Self-assembled complexes of oligopeptides and metalloporphyrins: Measurements of the reorganization and electronic interaction energies for photoinduced electron-transfer reactions

Cationic porphyrins form ground state electrostatically associated complexes with anionic oligo-electrolytes such as those formed by a series of glutamic acid (E) residues. Temperature dependencies were measured of the rate constants for intra-complex electron transfer to the triplet state of Pd(II)TMPyP⁴⁺ from a tyrosine (tyr, Y) or tryptophan (trp, W) moiety connected to a glutamic acid tetramer. In complexes such as YE₄, E₂YE₂, YE₄G₁₀E (G, glycine), and WE₄ these data were used to estimate the reorganization energy (λ) and electronic interaction energy (H(DA)) relevant to the process. For all tyr-peptide complexes, λ values were found to be large (λ~1.60±0.06 eV), reflecting a relatively high medium polarity in the vicinity of tyr residues. It further indicates that the tyr residues in all oligo-peptides are exposed to the aqueous medium in a similar way irrespective of the position of the aromatic moiety in the peptide chain. A significantly lower λ value (λ=1.08 eV) was derived for the tryptophan-containing peptide complex, indicating a relatively higher hydrophobic character of trp compared to tyr. The electronic coupling matrix elements (H(DA)) derived for tyr-peptide complexes (5.1 meV for YE₄, 5.4 meV for YE₄G₁₀E and 7.5 meV for E₂YE₂) were larger than that found for WE₄ (1.1 meV). Molecular dynamics calculations were employed to obtain structural features of the porphyrin-peptide complexes. These showed average distances between the center of mass (COM) of the porphyrin ring and the center of mass of the amino acid aromatic ring of 816±140 pm (YE₄), 800±80 pm (E₂YE₂), 900±130 pm (YE₄G₁₀E) and 970±160 pm (WE₄). The molecular dynamics calculations were shown to be in good agreement with the experimentally determined electronic interaction energies, strongly suggesting that H(DA) is primarily responsible for the dependence of the electron-transfer rate constant (k(ET)) on the donor-acceptor separation distance and relative orientation. The higher H(DA) (7.55 meV) derived for tyr incorporated into the middle of the peptide backbone (E₂YE₂) was presumed to be associated with a higher degree of orbital overlap due to a more favorable ring-ring orientation. Overlap parameters (β derived for all peptide-porphyrin complexes were similar (~0.95±0.06 A^-1), being in good agreement with most literature values for similar systems. Finally, the intra-complex electron-transfer ratio (k(trp)/k(tyr)) derived from flash photolysis experiments and the corresponding ratio derived from Marcus' theory combined with experimental data from the temperature-dependence investigations and electrochemical measurements were found to be in excellent agreement. This same consistency was found for the couple E₄Y and E₂YE₂. The empirical expression (Moser and Dutton) governing the intraprotein electron-transfer rate constant in native systems combined with our experimental data (k(ET), λ, ΔG⁰) yielded tunneling pathway distances in excellent agreement with those arising from the molecular modeling studies. The exception was for the long peptide YE₄G₁₀E, for which the Quenched Molecular Dynamic (QMD) sampling technique was complicated and is probably inadequate. Copyright (C) 2000 Elsevier Science B.V.