Pulse radiolysis studies of intramolecular electron transfer in model peptides and proteins.: 8.: Trp[NH•+] → Tyr[O•] radical transformation in H-Trp-(Pro)n-Tyr-OH, n=3-5, series of peptides

The kinetics of intramolecular long-range electron transfer (LRET) between the protonated tryptophan indolyl radical cation Trp[NH.+], remaining in equilibrium with its neutral Trp[N-.] form, and tyrosine in aqueous H-Trp-(Pro)(n)-Tyr-OH, n = 3-5, peptides (abbreviated 3, 4, and 5) has been studied by pulse radiolysis at pH 2-8 at 298 K and at pH 4 over the temperature range 283-328 K. LRET has been found to occur down to pH 2, contrary to expectations based on the electrochemical redox potentials for Trp and Tyr in the form of free amino acids. The first-order rate constants of LRET, k(obs), varied sigmoidally with pH, allowing evaluation of pK(a1)'s for deprotonation of Trp[NH.+]: 3.7, 4.1, and 4.3 for 3, 4, and 5, respectively, as well as the intrinsic rate constants, k(2), of LRET involving solely Trp[NH.+] radicals. Variation of pk(a1) was attributed to electrostatic interactions between the indolyl radical and the COOH group of terminal Tyr, expected to decrease in the shown order, and taken as an indication of variation, in the same order, of the electrochemical driving force Delta G degrees of LRET. In 4 and 5, k(2) proved to be about 60-fold larger than the corresponding k(1) values characteristic for Trp[NH.+], which indicates that the kinetics of LRET involving Trp[NH.+] exhibits similar through-bond distance dependence as that found earlier for the reaction with TrD[N-.]. For 3 the ratio of k(2)/k(1) was found to be about 3-fold lower than for its two longer-bridged analogues, 4 and 5. Arrhenius activation energies, E-a2, of LRET involving Trp[NH.+] proved to be rather low for 4 and 5, 8.6 and 7.2 KJ mol(-1), respectively. In the case of 3, however, k(obs) varied nonlinearly with temperature. A nonlinear fit of Arrhenius type function to k(obs)(T) data under the assumption delta E-a2/delta T = 0 and delta pK(a1)/delta T = 0 gave E-a2 of 31.6 kJ mol(-1) and indicated a decrease in pK(a1) of Trp[NH.+] by about one unit in the studied temperature range. Conformational preferences of 3 were thus studied by means of molecular dynamics modeling to understand why the parameters of LRET in this peptide were so different from those of 4 and 5. It has been shown that a beta - alpha transition at the central psi(Pro(3)) dihedral angle, which could bring NH (indole) and COOH groups into a close contact compatible with formation of a hydrogen bond, may occur in 3 on the time scale of the observed electron-transfer reaction. In longer-bridged analogues two such concerted transitions would be necessary to perturb LRET measurably, an event of extremely low probability. It is thus argued that this transition may explain the kinetic and energetic peculiarities of LRET in 3. Analysis of thermodynamic parameters indicated that in reactions involving both Trp[NH.+] as well as Trp[N-.] radicals the free energy barrier of activation of LRET, Delta G(#), is for the most part entropic in nature. A more detailed analysis of thermodynamics of these reactions must await experimental determination of Delta G degrees for both reactions in the peptides studied.