Distance dependence of electron transfer across polyproline bridges: Experimental evidence for the existence of two distinct mechanisms.

摘要

The distance dependence of long range electron transfer (LRET) was studied across polyproline bridges in different series of Donor(D)-polyproline bridge-Acceptor(A) complexes. The distance was systematically varied through the use of polyproline bridges with varying lengths. Rigid and flexible Donor(D)-polyproline/glycine bridge-Acceptor(A) complexes were designed and synthesized to elucidate the mechanism(s) of long range electron transfer across polypeptide bridges. Results of kinetic measurements for (bpy)₂RuII(cmbpy)-Pro _n-apyRuIII(NH₃)₅ series (where bpy = 2,2′-bipyridine, cmbpy = 4-carboxy, 4′ -methyl-2,2′-bipyridine, Pro = proline, apy = 4-aminopyridine) show two regimes of distance dependence correlated with the existence of two mechanisms for electron transfer across polyproline bridges: the strong exponential dependence is accounted for by the superexchange mechanism (direct tunneling), while the weaker slope is correlated with a bridge mediated mechanism (sequential hopping). Picosecond kinetics in a series of (bpy) ₂RuII(cmbpy)-Pro_n-apyRuIII(NH ₃)₄(L) (where L = NH₃, pyridine, isonicotinamide) show double exponential decay. The fast component is in good agreement with the rate predicted from the exponential distance dependence, the second slower component is analyzed in terms of excitation transfer in the ruthenium trisbipyridine donor unit and/or conformational dynamics in the system. In another series (bpy)₂RuII(cmbpy)-Pro_n-apyOsIII (bpy)₂(Cl) (where n = 1−4), kinetic results showed the same exponential distance dependence observed in (bpy)₂Ru II(cmbpy)-Pro_n-apyRuIII(NH₃) ₅ series for n ≤ 4, except that double exponential kinetics in (bpy) ₂RuII(cmbpy)-Pro₂-apyOsIII(bpy) ₂(Cl) suggest the existence of two conformational states of the polyproline bridge. The two regimes of distance dependence for electron transfer across polyproline bridges were also observed in kinetic measurements carried out using electrochemical techniques of self-assembled monolayers (SAMs). The kinetic experiments were studied using SAMs connected to polyproline bridges with pendant (4-aminopyridine) ruthenium pentammine redox centers adsorbed on gold surfaces. In another two series, (bpy)₂RuII(cmbpy)-hexapeptide-apyRu III(NH₃)₅ and (DTFMB)₂RuII (cmbpy)-hexapeptide-apyRuIII(NH₃)₅ series (where DTFMB = 4,4′-ditrifluoromethyl-2,2 ′-bipyridine), electron transfer across hexapeptide bridges with different peptide secondary structures were studied. Results suggest that due to the difference of the orbital energies between the donor and the peptide bridge in the two series, different mechanisms of electron transfer are at play in the two series.