Page 35 - Color Atlas of Biochemistry

P. 35

26 Basics

Water as a solvent of water clusters increases until the water
begins to crystallize. Under normal atmo-
Life as we know it evolved in water and is still spheric pressure, this occurs at 0 °C. In ice,
absolutely dependent on it. The properties of most of the water molecules are fixed in a
water are therefore of fundamental impor- hexagonal lattice (3). Since the distance be-
tance to all living things. tween the individual molecules in the frozen
state is on average greater than in the liquid
state, the density of ice is lower than that of
A. Water and methane
liquid water. This fact is of immense biological
The special properties of water (H 2 O) become importance—it means, for example, that in
apparent when it is compared with methane winter, ice forms on the surface of open
(CH 4 ). The two molecules have a similar mass stretches of water first, and the water rarely
and size. Nevertheless, the boiling point of freezes to the bottom.
water is more than 250 °C above that of
methane. At temperatures on the earth’s sur-
face, water is liquid, whereas methane is gas- C. Hydration
eous. The high boiling point of water results In contrast to most other liquids, water is an
from its high vaporization enthalpy, which in excellent solvent for ions. In the electrical
turn is due to the fact that the density of the field of cations and anions, the dipolar water
electrons within the molecule is unevenly molecules arrange themselves in a regular
distributed. Two corners of the tetrahedrally- fashion corresponding to the charge of the
shaped water molecule are occupied by un- ion. They form hydration shells and shield
shared electrons (green), and the other two the central ion from oppositely charged ions.
by hydrogen atoms. As a result, the H–O–H Metal ions are therefore often present as
2+
bond has an angled shape. In addition, the hexahydrates ([Me(H 2 O) 6 ], on the right). In
O–H bonds are polarized due to the high elec- the inner hydration sphere of this type of ion,
tronegativity of oxygen (seep. 6). Onesideof the water molecules are practically immobi-
the molecule carries a partial charge (δ)of lized and follow the central ion. Water has a
about –0.6 units, whereas the other is corre- high dielectric constant of 78—i. e., the elec-
spondingly positively charged. The spatial trostatic attraction force between ions is re-
separation of the positive and negative duced to 1/78 by the solvent. Electrically
charges gives the molecule the properties of charged groups in organic molecules (e. g.,
an electrical dipole. Water molecules are carboxylate, phosphate, and ammonium
therefore attracted to one another like tiny groups) are also well hydrated and contribute
magnets, and are also connected by hydrogen to water solubility. Neutral molecules with
bonds (B)(seep. 6). When liquid water vapor- several hydroxy groups, such as glycerol (on
izes, a large amount of energy has to be ex- the left) or sugars, are also easily soluble,
pended to disrupt these interactions. By con- because they can form H bonds with water
trast, methane molecules are not dipolar, and molecules. The higher the proportion of polar
therefore interact with one another only functional groups there is in a molecule, the
weakly. This is why liquid methane vaporizes more water-soluble (hydrophilic)it is. By con-
at very low temperatures. trast, molecules that consist exclusively or
mainly of hydrocarbons are poorly soluble or
insoluble in water. These compounds are
B. Structure of water and ice
called hydrophobic (see p. 28).
The dipolar nature of water molecules favors
the formation of hydrogen bonds (see p. 6).
Each molecule can act either as a donor or an
acceptor of H bonds, and many molecules in
liquid water are therefore connected by H
bonds (1). Thebonds arein a stateof constant
fluctuation. Tetrahedral networks of mole-
cules, known as water “clusters,” often arise.
As the temperature decreases, the proportion

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