Modulation of the Rate of Reversible Electron Transfer in Oxidized Tryptophan and Tyrosine Containing Peptides in Acidic Aqueous Solution

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) was used to investigate reversible intramolecular electron transfer (IET) in short-lived oxidized peptides, which had different structures and contained tryptophan and tyrosine residues, in an acidic aqueous solution with a pH below the p<i>K</i>_a of the tryptophanyl cation radical. The CIDNP kinetic data were obtained at the microsecond scale and were analyzed in detail to calculate the rate constants for electron transfer in both directions: from the tyrosine residue to the tryptophanyl cation radical, <i>k</i>_f, and from the tryptophan residue to the neutral tyrosyl radical, <i>k</i>_r. The charge of the terminal amino group and the presence of glycine and proline spacers were shown to strongly affect the rate constants of the reaction under study. Among these functional groups, the presence and the location of the positive charge on the amino group in close proximity to the cationic indolyl radical had the strongest effect on the rate constant of the forward IET from the tyrosine residue to the tryptophanyl radical cation, <i>k</i>_f. This effect was manifested as an increase of 2 orders of magnitude in <i>k</i>_f for a change in the linkage order between residues in the dipeptide: <i>k</i>_f = 4 × 10³ s^–1 for the oxidized Tyr-Trp increased to <i>k</i>_f = 5.5 × 10⁵ s^–1 in oxidized Trp-Tyr. The reverse rate constant for IET was less sensitive to the amino group charge. Moreover, the presence of glycine or proline spacers in the peptides with a tryptophan residue at the N-terminus not only reduced the IET rate constant but also shifted the equilibrium of the IET in the reaction under study toward the formation of tyrosyl radicals with respect to the peptide Trp-Tyr. That is, the glycine or proline spacers affected the difference in the reduction potential of the tryptophanyl and tyrosyl radicals

Effect of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino Acids

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>_q = 2.2 × 10⁹ M^–1 s^–1 (4.9 < pH < 9.4), <i>k</i>_q = 1.6 × 10⁹ M^–1 s^–1 (6.0 < pH < 9.2), and <i>k</i>_q = 1.5 × 10⁹ 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 <i>k</i>_q = 8.0 × 10⁸ M^–1 s^–1 (pH > 9.4), <i>k</i>_q = 3.0 × 10⁸ M^–1 s^–1 (pH > 9.2), and <i>k</i>_q = (4.0–10.0) × 10⁸ 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 <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