Chemistry Reference
In-Depth Information
OSCN H O H NC O SO
−
+
→
(
=
)
−
.
(3.10)
2
2
The concentrations of HOSCN and OSCN
−
were determined using molar
absorptivities of 95 and 3870/M/cm at 240 and 220 nm, respectively [77]. An
additional spectrum of OSCN
−
at 376 nm has also been reported (ε = 26.5/M/
cm) [82].
3.1.3.1 Kinetics of HOSCN.
The reactions of HOSCN with Cys, Met,
cystine, gSSg, lys, His, and Trp were not detected (
k
< 10
3
/M/s) [84]. However,
thiol side chains and proteins reacted rapidly [84]. Table 3.3 provides the
second-order rate constants for the reaction of HOSCN with thiol residues
[84, 85]. Rate constants varied with the structure of the molecule. The rate
constants for low-molecular-mass thiols varied from 7.3 × 10
3
/M/s for
N
-acetyl-
cysteine at pH 7.4 to 7.7 × 10
6
/M/s for 2-nitro-5-thiobenzoic acid (TNB) at
pH 6.0. The rate constants decreased with an increase in pH, which was more
noticeable in the reaction of TNB than the reaction of gSH. The results
suggest the HOSCN species is much more reactive than the
−
OSCN species.
The rate constants, with the exception of penicillamine, could be correlated
with the p
K
a
of thiols, which had an inverse relationship (Fig. 3.10) [84]. Under
the same conditions, anionic thiols are stronger nucleophiles than neutral
thiols, and therefore, thiols with a low p
K
a
were expected to have higher reac-
tivity. Pencillamine is a sterically hindered thiol, which may be the reason for
not following the trend.
TABLE 3.3. Second-Order Rate Constants for the
Reaction of Thiols with HOSCN in 0.1 M Phosphate at
22°C
Substrate
pH
k
(/M/s)
5-Thio-2-nitrobenzoic acid
7.4
3.8 × 10
5
(TNB)
6.7
1.6 × 10
6
6.0
7.7 × 10
6
7.8 × 10
4
l-Cys
7.4
Cysteamine
7.4
7.1 × 10
4
N
-Acetyl-Cys
7.4
7.3 × 10
3
l-Cys-methyl ester
7.4
1.6 × 10
5
Penicillamine
7.4
3.4 × 10
5
gSH
7.4
2.5 × 10
4
6.7
3.1 × 10
4
4.0 × 10
4
6.0
BSA
7.4
7.6 × 10
4
7.4
3.1 × 10
4
7.4
1.0 × 10
4
7.4
1.4
×
10
4
Data are taken from References 84 and 85.
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