Understanding the properties of water is key in biochemistry because the macromolecular components of cells proteins, polysaccharides, DNA, RNA and membranes - assume their characteristic shapes in response to water.
2.1
Structure and Polar Nature of Water
Review of Electronic Structure of Water:
a)
Oxygen has the
following electronic configuration: 1s22s22p4.
b)
The 2s and 2p
orbitals of the outer shell form four sp3 hybrid orbitals that can
accommodate 4 pairs of electrons.
c)
With 6 electrons in
the outer shell, the outer shell orbitals are populated such that two orbitals
are filled and two contain one electron.
d)
The filled orbitals
cannot form bonds and are called lone pairs of electrons.
e)
The half-filled
orbitals participate in the formation of a covalent bond between oxygen and
hydrogen.
f)
These orbitals are
tetrahedral in their orientation (although the ideal bond angle of 109° is
distorted to 104.5° by charge repulsion in the lone pair electrons).
g)
The ³bent² shape of
the water molecule is polarizing, creating a permanent dipole for the molecule
as a whole.
2.2
Hydrogen Bonds
1. Characteristics of H-Bonds
a)
Formation of
H-bonds is primarily an electrostatic attraction between:
·
A donor hydrogen atom attached to an electronegative
atom (e.g. NH or OH)
·
An acceptor electronegative atom (e.g. the lone pair electrons of nitrogen or oxygen in
the case of water) (See Campbell, Table 2.1 for electronegativities)
b)
Typical length:
1.8Å (from hydrogen to oxygen, 2.7 Å from nitrogen to oxygen)
c)
Typical angle: 180°
± 20° (the closer to linear the stronger the hydrogen bond)
d)
Typical energy: 20
kJ/mole. How does this compare to a covalent bond?
2. Significance of hydrogen bonds
a)
Defines the solvent
properties of water.
b)
Strong
intermolecular cohesion responsible for its high boiling point, melting point,
heat of vaporization and surface tension.
c)
Large heat
capacity, important for temperature regulation.
d)
Low density of
ice. H-bonds in ice are
directional and straight, resulting in strong H-bonds and an open, lattice
structure.
e)
Water molecules in
liquid water form a random H-bonded network with non-ideal orientations. (The average lifetime of an H-bond
between two water molecules is 10 picoseconds.) Thus each individual H-bond is easily made and broken,
resulting in fluidity.
f) Important role in recognition at the molecular
level as well as the formation of the structure of complex bio-molecules: most,
if not all hydrogen bonds must be satisfied in these interactions.
2.3 Solvation (Oil and Water don't mix, at least not very
well)
a) Hydrophobic (apolar) compounds (e.g. butane): do not form hydrogen bonds to water; rather, they are
excluded from water and are forced to interact with each other.
b) Hydrophilic (polar) compounds (e.g.
ethanol): interaction by hydrogen bonds.
c) Solvation of ions: Ions dissolve readily in water. Interaction by electrostatic forces.
The force between two charged particles is:
The force depends on the distance between the two
charges and the dielectric constant (D) of the medium. A high dielectric
constant, such as that found in water, is important because the forces between
charges are attenuated. Thus,
unlike charges will not be strongly attracted to each other, instead, they will
remain in solution and interact with the water.
The
dielectric constant is related to the dipole moment of the solvent, as
illustrated below:
|
Compound |
Dielectric Constant(D) |
Dipole Moment (m
) |
|
Formamide |
110 |
3.37 |
|
Water |
79 |
1.85 |
|
Methanol |
32 |
1.66 |
|
Benzene |
2 |
0.00 |
The dipole moment reflects the charge
distribution of a molecule. It is defined by the following equation: 
. A large dipole
moment means that the solvent molecules can interact favorably with charged
solute molecules. Consequently, a high dipole moment usually implies a high
dielectric constant.
d) 
Amphipathic (or amphiphilic) compounds are both polar (or charged) and
non-polar (e.g. fatty
acids). These can form micelles if
the non-polar part is sufficiently large. Micelles are aggregates of
amphipathic molecules that sequester the non-polar part on the inside, much
like the inside of an orange.
Take home question:
Draw, and then describe how water will interact with the following: