Biomedical Engineering Reference
In-Depth Information
7 Buffers
7.1 Theoretical Considerations
Macromolecules, e.g., proteins, need a distinct structure within the
aqueous surrounding to realize their biologic functions. This struc-
ture is stabilized inter-alia by ionic interactions between positively
and negatively charged amino acid side chains and between these
chains and other molecules. Optimal functionality needs a well-
balanced ratio of charged residues. Each disorder of this ratio re-
sults in alterations up to complete denaturation.
Since the side chains of a distinct amino acid do not show com-
Incomplete
dissociation of
amino acid side
chains in proteins
plete dissociation at common concentrations, these acids (when
forming an anion during dissociation) and bases (when forming
a cation) are called weak acids and bases, respectively, as opposed to
the completely dissociated strong acids and bases, e.g., hydrochloric
acid, trifluoroacetic acid, sodium hydroxide, or triethylammonium
hydroxide.
Weak acids and bases are not restricted to amino acids; many
other compounds also behave similar in aqueous solutions. The
dissociation into anion A
−
and proton H
+
(in aqueous solution
the dissociated protein is captured by a water molecule to form
a hydronium ion H
3
O
+
) in the case of acids, and the addition of
aprotontoabaseBtoformthecationBH
+
, is an equilibrium
←
A
−
+ H
3
O
+
AH + H
2
O
←
BH
+
+ H
2
O
B + H
3
O
+
This equilibrium can be described for dissociation by the thermo-
dynamic equation:
[A
−
]
[H
+
]
[AH]
k
−1
k
1
·
=
=
K
D
with the dissociation equilibrium constant K
D
and the concentra-
tions [A
−
], [H
+
], and [AH] of the reaction partners. As a ther-
modynamic parameter, the dissociation constant K
D
depends on
temperature and therefore the dissociation is also temperature de-
pendent.
In a more detailed view are factors of the equilibrium equation,
not the concentrations, but the activities a
x
, i.e., only a part of the
total amount of a reactant:
