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 lambda(max)(emission) similar to 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 10(7) s(-1) (L = bpy); 2.3(2) x 10(7) s(-1) (L = dmbpy); and 1.9(2) x 10(7) s(-1) (L = tmbpy). Activationless rates (-Delta G degrees similar to lambda similar to 0.70-0.75 eV) are consistent with coupling-limited tunneling through a beta sheet at an estimated Cu-Ru distance of 15.6 Angstrom(calcd k(ET) = 10(7) s(-1) for a tunneling decay constant of 1.1 Angstrom(-1)). Biphasic Cu+ --> Ru3+ ET kinetics (k(ET) > 10(7) and similar to 10(4) 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(Delta H double dagger = 43.1 kJ/mol; Delta S double dagger = -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.