DISTANCE DEPENDENCE OF INTRAMOLECULAR ELECTRON-TRANSFER ACROSS OLIGOPROLINES IN [(BPY)2RUIIL-(PRO)N-COIII(NH3)5]3+, N = 1-6 - DIFFERENT EFFECTS FOR HELICAL AND NONHELICAL POLYPROLINE-II STRUCTURES

A series of complexes of the type [(bpy)2RuIIL-(Pro)n-CoIII(NH3)5]4+, n = 1-6, where L = 4-carboxy-4'-methyl-2,2'-bipyridine, bpy = 4,4'-bipyridine, and Pro = l-proline, have been synthesized from the corresponding [(bpy)2RuIIL] and [(NH3)5CoIII(Pro)n] components. The compounds were characterized by metal analyses, electrochemical measurements, and absorption spectroscopy. For n = 4-6 prolines, the CD spectra of the complexes show a polyproline II helical structure. Intramolecular electron transfer within these complexes was studied by generating the [(bpy)2RuIIL.-(Pro)n-CoIII(NH3)5] intermediate by the reaction of e(aq) (generated by pulse radiolysis) with the [(bpy)2RuIIL-(Pro)n-CoIII(NH3)5] molecules. The driving force for this reaction is estimated to be \DELTAG-degrees\ approximately -1.1 V. The intramolecular electron transfer rates (k) and activation parameters (DELTAH(double dagger) (kcal/mol), DELTAS(double dagger) (eu)) found for these studies were (1.6 +/- 0.1) X 10(7) s-1, 6.0 +/- 0.6, -6 +/- 2; (2.3 +/- 0.2) x 10(5) s-1, 9.2 +/- 0.4, -3 +/- 1; (5.1 +/- 0.4) X 10(4) s-1, 9.4 +/- 0.2, -5.5 +/- 0.8; (1.8 +/- 0.1) x 10(4) S-1, 9.0 +/- 0.4, -9 +/- 1; and (8.9 +/- 0.6) X 10(3) S-1, 8.8 +/- 0.4, -11 +/- 1 for n = 2-6, respectively. For n = 1 proline, k is >5 X 10(8) s-1 and no temperature dependence could be determined. The rates of intramolecular electron transfer decrease rapidly with distance for n = 1-3 prolines but show a surprisingly weak decrease with distance for the n = 4, 5, and 6 prolines, which exhibit the polyproline II helical structure. The electron-transfer pathways within these molecules and the relationship of the electron-transfer rates to the helical polyproline II structure are discussed.