LONG-RANGE ELECTRON-TRANSFER IN HELICAL POLYPROLINE-II OLIGOPEPTIDES

A series of binuclear donor-acceptor complexes with helical polyproline bridges [ (bpy)2Ru(II)L-(Pro)n-apy-Ru(III)(NH3)5]5+ n=6, 7, 9, where L=4-carboxy-4'-methyl-2,2'-bipyridine, bpy=4,4'-bipyridine, and apy=4-aminopyridine, were synthesized and characterized by absorption spectra, electrochemistry and HPLC. The CD spectra of the complexes confirm that they exist in the helical polyproline II structure. Intramolecular electron transfer within these complexes was studied by generating the [ (bpy)2Ru(I)L.-(Pro)n-apy-Ru(III)(NH3)5] intermediate from the reaction of e(aq) (from pulse radiolysis) with the [ (bpy)2Ru(II)L-(Pro)n-apy-Ru(III)(NH3)5] species in aqueous solution. The driving force for this reaction is estimated to be \DELTAG0\ almost-equal-to 1.5 V. The rates (k, 25-degrees-C) and activation parameters (DELTAH(double dagger) (kcal/mol), DELTAS(double dagger) (eu) ) for the intramolecular electron transfer were found to be: 1.08 X 1O(5) s-1, 5.6, - 17; 6.40 x 1O(4) s-1, 5.1, - 19; 1.91 X 10(4) s-1, 4.0, and -26 for n=6, 7, 9 respectively. The rate (k, 25-degrees-C) and activation parameters (DELTAH(double dagger) (kcal/mol), DELTAS(double dagger) (eu)) for the intermolecular reaction between [(bpy)2Ru(I)L.] and [(NH3)5Ru(III)-apy-Pro] were found to be 2.l x 10(9) M-1 s-1, 3.3 and -5. This series extends our studies of the distance dependence of rate versus the number of helical prolines bridging a donor and acceptor ruthenium site to a metal-to-metal distance almost-equal-to 40 angstrom. The weak dependence of rate versus the number of prolines observed for n = 6, 7, and 9 is very similar to that observed earlier for [ (bpy)2Ru(II)L-(Pro)n-Co(III)(NH3)5], n=4-6. The rapid rates observed at these long distances show that long range electron transfer can be observed between an appropriate donor and acceptor directly connected to the proline bridge via peptide bonds at distances similar to the diameter of a small protein.