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

A series of binuclear donor-acceptor complexes with helical polyprolin e 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 spect ra, electrochemistry and HPLC. The CD spectra of the complexes confirm that they exist in the helical polyproline II structure. Intramolecul ar electron transfer within these complexes was studied by generating the [ (bpy)2Ru(I)L.-(Pro)n-apy-Ru(III)(NH3)5] intermediate from the re action 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 fo r this reaction is estimated to be \DELTAG0\ almost-equal-to 1.5 V. Th e rates (k, 25-degrees-C) and activation parameters (DELTAH(double dag ger) (kcal/mol), DELTAS(double dagger) (eu) ) for the intramolecular e lectron 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 re spectively. The rate (k, 25-degrees-C) and activation parameters (DELT AH(double dagger) (kcal/mol), DELTAS(double dagger) (eu)) for the inte rmolecular 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 extend s our studies of the distance dependence of rate versus the number of helical prolines bridging a donor and acceptor ruthenium site to a met al-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(II I)(NH3)5], n=4-6. The rapid rates observed at these long distances sho w that long range electron transfer can be observed between an appropr iate donor and acceptor directly connected to the proline bridge via p eptide bonds at distances similar to the diameter of a small protein.