Pyridine reduction potential range

The transferrin receptor TfR may have a role in reduction of the transferrin-bound Fe(III) within the endosome. Iron leaves the endosome to enter the cytoplasm via the divalent metal ion transporter DMTl, which only transports Fe(ll), not Fe(lll) (see Section 5). As noted above, the reduction potential of Fe(lll) bound to transferrin not associated with TfR is less than -500 mV, which is too low for reduction of the bound Fe(lll) by physiological systems such as reduced pyridine nucleotides. However, as Dhungana et al. have shown," the reduction potential of Fe(III)-transferrin bovmd to TfR is considerably higher at around -300 mV, which puts it in the range for reaction with physiological reductants. 

From their standard reduction potentials, in the range of 1.35 V versus NHE [37], complexes should not be able to oxidize methylbenzenes, the standard potentials of which exceed 1.75 V versus NHE [29]. The reaction, however, proceeds easily in the presence of a pyridine base, according to the following stoichiometry ... 

Reduction potential data support this prediction. Also, the inclusion of multiple substitutions of CH by N in benzene is expected to increase electron affinity. On the basis of reduction potentials the Ea of pyridine is about zero and that for the diazines ranges from 0.2 to 0.36 eV. The triazines vary from 0.5 to 0.9 eV. The predicted value for hexazine is 2.8 eV. These substitution and replacement effects can be used to predict electron affinities. Indeed, the first attempt at estimating the electron affinities of AGCUT was made using such correlations. These will be discussed in more detail in Chapter 12. 

The polarographic reduction of quinoxaline and its derivatives has been studied by a number of workers. Half-wave electrode potentials are pH dependent and over the pH range of 1 to 10, Ei has been reported to vary from —0.254 to -0.863 The half-wave electrode potential in anhydrous dimethylformamide is — l.bOV for quinoxaline, —1.06 V for cinnoline, —1.41V for phthalazine, and —1.22 V for quinazoline. The benzodiazines are more easily reduced than the corresponding diazines, thus pyrazine has a half-wave electrode potential of -1.57V. Pyridine with a half-wave potential of —2.15 V is still more difficult to reduce. The reversible reduction potential of quinoxaline as determined by cyclic voltammetry is -1.097 V. The observed energy differences between azine and radical anion are well correlated with the results of CNDO and SCF iT-electron calculations. 

Another classic reaction of pyridinium salts is reduction of the pyridine ring. Donohoe and co-workers reported the partial reduction of A-alkylpyridinium salts <060BC1071>, which is accompanied by subsequent alkylation and hydrolysis to furnish a range of 2,3-dihydropyrid-4-ones. This sequence has the potential to introduce a variety of functional groups at the C-2 position of 2,3-dihydropyrid-4-ones. Reduction of pyridinium ylides with sodium borohydride has also been reported in fair to good yields <06JHC709>. 

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