Evidence for through-space electron transfer in the distance dependence of normal and inverted electron transfer in oligoproline arrays

Abstract

Four new helical oligoproline assemblies containing 16, 17, 18, and 19 proline residues and ordered arrays of a Ru-II-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-Pro(6)-Pra(PTZ)-Pro(n)-Pra(Ru(II)b(2)M)(2+)-Pro(6)-NH2, where PTZ is 3-(10H-phenothiazine-10)propanoyl and (Ru(II)b'M-2)(2+) is bis(4,4'-diethyl amide-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 (Ru(II)b'M-2)(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 Angstrom (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 Ru-II --> b'(2)m MLCT excitation at 460 nm in water, excited-state PTZ --> Ru2+* quenching (k(2)) occurs by reductive electron transfer, followed by Ru+ --> PTZ(+) back electron transfer (k(3)), as shown by transient absorption and emission measurements in water at 25degreesC. Quenching with DeltaGdegrees = -0.1 eV is an activated process, while back electron transfer occurs in the inverted region, DeltaGdegrees = -1.8 eV, and is activationless, as shown by temperature dependence measurements. Coincidentally, both reactions have comparable distance dependences, with k(2) varying from = 1.9 x 10(9) (n = 2) to 2.2 x 10(6) s(-1) (n = 4) and k(3) from similar to2.0 x 10(9) (n = 2) to 2.2 x 10(6) s(-1) (n = 4). For both series there is a rate constant enhancement of similar to10 for n = 5 compared to n = 4 and a linear decrease in In k with the through-space separation distance, pointing to a significant and probably dominant through-space component to intrahelical electron transfer.