at chemical equilibrium, it would be dead! Nevertheless, the concepts of equi-
librium are useful in biochemical reactions. Applications include equilibria of
weak acids and bases in aqueous solution, buffer equilibria, and temperature
effects on equilibrium, among others.
Amino acids contain the organic acid (or carboxyl) group, –COOH, and a
basic amino group, –NH 2. The carboxyl group can ionize to –COOand H,
and the amino group can accept an Hand become the –NH 3 group. In solid
or neutral aqueous phase, the overall neutral amino acid is actually a doubly
charged species called a zwitterion:*
RC(NH 2 )COOH →RC(NH 3 )COO
where R represents the different R groups that distinguish different amino
acids. For all amino acids, a series of equilibria between different ions will ex-
ist whose equilibrium extents depend on the presence (or absence) of free H
ions from other sources (like other acids):
K 1 K 2
RC(NH 3 )COOH RC(NH 3 )COO RC(NH 2 )COO (5.21)
The equilibrium constant K 1 is the equilibrium constant for the acid dissocia-
tion involved in the ionization of the –COOH group. The equilibrium constant
K 2 is the equilibrium constant for the acid dissociation in the loss of Hfrom
the –NH 3 group. (The Hions have been left out of equation 5.21 for clar-
ity.) The presence or absence of H, though, will dictate the extent of each
equilibrium in equation 5.21.
For simplicity’s sake, typically the negative logarithm of the Kvalues are tab-
ulated. The negative logarithm (base 10) of the equilibrium constant is labeled
the pK(spoken as “pea-kay”):
pKlog K (5.22)
Values of the pK’s for the amino acids in proteins are listed in Table 5.1. When
the pH of the solution is such that the amino acid exists as the zwitterionic
form, this pH is called the isoelectric pointof that amino acid. In many cases,
the isoelectric point is midway between the two pK’s, but for amino acids that
have other acidic or basic groups, this is not the case. As Table 5.1 indicates,
amino acids have varying behavior in aqueous solution. The point here is that
equilibrium processes are important for amino acid chemistry and, by exten-
sion, protein chemistry.
The concept of equilibrium is also important in biochemical processes such
as O 2 /CO 2 exchange in hemoglobin (for example, see exercise 5.7 at the end of
this chapter), the binding of small molecules to DNA strands (as might occur
in the transcription process), and the interaction of substrates and enzymes.
Temperature effects are important in protein denaturation process. Clearly, the
ideas established in this chapter are widely applicable to all chemical reactions,
even very complex ones.5.7 Summary
Chemical equilibrium is defined in terms of a minimum of Gibbs free energy
with respect to the extent of a reaction. Because the Gibbs free energy is related
to the chemical potential, we can use equations involving chemical potential to
derive some equations that relate to equilibrium and nonequilibrium conditionsJQPJ JQPJ
136 CHAPTER 5 Introduction to Chemical Equilibrium
*The word zwitterioncomes from the German word zwitter, meaning “hybrid.”Table 5.1 pKvalues for amino acids
Amino acid pK 1 pK 2
Alanine 2.34 9.69
Arginine 2.17 9.04
Asparagine 2.02 8.80
Aspartic acid 1.88 9.60
Cysteine 1.96 10.28
Glutamic acid 2.19 9.67
Glutamine 2.17 9.13
Glycine 2.34 9.60
Histidine 1.82 9.17
Isoleucine 2.36 9.60
Leucine 2.36 9.60
Lysine 2.18 8.95
Methionine 2.28 9.21
Phenylalanine 1.83 9.13
Proline 1.99 10.60
Serine 2.21 9.15
Threonine 2.09 9.10
Tryptophan 2.83 9.39
Tyrosine 2.20 9.11
Valine 2.32 9.62