Quinonic chromophores

The UV spectrum (/Imax 283, 352, and 405 nm) of calothrixin B (378) was similar to that of calothrixin A (377) indicating the presence of a similar pentacyclic quinolino[4,3- z]carbazole-l,4-quinone chromophore. Close comparison of the HREIMS of calothrixin B (378) with that of calothrixin A (377) indicated that... 

More definite evidence for the transient existence of the un-cyclized l-(jS-aminoethyl)-3,4-benzoquinones has been obtained recently by Kodja and Bouchilloux,77 78 who noted that a transient yellow color (Amax ca. 385 mp) was occasionally observed during the enzymic oxidations of catecholamines (particularly in unbuffered systems at low temperatures). This phenomenon was probably due to the formation of the transient o-quinones. (The absorption maximum of o-benzoquinone, the effective chromophore of the open-chain quinones, is known to occur at ca. 390 mp.79) An absorption maximum at 390 mp is characteristic of the formation of the dopa-quinone chromophore during oxidation of small C -terminal tyrosine peptides in the presence of tyrosinase.37 48 Similar spectroscopic features were observed when the oxidations were carried out with lead dioxide in sulfuric acid solutions (pH> 1). If the initial oxidation was carried out for a short period of time, it was possible to regenerate the original catecholamines by reduction (e.g. with sodium bisulfite, potassium iodide, and zinc powder) and to show that the 385 mp peak disappeared.77,78 Kodja and Bouchilloux were also able to identify 2,4-dinitrophenylhydrazones of several of the intermediate non-cyclized quinones by paper chromatography and spectroscopy (Amax n weakly acid solution ca. 350 mp with a shoulder at ca. 410 mp).77,78... 

In lignin, there are attendant ketone (sometimes quinone) chromophoric moieties that give it a yellow/brown color. Cellulose is a white substance that provides the desired strength and brightness properties of paper products. The strength and whiteness of a paper product depends on the extent of removal of lignin from the pulp fibers. Raw woods contain as much as 20% to 30% lignin. 

In solid HPC film methoxy-p-benzoquinone Q shows absorption maximum at 352 nm (figure 5) ascribable to a nn transition [12]. The weak band observed at 530 nm is likely due to the dibenzofurane-l,4-quinone F formed in small yield during the preparation of the film. After 30 min irradiation, the bands of the quinone chromophore disappear completely (Figure 5) giving rise to absorption similar to that of hydroquinone A irradiated for one hour (Figure 1). Such behaviour is also observed, as is shown below, when D is adsorbed on filter paper. 

Several new, mostly very typical angucyclines or angucyclinones have been described which possess a partially saturated ring A and an anthra-quinone chromophore (rings B-D), the hatomarubigins A-D (201-204,... 

L Zhang and G Gellerstedt. Quinone Chromophores and Their Contribution to Photoyellowing in Lignin. 5th European Workshop on Lignocellulosics and Pulp, Aveiro, Portugal, 1998, pp. 285-289. 

The presence of antioxidants in polyethers has a negative influence on the colour. If the polyether is not very well purified and has a basic pH, the phenolic antioxidants are oxidised to quinonic chromophoric structures, which negatively affect the colour of polyether polyols. This is the reason why it is preferable to add antioxidants to the slightly acidic purified polyol and not to the alkaline polyether. 

In this section we examine structurally complex thiazines, where the heterocycle forms part of a polycyclic system. Many of these fascinating natural products also contain quinone chromophores, and in many cases the corresponding compound lacking the thiazine ring also occurs in Nature. Hence it appears that the thiazine ring is added late in the biosynthetic pathway, presumably by addition of cysteine, cysteamine or hypotaurine to the quinone. 

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