Chemistry Reference
In-Depth Information
acids at ionic strength of the biological environment in which amino acid side
chains are exposed (see below). The enthalpy of some amino acids in aqueous
NaCl solution has also been directly determined [54].
The dissociation constants are also affected by the ionic strength (
I
), defined
by Equation (2.8):
∑
I
=
1/2
m z
i
2
,
(2.8)
i
where
m
i
and
z
i
are the molality (mole per kilogram of solvent) and charge of
ion
i
. The dissociation constants at a given ionic strength of species
i K
i
p
in
NaCl solutions for Cys, as an example, are presented in Figure 2.2 [40]. Least-
squares fitting of the
p
K
i
*
results as a function of
I
is given in Equations
(2.9)-(2.11):
(
)
*
0 5
.
p
K
=
1 378 228 4
.
+
.
/
T
−
0 4044
.
I
+
0 2472
.
I
(2.9)
1
*
0 5
.
p
K
=
2 031 1833 4
.
+
.
/
T
−
0 1847
.
I
+
0 2190
.
I
(2.10)
2
*
0 5
.
p
K
=
2 861 2191 3
.
+
.
/
T
−
0 2170
.
I
+
0 2217
.
I
.
(2.11)
3
The
p
K
i
*
values at different
I
have been interpreted using the effect of ionic
strength on the activity coefficients [37-41, 55-58]. Various models have been
used to estimate activity coefficients of amino acids and peptides in aqueous
solutions [28, 32, 37, 38, 40, 41, 55, 57, 59-66]. The activity coefficients at dif-
ferent ionic strengths can be determined by using the suitable model. At low
ionic strength, the Davies equation has been applied (Eq. 2.12) [67]:
log
γ
i
= −
z AI
2
[
0
.
5
/ 1
(
+
I
0
.
5
)
−
bI
],
(2.12)
where
A
is the Debye-Huckel constant and
b
= 0.1.
A
is a function of tem-
perature and is given by [68]
A
=
1 8252 10
6
.
⋅
(
ρ
/
ε
3
T
3 1 2
)
/
,
(2.13)
W
where ρ
W
and ε are the density (gram per cubic centimeter) and the dielectric
constants of water, respectively. The dielectric constant depends on tempera-
ture and is expressed by Equation (2.14) [69]:
ε =
5321
/
T
+
233 76 0 9297
.
−
.
T
+
1 417 10
.
⋅
−
3
T
2
−
8 292 10
.
⋅
−
7
T
3
.
(2.14)
In addition to ionic strength, cations such as Na
+
, Rb
+
, Mg
2+
, and Ca
2+
also
influence p
K
i
values, which can also be interpreted using these models [37-39,
55, 56, 59, 60]. Overall, such information on the dissociation of amino acids can
be used to enhance understanding of complicated biological systems. For
example, quantitative information on the affinity of the sulfur side chain of
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