Effect
of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino
Acids
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Abstract
The
kinetics of the photooxidation of aromatic amino acids histidine (His),
tyrosine (Tyr), and tryptophan (Trp) by 3,3′,4,4′-benzophenonetetracarboxylic
acid (TCBP) has been investigated in aqueous solutions using time-resolved
laser flash photolysis and time-resolved chemically induced dynamic
nuclear polarization. The pH dependence of quenching rate constants
is measured within a large pH range. The chemical reactivities of
free His, Trp, and Tyr and of their acetylated derivatives, <i>N</i>-AcHis, <i>N</i>-AcTyr, and <i>N</i>-AcTrp, toward TCBP triplets are compared to reveal the influence
of amino group charge on the oxidation of aromatic amino acids. The
bimolecular rate constants of quenching reactions between the triplet-excited
TCBP in the fully deprotonated state and tryptophan, histidine, and
tyrosine with a positively charged amino group are <i>k</i><sub>q</sub> = 2.2 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (4.9 < pH < 9.4), <i>k</i><sub>q</sub> = 1.6 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (6.0 < pH < 9.2), and <i>k</i><sub>q</sub> = 1.5
× 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (4.9
< pH < 9.0), respectively. Tryptophan, histidine, and tyrosine
with a neutral amino group quench the TCBP triplets with the corresponding
rate constants <i>k</i><sub>q</sub> = 8.0 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> (pH > 9.4), <i>k</i><sub>q</sub> = 3.0 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> (pH > 9.2), and <i>k</i><sub>q</sub> = (4.0–10.0) × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> (9.0 < pH < 10.1) that are close to
those for the N-acetylated derivatives. Thus, it has been established
that the presence of charged amino group changes oxidation rates by
a significant factor; i.e., His with a positively charged amino group
quenches the TCBP triplets 5 times more effectively than <i>N</i>-AcHis and His with a neutral amino group. The efficiency of quenching
reaction between the TCBP triplets and Tyr and Trp with a positively
charged amino group is about 3 times as high as that of both Tyr and
Trp with a neutral amino group, <i>N</i>-AcTyr and <i>N</i>-AcTrp