title: 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 CH(3)CO-Pro(6)-Pra(PTZ)-Pro(n)()-Pra(Ru(II)b(2)m)(2+)-Pro(6)-NH(2), where PTZ is 3-(10H-phenothiazine-10)propanoyl and (Ru(II)b'(2)m)(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 (Ru(II)b'(2)m)(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 Ru(II) --> b'(2)m MLCT excitation at 460 nm in water, excited-state PTZ --> Ru(2+) 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 25 degrees C. Quenching with DeltaG degrees = -0.1 eV is an activated process, while back electron transfer occurs in the inverted region, DeltaG degrees = -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 approximately 2.0 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 approximately 10 for n = 5 compared to n = 4 and a linear decrease in ln k with the through-space separation distance, pointing to a significant and probably dominant through-space component to intrahelical electron transfer.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill, CB # 3290, Chapel Hill, North Carolina 27599-3290, USA.
authors.forename: Scafford A
authors.initials: SA
authors.lastname: Serron
authors.forename: W Stephen
authors.initials: WS
authors.lastname: Aldridge Iii
authors.forename: Cavan N
authors.initials: CN
authors.lastname: Fleming
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Danell
authors.forename: Mu-Hyun
authors.initials: MH
authors.lastname: Baik
authors.forename: Milan
authors.initials: M
authors.lastname: Sykora
authors.forename: Dana M
authors.initials: DM
authors.lastname: Dattelbaum
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meyer
mesh_terms: D004583:Electrons; D007700:Kinetics; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*; D010084:Oxidation-Reduction; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D009842:Oligopeptides; D010455:Peptides; C011083:polyproline