Chemistry Reference
In-Depth Information
constants of Met, Gly, L-Ala, L-Leu, L-Ile, L-Ser, and L-Phe in water-ethanol
and water-acetonitrile mixtures have also been determined to learn speciation
of these amino acids in a water-organic solvent medium [71, 72]. The ratios of
dipolar ionic form to neutral form vary with the polarity of the medium. As
discussed in subsequent chapters, the rates of the reactions vary with the spe-
ciation of functional groups of amino acids of proteins, and therefore, kinetic
studies under water-organic solvent conditions represent a more accurate
biological system.
2.2 SPECIATION
2.2.1 Protonation
The total amount of each amino acid dissolved in solution ([HA)]
total
) is the
sum of all existing dissolved amino acid (e.g., Cys) species (Eq. 2.15):
+
±
−
2
−
[
HA
]
total
=
[
H A
]
+
[
H A
]
+
[
HA
]
+
[
A
].
(2.15)
3
2
The fraction of these species (α
i
) is a function of pH and can be calculated
using Equations (2.16)-(2.19):
+
3
+
3
+
2
+
α
H A
=
[
H / H
] ([
]
+
[
H
]
K
+
[
H
]
K K
+
K K K
)
(2.16)
3
+
a
1
a
1
a
2
a
1
a
2
a
3
+
2
+
3
+
2
+
α
H A
=
[
H
]
K
/ H
([
]
+
[
H
]
K
+
[
H
]
K K
+
K K K
)
(2.17)
2
±
a
1
a
1
a
1
a
2
a
1
a
2
a
3
(
)
+
+
3
+
2
+
α
HA
=
[
H
]
K K
/ H
[
]
+
[
H
]
K
+
[
H
]
K K
+
K K K
(2.18)
−
a
1
a
2
a
1
a
1
a
2
a
1
a
2
a
3
+
3
+
2
+
α
A
=
K K K
/ H
([
]
+
[
H
]
K
+
[
H
]
K K
+
K K K
).
(2.19)
2
−
a
1
a
2
a
3
a
1
a
1
a
2
a
1
a
2
a
3
+
, H
3
A
+
;
The calculated fractions of species of Cys (
NH CH CH SH COOH
3
(
)
2
NH CH CH SH COO
3
( )
, HA
−
; NH
2
CH(CH
2
S
−
)
COO
−
, A
2−
) are depicted in Figure 2.4. The speciation of Cys at zero ionic
strength suggests the neutral and negatively charged forms occur at pH 7.4.
The neutral form predominates under this condition in which both the sulfur
and amino groups of the Cys molecule would be protonated. The speciation
of Cys would change slightly with an increase in ionic strength, but the neutral
species of Cys would still remain the major species at pH 7.4 (Fig. 2.4).
The calculated fractions of other selected amino acids relevant to redox
chemistry in the biological environment at pH 7.4 are given in Table 2.3. Most
of the amino acids exist in their monoprotonated forms (HA) as the major
species at nearly neutral pH. The major species of Tyr and Cys are in their
diprotonated forms (H
2
A). Deprotonated species (A) of Gly, His, Met, Trp,
and Phe are in very minor fractions. Tyr and Cys had an insignificant presence
of deprotonated forms (Table 2.3).
+
(
)
−
, H
2
A;
NH CH CH S COO
+
−
−
2
3
2
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