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, N-AcHis, N-AcTyr, and N-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 kq = 2.2 × 10(9) M(-1) s(-1) (4.9 < pH < 9.4), kq = 1.6 × 10(9) M(-1) s(-1) (6.0 < pH < 9.2), and kq = 1.5 × 10(9) M(-1) s(-1) (4.9 < pH < 9.0), respectively. Tryptophan, histidine, and tyrosine with a neutral amino group quench the TCBP triplets with the corresponding rate constants kq = 8.0 × 10(8) M(-1) s(-1) (pH > 9.4), kq = 3.0 × 10(8) M(-1) s(-1) (pH > 9.2), and kq = (4.0-10.0) × 10(8) M(-1) s(-1) (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 N-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, N-AcTyr and N-AcTrp.
The
photo-oxidation of purine nucleotides adenosine-5′-monophosphate
(AMP) and guanosine-5′-monophosphate (GMP) by 3,3′,4,4′-benzophenone
tetracarboxylic acid (TCBP) has been investigated in aqueous solutions
using nanosecond laser flash photolysis (LFP) and time-resolved chemically
induced dynamic nuclear polarization (CIDNP). The pH dependences of
quenching rate constants and of geminate polarization are measured
within a wide range of pH values. As a result, the chemical reactivity
of reacting species in different protonation states is determined.
In acidic solution (pH < 4.9), the quenching rate constant is close
to the diffusion-controlled limit: kq =
1.3 × 109 M–1 s–1 (GMP), and kq = 1.2 × 109 M–1 s–1 (AMP), whereas in neutral
and basic solutions it is significantly lower: kq = 2.6 × 108 M–1 s–1 (GMP, 4.9 < pH < 9.4), kq = 3.5
× 107 M–1 s–1 (GMP,
pH > 9.4), kq = 1.0 × 108 M–1 s–1 (AMP, pH > 6.5).
Surprisingly,
the strong influence of the protonation state of the phosphoric group
on the oxidation of adenosine-5′-monophosphate is revealed:
the deprotonation of the AMP phosphoric group (6.5) decreases the
quenching rate constant from 5.0 × 108 M–1 s–1 (4.9 < pH < 6.5) to 1.0 × 108 M–1 s–1 (pH > 6.5).
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