Electron Transfer in Ruthenium-Modified Plastocyanin

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 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.

The Importance of the Long Type 1 Copper‐Binding Loop of Nitrite Reductase for Structure and Function

“…Protonation of the His ligand at a T1 site results in Cu I becoming three-coordinate, an arrangement that stabilises this oxidation state of the metal and leads to a striking increase in E m [13,27,[46][47][48][49] and a dramatic decrease in ET (owing to an increased reorganisation energy). [50,51] The E m value for the T1 site of NIRAMI is approximately 200 mV higher than that of WT NIR and agrees with the value reported for the influence of protonation and dissociation of the His ligand on the E m value of AMI, [47] indicating that structural changes occur in NIRAMI upon reduction to stabilize Cu I over Cu II . The structure of the T2 centre (the site of nitrite binding and reduction) is almost identical in NIRAMI and WT NIR, and its properties, including the E m value, should not have been significantly influenced by the loop contraction.…”

Plastocyanin

“…Posi- Intra-molecular electron transfer rates have been measured in Scenedesmus obliquus plastocyanin by attaching a Ru 2+ complex to a non-ligand histidine side chain remote from the copper site, and using photo-generated Ru 3+ to oxidize the Cu + atom. 62 The results are consistent with the coupling-limited tunneling through a b-sheet. In acidic solutions, the kinetics are biphasic, suggesting that fast electron transfer is preceded by a slower re-organization of the redox-inactive low-pH form HCu + Pc to the redox-active form Cu + Pc.…”

Toward Ab Initio Theory of Longdistance Electron Tunneling in Proteins: Tunneling Currents Approach

“…The expressions for self-energies C, , and Cdd have the same form as T f l =&, and can be computed using the same technique of sparse matrices as for Tfi discussed above. Calculations tested on Ru-modified proteins [4,5] showed that the new expression gives results which are nearly identical to those obtained with exact diagonalization of the Hamiltonian matrix, even when intermediate resonances are present in the medium (in contrast to Tfl, which diverges for such cases). The advantage of the former method, of course, is that it can be applied to systems that may be too large for a direct diagonalization.…”

“…19, 55, and 561, where the coupling is not difficult to calculate. The tunneling currents '4,A. STLCHEBRUKHOV approach provides yet another perspective on the problem and introduces new computational techniques for studies of electron tunneling in molecular systems.…”