Electron self-exchange in azurin: calculation of the superexchange electron tunneling rate.

Abstract

Electronic coupling between the copper atoms in an azurin dimer has been calculated in this conformationally well-defined system by using many-electronic wave functions. When one of the two water molecules forming intermolecular hydrogen bonds between the copper-ligating His-117 of the two azurins is removed, the calculated coupling element is reduced from 2.5 x 10-6 to 1.1 x 10-7 eV (1 eV = 1.602 x 10-19 J). Also, the effects of the relative orientations of the two water molecules have been analyzed. The results show that water molecules may play an important role as switches for biological electron transfer. The rate of electron self-exchange between two azurins has been calculated, and the result is in very good agreement with the rate found experimentally.Long-range electron transfer (ET) plays a major role in many biological processes-e.g., in photosynthesis and other energy conversion systems (1, 2). Many recent reports have documented that ET can take place over large distances in both native and modified protein systems and investigations were made on the effects of driving force, reorganization energy, distance, and the nature of the intervening medium on the rate of long-range .Azurins are blue single copper proteins found in many bacterial systems, where they function as mobile electron carriers in the respiratory system. The three-dimensional structure is now available for a number of both natural (wild type) and single-site mutated azurins (9-12), which makes this group of proteins highly interesting for studies of the different factors that control the rate of long-range ET (13). The rate of self-exchange reactions in both Pseudomonas aeruginosa azurin (14-16) and Alcaligenes denitrificans azurin (17) have been determined by 'H NMR line broadening and by rapid-freeze ESR as a function of temperature, pH, and ionic strength. The structure ofthe binary complex in the self-exchange reaction is unknown, but the NMR studies described above as well as mutational studies implicate the hydrophobic surface patch surrounding the Cu ligand His-117 (12). The association along this patch would minimize the distance between the two metal centers. Crystallographic data support this notion, since in several azurin crystal polymorphs the molecules are pairwise packed in this manner (9-12). The three-dimensional structure of P. aeruginosa azurin shows that the imidazole rings of His-117 in the azurin dimer are interconnected via hydrogen-bond formation to two water molecules (10). NMR 1H/2H exchange experiments (18) indicate that these water molecules are also tightly bound in solution. On the basis of these findings, an involvement of the water molecules in the ET pathway of the self-exchange reaction as well as the cross-exchange reacThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.tions with cytochrome c and nitrite reductase has been suggested (10...

Photogeneration and Quenching of Tryptophan Radical in Azurin

“…When the electron acceptor is Cu II Az, the situation is different because an azurin dimer has been proposed to form via the large hydrophobic patch on the protein surface. 65,66 Experiments that illustrated relatively fast electron self-exchange rates for the dimer focused on a high concentration of azurin, on the order of 1–2 mM. 67 Recently, a noncovalent dimer of rhenium-labeled azurin was reported to undergo tryptophan-mediated electron hopping through the protein interface; the dimer was found to exist at low concentrations near 70 μ M. 68 The present observation that reduction of Cu II Az proceeds quantitatively with formation of W48• strongly supports the presence of a dimer that enables interprotein ET under the present experimental concentrations of ∼50 μ M. An ET path through a dimer protein interface is proposed in the Supporting Information.…”

Engineering type 1 copper sites in proteins

“…Possibly this water molecule is part of the electron transfer pathway. Theoretical estimates show that in the absence of this water molecule electron transfer may be slowed down by as much as an order of magnitude [81].…”

Quantum Stochastic Synchronization

“…For ET occurring in an azurin dimer, the values are 0:25 eV and @ 5 10 ÿ6 eV [19]. In molecular systems, the cutoff frequency of low-frequency molecular vibrations ranges between @!…”

“…This driven spin-boson Hamiltonian (1) describes an abundance of applications [18] such as, e.g., the electron transfer (ET) in a molecular dimer in azurin crystals [19]. There the low-frequency molecular vibrations provide the bath, and the timedependent energy bias is given by t reEt, where r is the tunneling distance, e denotes the charge transferred, and Et is the time-dependent, applied electric field.…”