Quinone group containing structures

To prove that the quinonic structure is responsible for dehydrogenation activity in organic polymers, two polymers were compared, similar in structure, but one containing quinonic groups and the other without 96). 

It is important to appreciate that these ditopic macrocycles contain a polyether moiety on one side and present a diamido motif on the other. Addition of a charged pyridinium cation to the substrate-free form of these macrocycles then generates a highly efficient receptor for chloride, presumably as the result of both template-induced organization and electrostatic effects. The further incorporation of hydro-quinone groups and polyether functionalities into the cyclic framework presumably contributes to the stabilization of the cationic pyridinium component within the final interpenetrated structure. Interestingly, while titration of the pyridinium cation building block alone revealed a preference for oxoanions, the final receptor-anion... 

The chromophore theory originated with Bernthsen and Friedlander, who proposed, simultaneously and independently, that the colorless form of phenolphthalein present in acid solutions possessed a lactone structure, and that the red salt formed by alkalis contained a chromophore quinone group. Hence the color transformation was accompanied by a change in constitution. Later, Hantzsch and his students extended the theory, and ascertained that unless a structural change occurred, the color remained the same. They stated, however, that in many cases the demonstration of a structural change was too difficult to be decisive. 

It can be seen that the only apparent similarity in these structures is that all of them contain a quinone group. The number of such compounds that has been studied by advanced techniques of pulse radiolysis and flash photolysis is increasing rapidly over nearly last three decades. Around the main objectives cited above, a large number of studies [7-12] have simply been made on the structure-property relationship. 

Plastoquinone a polyisoprenoid quinone. P. is structurally similar to ubiquinone, but contains a methyl group (not a methoxy group) in the aromatic ring. P. can be isolated from chloroplasts it acts as a reversible redox component in photosynthetic electron transport (see Photosynthesis). 

Vitamin K (phylloquinone) structure contains quinone groups that can be electrochenti-cally reduced to hydroquinone counterparts. Therefore, Koivu et al. [110-113] employed isocratic reversed-phase HPLC-ED schemes for vitamin K determination in digested extracts of different foods. They used Coulochem cells in the redox mode with the first cell unit at a large negative reduction potential and the second unit detecting the vitamin K reduction products at very low potential. 

Oxidation of P-nicotinamide adenine dinucleotide (NADH) to NAD+ has attracted much interest from the viewpoint of its role in biosensors reactions. It has been reported that several quinone derivatives and polymerized redox dyes, such as phenoxazine and phenothiazine derivatives, possess catalytic activities for the oxidation of NADH and have been used for dehydrogenase biosensors development [1, 2]. Flavins (contain in chemical structure isoalloxazine ring) are the prosthetic groups responsible for NAD+/NADH conversion in the active sites of some dehydrogenase enzymes. Upon the electropolymerization of flavin derivatives, the effective catalysts of NAD+/NADH regeneration, which mimic the NADH-dehydrogenase activity, would be synthesized [3]. 

In aromatic diazonium compounds containing an ionized hydroxyl group ( —O-) in the 2- or 4-position, it is necessary to consider delocalization of electrons and, therefore, two mesomeric structures (1.7a-1.7b) (see Sec. 4.2). This fact has implications for nomenclature compounds of this type are considered as quinone derivatives following IUPAC Rule C-815.3 (Exception) compounds of this class are called quinone diazides. As a specific compound 1.7a-1.7b is indexed in Chemical Abstracts as 4-diazo-2,5-cyclohexadien-l-one. If reference is made specifically to mesomeric structure 1.7b, however, it is called 4-diazoniophenolate. 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

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