Chemistry Reference
In-Depth Information
slow exchange [10]. HDX-MS spectrometry is applied to determine the con-
formation of membrane proteins in phospholipid bilayer nanodiscs [15].
In addition to the involvement of protonation in redox reactions, interac-
tions of metals-amino acids and metals-peptides also play significant role
in the oxidative chemistry of proteins. For example, the addition of glutathione
to the Cu(II) ion immediately formed the Cu(I)-(GSH)
2
complex [16], which
was able to generate superoxide by its reaction with oxygen [17, 18]. The
formation of iron, cobalt, copper, and nickel complexes has been shown to
influence the production of superoxide and hydroxyl radicals [19, 20]. Com-
plexation can also enhance or retard the reactivity of reactive species with
metal centers, which is described in Chapters 4-6. Complexes of metals may
stabilize reactive intermediates such as the formation of high-valent iron
(Fe(IV) and Fe(V)) complexes with oxo- and nitrogen donor ligands [21],
which then react with amino acid side chains of proteins (see Chapter 6). The
chemistry of metal complexation is discussed later in this chapter.
2.1 DISSOCIATION CONSTANTS
The acid-base properties of amino acids/peptides are generally characterized
using macroscopic protonation (or dissociation) constants, log
K
i
(or p
K
i
),
which are composites of the microscopic constants (log
k
i
or p
k
i
) for the indi-
vidual groups [22]. The microscopic constants represent a particular molecular
subunit in a defined protonation state for all other moieties in that ligand [22].
Microspecies remain in incessant interconversion due to the instantaneous
nature of protonation reactions; therefore, a direct approach to determine log
k
i
is not feasible. Thus, an indirect approach is applied with the use of at least
two experimental techniques and/or auxiliary compounds, followed by an
appropriate evaluation technique. This approach has been described in detail
in the microequilibrium analysis of tetrabasic acid [22, 23].
The dissociation of a triprotic acid (e.g., cysteine) is described by Equations
(2.1)-(2.6):
AH
+
H AH
+
+
±
(2.1)
3
2
AH
±
H AH
+
+
−
(2.2)
2
−
+
2
−
(2.3)
AH
H A
+
K
a
=
a
a
/a
=
(
γ γ
/
γ
)([
H AH / AH
+
][
±
] [
+
])
(2.4)
1
H AH
+
2
AH
3
+
H AH
2
AH
3
2
3
K
a
=
a
a
/a
=
(
γ
γ
/
γ
)([
H AH / AH
+
][
−
] [
±
])
(2.5)
2
H AH
+
−
AH
2
±
H
+
AH
AH
2
2
K
a
=
a
a
/a
=
(
γ γ
/
γ
)([
H A / AH
+
][
2
−
] [
−
]),
(2.6)
3
H A
+
2
−
AH
−
H A
AH
where
K
i
, a
i
, γ
i
, and terms in brackets are thermodynamic dissociation con-
stants, activities, activity coefficients, and molarities of the respective species.
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