Pyridine coenzymes. VII. Catalysis of the hydration of the reduced form by iodoaromatic compounds

Abstract

Paulo, in partial fulfillment of the requirements for the degree of Doctor of Science. Paper VI of this series is Cilento and Sanioto (1965).

Pyridine coenzymes. VIII. Autoxidation of the reduced form catalyzed by 4-amino-2,6-diiodophenol

“…Manometrically, the behavior was very similar to that of the NADH system. However, DIPAP, as expected on the basis of the precedent work (Schreier and Cilento, 1969) does also catalyze solvent addition to the 5,6-double bond of the model coenzyme. The latter reaction could be properly followed spectrophotometrically, using evacuated Thunberg cells, as it leads to disappearance of the 355-µ band and appearance of a stronger one at 290 µ.…”

“…A more complex scheme obtains for the model compound, because differently from the natural coenzymes, the model is also prone to solvent addition (Schreier and Cilento, 1969); as a result, a variety of products is formed.…”

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The reduced pyridine coenzymes are stoichiometrically and directly oxidized by molecular oxygen to the pyridinium form, under the catalytical effect of protonated 4-amino-2,6-diiodophenol. With the model coenzyme, 1 -benzyl-1,4-dihydronicotinamide, two processes take place simultaneously: (i) the direct oxidation and (ii) addition to the 5,6-double bond of the dihydronicotinamide followed by autoxidation of the rein the precedent paper (Schreier and Cilento, 1969) it was reported that o-halogenophenols, including thyroid hormones, catalyze the hydration of 1,4dihydronicotinamides. We have now found that DIPAP1 not only catalyzes solvent addition to 1-benzyl-1,4-dihydronicotinamide, a model coenzyme, but also the autoxidation of the resulting or intermediate products as well as of the model itself.

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Autoxidation of reduced pyridine coenzymes and of their models promoted by N,N,N',N'-tetramethyl-p-phenylenediamine

“…In the case of the benzyl model, the spectrum of the final reaction mixture in Tris buffer clearly showed the pyridinium absorption at 265 µ and another absorption band at ca. 290 µ, peculiar to the products resulting from saturation of the 5,6 double bond of 1,4-dihydronicotinamides (Schreier and Cilento, 1969). In the case of the propyl model in Tris only the latter absorption was discernible.…”

“…Whereas with the 2,6-dihalogeno-4-aminophenol catalysts a hydride ion seems to be transferred from the 1,4-dihidronicotinamide to 02 (Silva Araujo et al, 1970), with TMPD as catalyst a homolytic mechanism will be shown to be operative. Actually, since 1,4-dihydronicotinamides may be oxidized in two one-electron steps and as the pyridinyl radical is highly reactive toward oxygen (Kosower and Pozionek, 1964), a chain mechanism is very likely.…”