Electron transfer rate saturation

This apparent low efficiency for reducing N2 was observed under conditions of saturating N2 and is not due to restrictions in electron transfer rate. Furthermore, the low efficiency is mirrored in bacterial cultures at 30°C (185). 

Early attempts at observing electron transfer in metalloproteins utilized redox-active metal complexes as external partners. The reactions were usually second-order and approaches based on the Marcus expression allowed, for example, conjectures as to the character and accessibility of the metal site. xhe agreement of the observed and calculated rate constants for cytochrome c reactions for example is particularly good, even ignoring work terms. The observations of deviation from second-order kinetics ( saturation kinetics) allowed the dissection of the observed rate constant into the components, namely adduct stability and first-order electron transfer rate constant (see however Sec. 1.6.4). Now it was a little easier to comment on the possible site of attack on the proteins, particularly when a number of modifications of the proteins became available. 

In the absence of substrate at MU-modified Au electrode oxidation and reduction was measured which corresponded to the Fe /Fe redox couple of the heme site with E° = -120 mV [120]. The heterogeneous electron transfer rate constant is in the order of = 15 s. The oxidation peak current increases proportionally to sulfite concentrations between 10 and 100 /aM. The current increase is approaching saturation at concentrations higher than 3 mM. 

In contrast to the cobalt-based system, small amounts of H2 and no CO are produced when nickel cyclam or other saturated 14-membered tetraazamacrocycles (L) in Figure 3 are used to replace the cobalt complex in the above system [22]. Flash photolysis studies indicate that the electron-transfer rate constant (kn) for the reaction of the />-terphenyl radical anion with Nil (cyclam)2 is 4.3 x 10 M s. However, when CO2 is added to the solution, the decay of the TP anion becomes slower Flash photolysis studies of the acetonitrile solutions... 

Bimolecular electron transfer proceeds by a number of steps prior and subsequent to the actual electron transfer, any one of which can become rate determining and cause the rate to saturate or plateau below the diffusion-controlled limit. Rate saturation means that the electron-transfer rate no longer increases with increasing driving force in limiting cases the rate also may become independent of the concentration of one of the reactants. 

Clear-cut experimental evidence for the existence of an inverted region was provided in 1984 by the work Miller et al.352 on long-distance intramolecular electron transfer in the rigidly linked bichromophoric radical anions 9 (Figure 5.4), which were generated by pulsed electron injection. The electrons are initially captured with nearly equal probability by either the donor chromophore biphenyl or the acceptor chromophore mounted on the other side of the saturated 5a-androstane spacer. Electron transfer rates to attain equilibrium were determined by time-resolved observation of the ensuing absorbance changes. 

Changes in the degree of saturation of the macrocycle affect the rate constants in a minor way. Similar rate constant values are obtained for both the porphyrin and the octaethylisobacteriochlorin (OEIBC) complexes, thus indicating little effect of the macrocycle on the electron-transfer rate, perhaps as a result of counterbalancing effects in either the inner or outer sphere reorganization and orbital occupation. 

Intervalence transfer bands have been noted in a series of saturated spiro sulfur-bound ruthenium (III) (II) mixed-valence dimers [(NH3)sRuS C2n+2H4 S Ru(NH3)5] , wherc n = 2,3, or 4 indicates the number of spiro rings. Calculated electron transfer rates decrease with intermetal distances giving values of 8.0 x lo s at 11.3 A for n = 2, 4.9 x 10 s at 14.4 A for n = 3, and 3.5 x lO s at 17.6 A for = 4 indicating very effective tunneling even at 17.6 A with only a cr-bonded framework. 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

The half-wave potentials of (FTF4)Co2-mediated O2 reduction at pH 0-3 shifts by — 60 mV/pH [Durand et ah, 1983], which indicates that the turnover-determining part of the catalytic cycle contains a reversible electron transfer (ET) and a protonation, or two reversible ETs and two protonation steps. In contrast, if an irreversible ET step were present, the pH gradient would be 60/( + a) mV/pH, where n is the number of electrons transferred in redox equilibria prior to the irreversible ET and a is the transfer coefficient of the irreversible ET. The —60 mV/pH slope is identical to that manifested by simple Ee porphyrins (see Section 18.4.1). The turnover rate of ORR catalysis by (ETE4)Co2 was reported to be proportional to the bulk O2 concentration [Collman et ah, 1994], suggesting that the catalyst is not saturated with O2. 

Bimolecular Rate constants for Electron Transfer between Carotenoid Pairs in Argon Saturated Hexane (CAR/- + CAR2 CAR, + CAR/-)... 

Bimolecular Rate Constants for Electron Transfer between Carotenoid Pairs in Argon Saturated Benzene... 

---