Electron Transfer in Ruthenium-Modified Plastocyanin

Reaction of Scenedesmus obliquus plastocyanin with excess [Ru(trpy)(L)(H2O)]2+ (trpy = 2,2‘:6‘,2‘‘-terpyridine; L = 2,2‘-bipyridine, 4,4‘-(CH3)2-2,2‘-bipyridine, 4,5,4‘,5‘-(CH3)4-2,2‘-bipyridine) affords Ru(trpy)(L)(His59)Pc as the main product. These RuPc derivatives are luminescent with λmax(emiss...

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Published in:Journal of the American Chemical Society 1998-08, Vol.120 (30), p.7551-7556
Main Authors: Di Bilio, Angel J, Dennison, Christopher, Gray, Harry B, Ramirez, Benjamin E, Sykes, A. Geoffrey, Winkler, Jay R
Format: Article
Language:English
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Summary:Reaction of Scenedesmus obliquus plastocyanin with excess [Ru(trpy)(L)(H2O)]2+ (trpy = 2,2‘:6‘,2‘‘-terpyridine; L = 2,2‘-bipyridine, 4,4‘-(CH3)2-2,2‘-bipyridine, 4,5,4‘,5‘-(CH3)4-2,2‘-bipyridine) affords Ru(trpy)(L)(His59)Pc as the main product. These RuPc derivatives are luminescent with λmax(emission) ∼ 650 nm and lifetimes of (Cu+) in the range 110−140 ns. Photogenerated *Ru2+PcCu2+ is quenched by *Ru2+ → Cu2+ electron transfer (ET) to produce Ru3+PcCu+; intramolecular ET was monitored by transient absorption at 590 (Cu+ → Cu2+) and 424 nm (Ru3+ → Ru2+). The Cu+ to Ru3+ ET rate constants (k ET) are as follows:  2.9(2) × 107 s-1 (L = bpy); 2.3(2) × 107 s-1 (L = dmbpy); and 1.9(2) × 107 s-1 (L = tmbpy). Activationless rates (−ΔG° ∼ λ ∼ 0.70−0.75 eV) are consistent with coupling-limited tunneling through a β sheet at an estimated Cu−Ru distance of 15.6 Å (calcd k ET = 107 s-1 for a tunneling decay constant of 1.1 Å-1). Biphasic Cu+ → Ru3+ ET kinetics (k ET > 107 and ∼104 s-1) were observed after flash-quench generation of Ru3+PcCu+ in acidic solutions. The slow phase kinetics are markedly temperature and pH dependent:  the activation parameters (ΔH ⧧ = 43.1 kJ/mol; ΔS ⧧ = −17 J/(K·mol) for L = bpy) suggest that the trigonal low-pH form of Cu+ reorganizes to the tetrahedral form prior to oxidation to the blue Cu2+ state.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja972625b