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 complexe s with anionic oligo-electrolytes such as those formed by a series of gluta mic acid (E) residues. Temperature dependencies were measured of the rate c onstants 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, YE(4)G(10)E (G, glycine), and WE4 these data were used to estimate the reorganization ener gy (lambda) and electronic interaction energy (H-DA) relevant to the proces s. For all tyr-peptide complexes, lambda values were found to be large (lam bda similar to 1.60 +/- 0.06 eV), reflecting a relatively high medium polar ity in the vicinity of tyr residues. It further indicates that the tyr resi dues in all oligo-peptides are exposed to the aqueous medium in a similar w ay irrespective of the position of the aromatic moiety in the peptide chain . A significantly lower lambda value (lambda = 1.08 eV) was derived for the tryptophan-containing peptide complex, indicating a relatively higher hydr ophobic character of trp compared to tyr. The electronic coupling matrix el ements (H-DA) derived for tyr-peptide complexes (5.1 meV for YE4, 5.4 meV f or YE(4)G(10)E and 7.5 meV for E2YE2) were larger than that found for WE4 ( 1.1 meV). Molecular dynamics calculations were employed to obtain structura l features of the porphyrin-peptide complexes. These showed average distanc es 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 p m (E2YE2), 900 +/- 130 pm (YE(4)G(10)E) and 970 +/- 160 pm (WE4). The molec ular dynamics calculations were shown to be in good agreement with the expe rimentally determined electronic interaction energies, strongly suggesting that H-DA is primarily responsible for the dependence of the electron-trans fer rate constant (k(ET)) on the donor-acceptor separation distance and rel ative orientation. The higher H-DA (7.55 meV) derived for tyr incorporated into the middle of the peptide backbone (E2YE2) was presumed to be associat ed with a higher degree of orbital overlap due to a more favorable ring-rin g orientation. Overlap parameters (beta derived for all peptide-porphyrin c omplexes were similar (similar to 0.95 +/- 0.06 Angstrom(-1)), being in goo d agreement with most literature values for similar systems. Finally, the i ntra-complex electron-transfer ratio (k(trp)/k(tyr)) derived from flash pho tolysis experiments and the corresponding ratio derived from Marcus' theory combined with experimental data from the temperature-dependence investigat ions and electrochemical measurements were found to be in excellent agreeme nt. This same consistency was found for the couple E4Y and E2YE2. The empir ical expression (Moser and Dutton) governing the intraprotein electron-tran sfer rate constant in native systems combined with our experimental data (k (ET), lambda, Delta G(0)) yielded tunneling pathway distances in excellent agreement with those arising from the molecular modeling studies. The excep tion was for the long peptide YE(4)G(10)E, for which the Quenched Molecular Dynamic (QMD) sampling technique was complicated and is probably inadequat e. (C) 2000 Elsevier Science B.V. All rights reserved.