Essential Cell Biology 5th edition

71
ELECTROSTATIC ATTRACTIONS
Electrostatic attractions occur both between
fully charged groups (ionic bond) and between
partially charged groups on polar molecules.
δ + δ –
The force of attraction between the two partial
charges, δ + and δ – , falls off rapidly as the
distance between the charges increases.
In the absence of water, ionic bonds are very strong.
They are responsible for the strength of such
minerals as marble and agate, and for crystal
formation in common table salt, NaCl.
Cl –
Na +
a crystal of
NaCl
ELECTROSTATIC ATTRACTIONS
IN WATER
Charged groups are shielded by their
interactions with water molecules.
Electrostatic attractions are therefore
quite weak in water.
H
O
H
O H O
H
H
H
H O
O P O
O H H
O H H
H
O
H
H O O
H
H
+
O
O
H
+ Mg
H O
H O
H
H
H
Inorganic ions in solution can also cluster around
charged groups and further weaken these electrostatic
attractions.
Na Cl
+ Na
+
Cl H
O Na
+
H N
C
+
O Na Cl H
+
+
Na
Cl
Cl
Despite being weakened by water and inorganic
ions, electrostatic attractions are very important
in biological systems. For example, an enzyme
that binds a positively charged substrate will
often have a negatively charged amino acid side
chain at the appropriate place.
HYDROPHOBIC FORCES
substrate
+
H
C
H
H
C
H
enzyme
–
H
H
H
H
H H H
C H C
Water forces hydrophobic groups together
in order to minimize their disruptive effects on
the water network formed by the hydrogen bonds
between water molecules. Hydrophobic groups
held together in this way are sometimes said
to be held together by “hydrophobic
bonds,” even though the attraction is
actually caused by a repulsion from water.
Panel 2.03b