Chemistry Reference
In-Depth Information
•
2
. This general trend does not necessarily
represent one-electron oxidizing capability as the
SO
•−
radical is a stronger
oxidant (2.5-3.1V vs. NHE) than the
•
OH (2.3V vs. NHE in neutral solution).
Therefore, the relative reactivity of the sulfate and phosphate radicals with
inorganic and compounds predict the reduction potential tendencies as
E(SO ) E(H PO ) E(HPO ) E(PO )
4
OH SO
>
•−
>
H PO HPO
•
>
•−
>
PO
• −
4
2
4
4
4
•−
>
•
>
•−
>
• −
2
[386]. The selectivity of
SO
•−
2
4
4
4
and
H PO
2
•
radicals were higher than the
•
OH radical. Reactions of substituted
benzenes go through addition and hydrogen atom abstractions, while sulfate
radicals involved electron transfer reactions. The effect of the substituents has
been demonstrated in a Hammett-type plot for substituted benzenes [389,
391]. The Hammett parameters (ρ
+
) were determined as −1.5, −1.2, −1.3, and
−1.7 for reactions of
SO
•−
,
H PO
2
4
•
,
HPO
•−
, and
PO
4
• −
, respectively [387, 392].
The negative values of ρ
+
suggests that the reactions were favored by high
electron density at the site of the reaction [387]. Comparatively, the parameter,
ρ, for the reaction of
•
OH with aromatic compounds was determined between
−0.5 and −0.4 [393].
The reactivity of phosphate radicals with amino acids and peptides is pre-
sented in Table 5.10 [349, 377, 378]. Similar to
SO
•−
radical,
HPO
•−
and
PO
4
• −
radicals reacted faster with peptides than free amino acids. Also, aromatic
amino acids had a higher reactivity than aliphatic amino acids. Significantly,
4
TABLE 5.10. Second-Order Rate Constants for the Rate Constants of
HPO
•-
and
PO
4
• -
with Amino Acids and Peptides [349, 377, 378]
k
(/M/s)
Amino Acids/Peptides
PO
4
• −
HPO
•−
(5.2 ± 0.6) × 10
5
gly
-
Ala
(9.4 ± 0.6) × 10
5
-
Val
4.1 × 10
7
1.6 × 10
7
Trp
(2.7 ± 0.2) × 10
8
-
Tyr
4.0 × 10
8
1.2 × 10
8
Ala-Ala
(2.1 ± 0.2) × 10
6
-
gly-Ala
(5.0 ± 0.2) × 10
6
-
(5 ± 1) × 10
9
gly-Trp
-
Trp-gly
(2.6 ± 0.5) × 10
9
-
gly-Tyr
(2.0 ± 0.9) × 10
9
-
Tyr-gly
(2.1 ± 0.1) × 10
9
-
Val-Tyr
2.2 × 10
8
1.0 × 10
8
Tyr-Val
5.9 × 10
8
1.2 × 10
8
gly-gly-gly
(1.6 ± 0.1) × 10
7
-
Ala-gly-gly
(1.3 ± 0.1) × 10
7
-
4.7 × 10
8
1.4 × 10
8
Val-Tyr-Val
gly-gly-gly-gly
(1.3 ± 0.1) × 10
7
-
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