Diphenols, degradation

In an attempt to identify more biocompatible diphenols for the design of degradable biomaterials, we studied derivatives of tyrosine dipeptide as potential monomers. After protection of the amino terminus and the carboxylic acid terminus, the reactivity of tyrosine dipeptide (Figure 1) could be expected to be similar to the reactivity of industrial diphenols. Thus, derivatives of tyrosine dipeptide could be suitable replacements for BPA in the synthesis of a variety of new polymers that had heretofore not been accessible as biomaterials due to the lack of diphenolic monomers with good biocompatibility. 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

We assume that degradation of diphenols is also initiated by radical reactions like dimerization. The primary step in this case also seems to be a one-electron transfer from the diphenol giving a resonance-stabilized free monoradical as shown in Figure 11. 

These findings show that by dimerization to diphenol, structures are being formed which are less stable towards oxidation (radical one-electron transfer reactions), explaining dimerization by radical coupling during oxidative degradation from the energetic point of view. 

Animals. Degradation in animals is principally by O-dealkylation to the corresponding phenol and diphenol, and by dehydrochlorination to 4,4 -dihydroxybenzophenone... 

CEs are known to react with phenols to form iminocarbonates which eventually lead to polycyanurates with the liberation of more acidic phenol moiety. This can be a method to alter the gel point of the resin, Tg, and thermal stability of the network by co-curing diphenol with CE. Thus, copolymerization of dicyanate with diphenols resulted in polycyanurates with altered network structure and diminished crosslink density [237]. However, an earlier report claims poly(imi-nocarbonate) by reaction of these two in equimolar quantities. The thermoplastic so formed was reported to retain the mechanical properties like a polycarbonate. This approach can produce strong, non-toxic, biodegradable films and molded plastics that are degradable at temperatures above 140 °C [169,238]. Except for a few very early reports [239], the reaction of CE with anhydrides to form poly(iminocarbamates) has not been explored much. 

Identified by Tressl et al. (1978a) among diphenols and other caramel constituents after silylation of roasted coffee. They gave concentrations of 15 ppm in arabica, 6 in robusta (13 in arabusta). Contrary to maltol (1.146), hydroxymaltol is degraded with over-roasting temperatures the concentration after 5 min being ca 15 ppm at 170 °C, ca 20 at 230 °C and <10 ppm at 260 °C in a robusta (Silwar and Lullmann, 1993b). 

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