Quinones structural characteristics

Carbohydrates (specifically polysaccharides), unsubstituted alkyl carbons, and carboxyl/ester carbons are prevalent in all samples [20]. Other recent evidence strongly implicates quinone functionalities in the photochemical activity of DOM in sunlit seawater [4,76]. In one model of DOM formation incorporating polymerization from reactive monomers, one important class of monomers are the polyphenols or quinones, which are both synthesized by microorganisms and released in Hgnin degradation [21]. Possible structural characteristic features of marine DOM are outHned in Fig. 5. 

Quantitative information about the elementary contributions of the native system structure to X. at the site of the RC from Rhodobacter sphaeroides can be obtained by comparing the electron transfer characteristics that have been already described for a family of quinones [8,13], with non-quinone, I.e. "exotic", cofactors [14,15]. The strategy to reveal specific contributions to X is to compare the rates of electron transfer mediated by quinone and exotic cofactors at comparable values of -AG°et. over the temperature range from 14 to 295 K. Figure 1 depicts the way in which alteration of the basic quinone structure could influence the appearance of the experimental rate/-AG°et relation through changes in X, In practice, the scatter in the quinone rate/-AG°et data [8], which has been attributed to small variations in V(r) for... 

Characteristic fragmentations of quinone structures and cleavages involving cyanide rearrangement are responsible for most of the total ion current (see Scheme 1.7). 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

Analysis of the spectral characteristics of photochromic quinones showed that the absorption band of the photoinduced form strongly depended on the compound structure. The substituents in the quinone nucleus most strongly affected the spectral characteristics of the photoinduced form. 

Condensed tannins, on the other hand, occur in the bark of all conifers and hardwoods examined to date, and they are frequently present in the wood. They are primarily responsible for the tan to brown color of wood after it is exposed to air. In their purest form, condensed tannins are colorless, but they become colored very readily once isolated because of their propensity to oxidize to quinones. The primary characteristic of the water-soluble condensed tannins (4) is dehydration/oxidation to intensely colored anthocyanidin pigments (5) when refluxed in butanol and hydrochloric acid (Figure 2). For this reason, there has been a tendency to refer to these compounds as proanthocyanidins in the last few years. Prior to that, they were referred to as leucoanthocyanidins (i.e., the colorless chemical form of anthocyanidins). All references earlier than the late 1950 s, when the structure of these substances was just beginning to be understood, used the term condensed tannin. 

The fact that the pKa of the various oxidation states of quinones differ dramatically has been exploited in a recent design of artificial systems that mimic the light-driven transmembrane proton transport characteristic of natural photosynthesis [218]. Triad artificial reaction centers structurally related to 41 were vectorially inserted into the phospholipid bilayer of a liposome (vesicle) such that the majority of the quinone moieties are near the external surface of the membrane, and the majority of the carotenoids extend inward, toward the interior surface. The membrane... 

Oximes.—Evidence in favor of the ketone structure is that quinone undergoes the characteristic aldehyde or ketone reaction with hydroxyl amine forming oximes. Furthermore, it forms both a mono- and a... 

Compounds containing the Sn=P double bond (stannaphosphenes) have been characterised, where very bulky ligands prevent oligomerisation,79 81 the double bond being introduced by the elimination of HF as shown in equation 16-78. The products are isolated as deep red solids no crystal structure has yet been determined, but the NMR spectra are very characteristic. With 1,2-diones, o-quinones, or enones, [2+4]-cycloaddition occurs to give six-membered rings. 

The preliminary results showed a correlation between physicochemical characteristics of inhibitor (activator) molecules and changes in kinetic parameters of bioluminescent reaction. For example the comparison of the effects of the quinones and phenols on three bacterial bioluminescence systems of different complexity indicates that the influence of the compounds on the bioluminescence intensity depends on the structure and redox characteristics. The inhibitory activity of quinones depends on their hydrophobic substituents and the size of the aromatic part. Such correlations are closely related to the physical mechanism of bioluminescence they are the biophysical basis for bioluminescent ecological monitoring. 

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