8
Bioorganic Chemistry
electrostatic interaction, or both should be
nonpolar (uncharged) to allow for effective
hydrophobic interaction (Fig. 5).
2.2.1
Electrostatic
Electrostatic interactions refer to charge–
charge interactions. An example of this is
the close contact of a positively charged
amino group and a negatively charged
carboxyl group (this arrangement is also
known as a salt bridge). Coulomb’s law
describes the energy that is released when
two charges are brought together (Fig. 6).
From the form of the equation, we can see
that the energy is directly proportional to
the charges on the species and inversely
proportional to the distance separating
them. In addition, the dielectric constant
indicates the ease with which the elec-
trostatic ±eld is propagated through the
medium (a vacuum has a dielectric con-
stant of 1, whereas the value for water is
79). Misuse of this equation can lead to
bizarre conclusions. For example, if one
calculates the energy released upon for-
mation of a salt bridge using the distance
of van der Waals’ contact and the dielec-
tric constant of a vacuum, the result is an
energy on the order of 60 kcal mol
1
.In
H
H
O
H
H
H
H
O
O
H
H
O
H
H
H
H
H
H
H
H
O
O
O
O
H
H
H
H
H
H
H
H
H
H
O
H
H
H
H
H
H
H
H
H
H
H
H
H
O
O
O
O
O
O
H
O
O
O
O
O
d
d
+
d
d
d
d
+
+
+
Electrostatic
interaction
energy
(Charge 1)
×
(charge 2)
Distance
×
dielectric constant
=
(a)
(b)
+
+
Fig. 6
(a) Electrostatic interaction energy. (b) Schematic representation
of the salt bridge–forming process showing the desolvation of the
charged species and its interaction with water dipoles.
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