, chlorine, nitrogen, oxygen, carbon dioxide, etc.). Sparingly soluble substances also bind water via clathrate formation (often referred to as semiclathration), where the molecules can act as Hydrogen bond donors or acceptors. These substances include alkanes, mercaptans, cyclic ethers, aliphatic amines, ketones, and alcohols. In particular, ethyl alcohol at -80 ⁰C crystallizes as a mixture of clathrates, semiclathrates, and semihydrates (in the latter, water molecules no longer form their own networks). Crystal hydrates of water-soluble substances contain even less water. The water content decreases in the series: hydrated phenols, aldehydes, Purines, Pyrimidines, Amino Acids, Peptides, and CARBOHYDRATES. Clathration is practically non-existent for such substances. Water molecules are typically bound via H-bonds only to active centers—H-bond donors and acceptors—On the surface of hydrophilic molecules, or they act as fillers for channels and voids within organic or inorganic frameworks.

Concurrently with the decrease in water content, the stability of hydrates—the "binding strength" of water by a given substance—increases. While clathrate hydrates of hydrophobic substances decompose near 0 ⁰C, the decomposition (melting) Temperature for hydrates of poorly soluble substances reaches 20–30 ⁰C. Hydrated water-soluble substances are stable at higher temperatures. Some of them melt in their own water of crystallization (~100 ⁰C). Certain traps retain water at 500 ⁰C and above, such as in silicates. At very high temperatures (1000–1200 ⁰C), dehydration is not caused by the adhesion of water molecules to the structure, but rather by a very narrow "throat" (0.2 nm) leading into the interior of a cavity that houses a single H₂O molecule.

When approaching the question of The Role of bound water in biosystems, it must be noted that There is a severe shortage of quantitative empirical data amenable to physical analysis. Therefore, the main efforts in this direction are devoted to constructing various hypotheses and extrapolating everything known about water in simpler structures, including minerals, to living systems. According to one such Classification, bound water is retained in the rock by chemical and physical binding forces (0.1–800 kJ/mol) exerted by minerals, which alter the Structure and properties of water.

Bound water can be divided into two types. The first is crystal lattice water, which includes constitutional and crystallization-bound water in various crystal hydrates, as well as water bound by coordinatively unsaturated atoms and ions of the crystal lattice. The second type is adsorption water, formed through the adsorption "attraction" of water molecules to active adsorption centers on molecular surfaces. Within this type, There are two varieties of bound water: a) monomolecular adsorption water with a surface attraction energy > 40 kJ/mol; b) polymolecular adsorption water with a binding energy < 40 kJ/mol. Bound water forms adsorption films one or several molecular layers thick. The Physical Properties of this water type differ most significantly from those of free water.

Transition-type water (intermediate between bound and free water) is less subject to surface forces. It is retained near molecular surfaces by weaker bonds. Consequently, its structure is less altered, and differences in physical properties compared to free water are insignificant. Within the transition type, two kinds of water are also distinguished: osmotically absorbed water and capillary water. The former is formed through selective Diffusion of Water molecules toward the molecular surface, driven by the presence of an "ionic atmosphere"—an electrical double layer—around the molecule. This layer consists of solution cations that compensate for the negative charge of the particles (Fig. 2.6).

Fig. 2.6. Formation of osmotically absorbed water on a charged particle: 1 - anions; 2 - cations; 3 - region of the adsorption layer occupied by tightly bound water; 4 - diffuse part of the electrical double layer occupied by osmotic water; 5 - region beyond the electrical double layer occupied by free water

The electrical double layer has two parts: the inner part, called the adsorption layer (3), and the outer part, the diffuse layer (4). The concentration of cations increases exponentially along the normal to the particle surface. This gives rise to a concentration gradient that induces "osmotic" Movement of water molecules from the bulk free pore solution (5) into the BOUNDARIES OF THE electrical double layer (4). The osmotic water formed in this manner occupies the outer part of the electrical double layer—the diffuse layer (4). This water is termed osmotic because its formation is associated with The phenomenon of microscopic surface osmosis, which resembles conventional macroscopic osmosis—the movement of water through a semipermeable membrane (water passes through while larger cations are retained) driven by a concentration gradient.

The outer boundary of the electrical double layer acts as such a "semipermeable membrane" in macromolecules (Fig. 2.6). The ability of many systems to swell—to increase in volume upon absorbing moisture—is closely associated with this category of water.

The second kind of transition-type water is capillary water. It forms in pores of capillary dimensions (10^-3–10³ μm in diameter) due to capillary pressure and is retained by capillary forces of water menisci (surface tension forces) formed at the water–air–solid surface interface. Capillary forces virtually do not alter the structure of water; therefore, in terms of its basic physical properties, capillary water is practically indistinguishable from free water.