Evidence for Through-Space Electron Transfer in the Distance Dependence of Normal and Inverted Electron Transfer in Oligoproline Arrays

Four new helical oligoproline assemblies containing 16, 17, 18, and 19 proline residues and ordered arrays of a RuII−bipyridyl chromophore and a phenothiazine electron-transfer donor have been synthesized in a modular fashion by solid-phase peptide synthesis. These arrays are illustrated and abbreviated as CH3CO-Pro6-Pra(PTZ)-Pron-Pra(RuIIb2m)2+-Pro6-NH2, where PTZ is 3-(10H-phenothiazine-10)propanoyl and (RuIIb‘2m)2+ is bis(4,4‘-diethylamide-2,2‘-bipyridine)(4-methyl,4‘-carboxylate,2,2‘-bipyridine)ruthenium(II) dication with n = 2 (2), 3 (3), 4 (4), and 5 (5). They contain PTZ as an electron-transfer donor and (RuIIb‘2m)2+ as a metal-to-ligand charge transfer (MLCT) light absorber and are separated by proline-to-proline through-space distances ranging from 0 (n = 2) to 12.9 A (n = 5) relative to the n = 2 case. They exist in the proline-II helix form in water, as shown by circular dichroism measurements. Following laser flash RuII → b‘2m MLCT excitation at 460 nm in water, excited-state PTZ → Ru2+* quenc...