Chemistry Reference
In-Depth Information
3
13
10
12
3
11
8
2
10
6
2
9
1
8
4
2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6
2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6
2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6
10
3
/
T
(K)
10
3
/
T
(K)
10
3
/
T
(K)
Figure 2.1.
p
K
1
, p
K
2
, and p
K
3
values of lysine, histidine, arginine, glutamic acid, and
tyrosine extrapolated to zero ionic strength at 283.1, 298.1, 313.1, and 333.1 K. Symbols
are
, Lys;
, His;
, Arg;
, Glu; Δ, Tyr;
, Thr;
, Phe; and
, Trp (data were taken
from Nagai et al. [32]).
amino acids with reactive species (Chapters 3-6). Most of available experi-
mental values of p
K
a
are at 25°C in aqueous solutions and knowing values at
37°C would be closer to human
in vivo
conditions. The effect of temperature
on dissociation constants can be determined by van't Hoff's equation:
K T K T
( )
=
(
)[ (
−
∆
h R
/
)(
1/
T
−
1/
T
),
(2.7)
a
a
o
diss
o
where
T
o
= 298.15 K and Δ
h
diss
is the dissociation enthalpy. Several studies have
measured the effect of temperature on the p
K
a
of amino acids in aqueous
solution [32, 50-53]. The temperature dependence of the thermodynamic dis-
sociation constants of amino acids is shown in Figure 2.1. With the exception
of Glu, the effect of temperature on the p
K
a
values was insignificant regarding
deprotonation of the carboxylic acid group of amino acids (Fig. 2.1). Glu
appeared to have temperature influence on the dissociation of the carboxylic
group attached to the side chain. The values of Δ
h
diss
/
R
at zero ionic strength
varied from −285 to 40 K [32]. Arg had a decrease in p
K
a1
with an increase in
temperature. The p
K
a2
values varied strongly with temperature, and the Δ
h
diss
/
R
values at zero ionic strength ranged from 3380 to 5155 K (Fig. 2.1). The p
K
a3
values decreased with an increase in temperature (Fig. 2.1). The temperature
greatly affected the p
K
a3
values of the aromatic hydroxyl group on the side
chain of Tyr, and Δ
h
diss
/
R
at zero ionic strength was obtained as 5713 K. Com-
paratively, the values of Δ
h
diss
/
R
at zero ionic strength were 1604, 2789, 4510,
and 5226 K for Arg, Glu, His, and Lys, respectively. Knowing the values of
Δ
h
diss
/
R
at zero ionic strength can be used to calculate p
K
i
at zero ionic strength
(Eq. 2.7), which are needed to determine accurately the dissociation of amino
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