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Electron transfer in ruthenium-modified plastocyanin

Lookup NU author(s): Professor Christopher Dennison, Emeritus Prof Alfred Sykes

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Abstract

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 λ(max)(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) x 107 s-1 (L = bpy); 2.3(2) x 107 s-1 (L = dmbpy); and 1.9(2) x 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; ΔSα= = -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.


Publication metadata

Author(s): Di Bilio AJ, Dennison C, Gray HB, Ramirez BE, Sykes AG, Winkler JR

Publication type: Article

Publication status: Published

Journal: Journal of the American Chemical Society

Year: 1998

Volume: 120

Issue: 30

Pages: 7551-7556

Print publication date: 21/07/1998

ISSN (print): 0002-7863

ISSN (electronic): 1520-5126

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/ja972625b

DOI: 10.1021/ja972625b


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