Carboxylate reduction potential

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

A biological example of E° is the reduction of Fe(III) in the protein transferrin, which was introduced in Figure 7-4. This protein has two Fe(III)-binding sites, one in each half of the molecule designated C and N for the carboxyl and amino terminals of the peptide chain. Transferrin carries Fe(III) through the blood to cells that require iron. Membranes of these cells have a receptor that binds Fe(III)-transferrin and takes it into a compartment called an endosome into which H is pumped to lower the pH to —5.8. Iron is released from transferrin in the endosome and continues into the cell as Fe(II) attached to an intracellular metal-transport protein. The entire cycle of transferrin uptake, metal removal, and transferrin release back to the bloodstream takes 1-2 min. The time required for Fe(III) to dissociate from transferrin at pH 5.8 is —6 min, which is too long to account for release in the endosome. The reduction potential of Fe(IH)-transferrin at pH 5.8 is E° = —0.52 V, which is too low for physiologic reductants to reach. 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

Br0nsted Acids. Carboxylic acids, phenols, and alcohols are electrochemi-cally reduced by means of their Br0nsted acidity at a reduction potential that is a direct measure of their acidity (p/fj in a given solvent (see Chapter 8) ... 

A number of other cytochromes are located in the intermembrane space and the inner surface of the outer mitochondrial membrane. Cyt bs (cytochrome b ) is a small heme protein with a hydrophihc domain of about 95 amino acids and a carboxyl terminus of about 45 hydrophobic amino acids that serve to anchor the protein to the inner surface of the outer mitochondrial membrane. Cyt bs is reduced by NADH-cyt bs reductase, also located on the inner surface of the outer membrane. Under conditions of high intermembrane ionic strength, Cc is released from the inner membrane, and can transport electrons from cyt bs on the outer membrane to CcO on the inner membrane. Cyt bs contains protoheme b with the iron atom hgated by two histidine nitrogen atoms. The heme group has a reduction potential of -1-10 mV versus NHE. Cyt bs is also found on the liver endoplasmic reticulum membrane, where it transfers electrons from NADH-cyt b ... 

Several conclusions can be drawn from the model studies. First, it is clear that Mn is just as versatile as Fe or Cu and that Mn complexes can, with appropriate design, be synthesized in a variety of oxidation states and coordination enviromnents. A coimnon feature of the model chemistry is the observation of (/u.-0)(/u.-carboxylate)2Mn2 and (fx-0)2(ii-carboxylate)Mn2 core structures similar to those suggested for Mn catalase. By judicious protonation of the oxo bridge(s), it is possible to vary the Mn reduction potential of these structures over a wide range. 

CAR was discovered in acetate-producing bacteria based on its ability to catalyze the reverse reaction, the reductive activation of carboxylic acids, though it can also catalyze aldehyde oxidation (Table 2). The acid/aldehyde couple has an extremely low reduction potential, for example. Eg value for acetaldehyde/acetate is -580 mV (SHE) and therefore, aldehyde oxidation is much more thermodynamically favorable than acid reduction. The electron carrier for the enzyme inside the cell is not known. CAR isolated from C.formicoaceticum has molecular... 

Pyridine-4-thiocarboxamide is reduced in two, two-electron steps in acidic medium at the potential of the second wave, reduction gives 4-aminomethylpyridine. The product of reduction at the first wave is believed to be the aminothiol [126]. These three cases show that the preparative outcome of an amide reduction is strongly dependent on the substituents at the amide group and that the changes manifest themselves in the chemical step [Eq. (32)]. For the amides of aromatic carboxylic acids, the reduction potential may be gleaned from a comprehensive compilation [127]. 

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