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

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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.
Publisher
AMER CHEMICAL SOC
Issue Date
2004-11
Language
English
Article Type
Article
Keywords

MLCT EXCITED-STATES; POLY-L-PROLINE; AMINO-ACIDS; SOLVENT DEPENDENCE; CHARGE-TRANSFER; COMPLEXES; ENERGY; OLIGOPEPTIDES; POLYPEPTIDES; OSMIUM(II)

Citation

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, v.126, no.44, pp.14506 - 14514

ISSN
0002-7863
DOI
10.1021/ja030659f
URI
http://hdl.handle.net/10203/203354
Appears in Collection
CH-Journal Papers(저널논문)
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