Intramolecular Electron Transfer in Peptides Containing Methionine, Tryptophan and Tyrosine: A Pulse Radiolysis Study

Summary

The kinetics of pulse radiolytically induced intramolecular radical transformations: Trp/N^·→Tyr/O^·, Tyr/O^·→Trp/N^·, Met/S∴Br→Trp/N^· and Met/S∴Br→Tyr/O^· has been investigated at various pH levels and temperatures in model peptides: Trp-(Pro)_n-Tyr, Trp-(Gly)_n-Tyr, Trp-(Pro)_n-Met (n = 0–3), Tyr-Phe-Met-Arg-Phe-NH₂·2AcOH, Met⁵-enkephaline and [d-Ala]²Met⁵-enkephaline. The rate constants of the Trp/N^·→Tyr/O^· transformation at pH 8 were found to decrease exponentially with the distance between Trp and Tyr in the proline peptides, while in the glycine peptides the rate of the transformation is less dependent on the number of bridging glycine residues. The activation energies determined fall into the range 10–20 kJ mol⁻¹ irrespective of: (i) the ionization state of tryptophyl radical and tyrosine, (ii) type of bridging amino acids, and (iii) reversal of the direction of the electron transfer upon tyrosine OH group ionization. The activation entropies at 298 K for the peptides of the glycine and proline series are negative and rather high, and fall into the relatively narrow range of −90 to −140 J mol⁻¹ deg⁻¹. These activation parameters seem to indicate that a tunnelling mechanism is involved in the electron transfer between strictly oriented aromatic moieties of Trp and Tyr. The variation of the activation parameters with average separation distance in the case of Trp-(Pro)_n-Tyr shows a predominance of the electronic factor over the nuclear in determining the distance dependence of the electron transfer rate. The intramolecular Met/S∴Br→Tyr/O^· transfer proceeds with the rate constant of 1·1 × 10⁵ s⁻¹ in Met⁵-enkephaline and 5·7 × 10⁴ s⁻¹ in [d-Ala]²Met⁵-enkephaline. The activation parameters for this transformation E_a = 30 kJ mol⁻¹ and ΔS⁴₂₉₈ = −62 J mol⁻¹ deg⁻¹ are close to those of the Trp/N^·→Tyr/O^· transformation, suggesting a similar mechanism for the electron transfer.