Intermolecular complex electron transfer process

The aim in solution studies on metalloprotein is to be able to say more about intermolecular electron transfer processes, first of all by studying outer-sphere reactions with simple inorganic complexes as redox partners. With the information (and experience) gained it is then possible to turn to protein-protein reactions, where each reactant has its own complexities... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

In inorganic compounds or complexes with variable valence system, a redox reaction may be set up by intramolecular or intermolecular electron transfer processes (redox reactions). 

Light absorption, by markedly affecting the electronic properties of molecules and metal complexes, may induce intra- or intermolecular electron transfer processes leading to electron-hole separation. 

Reaction of Cytochrome cimu with Tris(oxalato)cobalt(III) The cytochrome c protein was also used as reductant in a study of the redox reaction with tris (oxalato)cobalt(III).284 Selection of the anionic cobalt(III) species, [Conl(ox)3]3 was prompted, in part, because it was surmised that it would form a sufficiently stable precursor complex with the positively charged cyt c so that the equilibrium constant for precursor complex formation (K) would be of a magnitude that would permit it to be separated in the kinetic analysis of an intermolecular electron transfer process from the actual electron transfer kinetic step (kET).2S5 The reaction scheme for oxidation of cyt c11 may be outlined ... 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

Solutions of indium (I) can be prepared by treatment of indium amalgam with silver triflate in dry acetonitrile in the absence of oxygen, and then diluted with water to give the low-concentration aqueous solution, which plays a sizable role in the study of the details of intermolecular electron transfer processes in solution. Aqueous In(I) solution has been used to examine the behavior of this hypovalent center in inorganic redox transformations. Reactions with complexes of the type [(NH3)5Co (Lig)] and [(NH3)5Ru (Lig)] (Tig = Cl, Br , I or HC2O4 ) show two consecutive one-electron reactions initiated by the formation of the metastable state In , which is then rapidly oxidized to In , and the first of which is predominating an inner-sphere mechanism. ... 

Data are processed on a computer. The method is widely used for studying the kinetics of the acid-base equilibrium, intermolecular transfer, formation of metal complexes, electron transfer reactions, and enzymatic catalysis. The method measures rate constants of up to lO" l/(mol s). 

We will use here the main results obtained for two complex and distinct situations the structural and spectroscopic information gathered for D. gigas [NiFe] hydrogenase and AOR, in order to discuss relevant aspects related to magnetic interaction between the redox centers, intramolecular electron transfer, and, finally, interaction with other redox partners in direct relation with intermolecular electron transfer and processing of substrates to products. 

In the presence of oxygen, the lifetimes of both radical ion pairs (i.e., ZnP +-C6o and ZnP +-H2P-C6o ) are decreased significantly due to oxygen-catalyzed back-electron transfer (BET) processes between Ceo and ZnP " [76]. The catalytic participation of O2 in an intramolecular BET between Ceo and ZnP + in ZnP-linked Ceo is depicted in Scheme 6 [76]. The intermolecular ET from Ceo to O2 is facilitated by the partial coordination of O2 to ZnP " in the transient state (denoted as in Scheme 6) [76]. Consequently, the one-electron reduction potential of the resulting 02 is shifted toward positive values, namely in favor of the ET event. The strong coordination of O2 to Zn(II) ion has been well established [77]. The complexation is then followed by a rapid intramolecular... 

Intermolecular charge- and proton-transfer processes, that is, the motion of the proton associated with the change of electronic structure in hydrogen-bonded complexes, are involved in many chemical reactions in solution. The study of such processes in small clusters leads to very detailed information on the reactive event. 

Despite the clear implication of the involvement of intramolecular electron transfer in the chemiluminescence of certain dioxetanes, there have been no clear examples of intermolecular electron exchange luminescence processes with dioxetanes. In a recent note, however, Wilson (1979) reports the observation of catalysis of the chemiluminescence of tetramethoxy-1,2-dioxetane by rubrene and, most surprisingly, by 9,10-dicyanoanthracene. While catalysis by the added fluorescers was not kinetically discernible, a lowering of the activation energy for chemiluminescence was observed. These results were interpreted not in terms of an actual electron transfer with the formation of radical ions, but rather in terms of charge transfer interactions between fluorescer and dioxetane in the collision complex. In any event, these results certainly emphasize the need for caution in considering the fluorescer as a passive energy acceptor in dioxetane chemiluminescence. 

Figure 5. Schematic representation of bridge-mediated superexchange of the "electron" (top, left to right) and hole (bottom, right to left) type, illustrated for the case of intermolecular electron transfer between two metal/ligand (M/L) complexes, where D = M/ B = L/ L, and A = M,. The virtual intermediate states for the hole and eleetron processes involve, respectively, charge localization based on the filled ( valence") and empty ("conduction ) bands of the bridge.

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