REACTIVE INORGANIC OXY-SPECIES OF C, N, S, AND P - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 245

atom abstraction from the phenol to the SO •− radical. This mechanism was
supported by dFT calculations in the gas phase and in the aqueous solution
[368]. Studies on the oxidation of fulvic and humic acids by SO •− showed the
formation of H-bonds to SO •− to yield adducts [371, 372]. Formation of radical
adducts has been suggested in the reactions of pyrimidine and pyridine [367,
374]. The rate constants for the reactions of the SO •− radical with a wide range
of β-lactum antibiotics in water were determined as (2.1 ± 0.6) × 10 9 /M/s and
(1.6 ± 0.9) × 10 9 /M/s for five- and six-member antibiotics at zero ionic strength
and 20-22°C [353]. The transient spectra obtained during the reactions showed
the SO •− radicals reacted at the five- or six-member rings adjacent to the β-
lactum moiety, mainly at the sulfur atom and the double bond, respectively
[353].
The kinetics for the oxidation of amino acids and peptides by SO •− radicals
have been performed (Table 5.9) [352, 377-380]. Peptides were found to be
more reactive than their parent free amino acids (Table 5.9). Aromatic amino
acids had a higher reactivity than aliphatic amino acids. The aromatic amino
acids confer a higher reactivity than those of aliphatic acids in the peptides
(Table 5.9). For example, Tyr and Trp amino acids containing peptides reacted
at diffusion-controlled rate constants, which are much faster than those con-
taining only gly and/or Ala [377].
The spectra of transients during the oxidation of amino acids and peptides
by SO •− radicals were used to understand the mechanisms of the reactions

TABLE 5.9. Rate Constants for Reactions of Amino Acids and Peptides with SO •-
Amino Acids

k (/M/s) Peptide k (/M/s)

gly gly-gly

(8.3 ± 1.0) × 10 7 [377] (3.7 ± 0.1) × 10

[6, 377]

(9.8 ± 0.8) × 10 7 [377] ∼9 × 10 [6, 352] gly-Ala

Ala (4.9 ± 0.1) × 10

[6, 377]

Ala-Ala (4.8 ± 0.4) × 10 7 [377]

Val 2.1 × 10 7 [378] gly-Trp (1.1 ± 0.1) × 10 10 [377]

Lys 1.6 × 10 7 [379] Trp-gly (1.5 ± 0.2) × 10 10 [377]

Arg 7.5 × 10 7 [380] gly-Tyr (1.2 ± 0.4) × 10 [10,

377]

Trp 2.9 × 10 [9, 377] Tyr-gly (2.7 ± 0.2) × 10 [10,

377]

Tyr-Val

8.2 × 10 8 [378] ∼2 × 10 [9, 352]

1.2 × 10 9 [378] Tyr ∼3 × 10 [9, 352] Val-Tyr

5.8 × 10 [9, 378] gly-gly-gly (1.2 ± 0.2) × 10 8 [377]

His ∼2.5 × 10 [9, 352] Ala-gly-gly (8.4 ± 0.8) × 10 [7, 377]

Val-Tyr-Val 4.4 × 10 8 [378]

gly-gly-gly-gly (1.3 ± 0.2) × 10 8 [377]