On the other hand, pristine PPy is insoluble, infusible, brittle, and amorphous due to the rigidity of its molecular chains from the quinoidal
and aromatic states.
Xu, "Direct visual evidence for quinoidal
charge delocalization in poly-p-phenylene cation radical," The Journal of Physical Chemistry B, vol.
For instance, when phenolic stabilizers decompose (either by radical transfer reactions with a polymer macroradical or by oxidation), they may form byproducts such as quinoidal
compounds that greatly discolor the material .
Segura-Aguilar, "Quercetin may act as a cytotoxic prooxidant after its metabolic activation to semiquinone and quinoidal
product," Free Radical Biology and Medicine, vol.
Semiquinones transfer electrons to molecular oxygen ([O.sub.2]) and return to their original quinoidal
structure, thus generating a superoxide anion radical ([O.sub.2.sup.-*]).
Com relacao a estabilidade das antocianinas, as mesmas se encontram comumente na forma de uma mistura de diferentes estruturas quimicas em equilibrio: cationflavilium (vermelho), base anidra quinoidal
(azul), pseudobase carbitol (incolor) e chalcona (incolor ou levemente amarela).
The great interest in its study, including the elucidation of mechanisms of action of compounds with quinoidal
structure is due to their multiple biological targets and their widespread presence in nature (Hussain et al., 2007).
A Reinvestigation of the quinoidal
effect in N-n-propyl-2-oxo-1-naphthylidene-methylmine, Acta Crystallogr., C52, 2573-2575(1996).
This process of deterioration is mediated by endogenous enzymatic activities such as polyphenol oxidase (PPO) and peroxidase (POD), which leads to the formation of dark polymers of a quinoidal
nature (ESCRIBANO et al., 2000; FRICKS et al., 2006; PEREZ-TELLO et al., 2001; VALENTINES et al., 2005; ZHOUA et al., 2003).
Under the conditions described above the quinoidal
forms for para- and ortho-species can be assigned by characteristic chemical shifts in [sup.13]C NMR spectra .