Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Unknown chromophore

Figure 16.2 Pathways for indirect photolysis of an organic compound i involving excited natural organic matter constituents. UC refers to unknown chromophores. Wavy arrows symbolize radiationless transition (adapted from Zafiriou et al., 1984). Figure 16.2 Pathways for indirect photolysis of an organic compound i involving excited natural organic matter constituents. UC refers to unknown chromophores. Wavy arrows symbolize radiationless transition (adapted from Zafiriou et al., 1984).
The syntheses of methyl bixin (24) and other natural carotenoids were mentioned above. A biogenetically inspired synthesis of e-carotene used the titanium tetrachloride complex of lycopene (16). Dehydrolycopene was also isolated. Two more syntheses of ) -carotene (2) have been reported which use intermediates in the synthesis of vitamin A (see Scheme 2). Although ll-cis-j5-carotene was produced it is rapidly isomerised to the all-frons form. The cross-conjugated system (72) has a previously unknown chromophore. Another example of this system, but with an additional 4 -oxo-group, was synthesised by Surmatis et al. in a study of the synthesis of keto carotenoids. They prepared echinenone (73) and the two protected 3,3 -dioxo-j8-carotene compounds (74) and (75). Treatment of the ketal with sulphuric acid gave mainly 3,3 -dioxo- -carotene (76) while hydrochloric acid gave 3,3 -dioxo-c-carotene (77). Under both conditions the enol ether gave the latter product. [Pg.211]

The mechanism proposed for the pyruvate ferredoxin oxidoreductase is drastically different from that of the pyruvate dehydrogenase enzyme complexes, as radical intermediates are proposed, suggesting stepwise one-electron transfers. Furthermore, the preliminary work by Uyeda and Rabinowitz (211) on the mechanism of C. acidiurici pyruvate ferredoxin oxidoreductase indicates that there may be subtle differences in the mechanism of the oxidoreductase from different organisms. The key points and contrasts in the mechanism of these enzymes may be summarized as follows. Addition of pyruvate to a stoichiometric amount of enzyme leads to the formation of an equimolar amount of CO2. In H. halobium (213) a enzyme-mediated one-electron transfer to an exogenous electron acceptor occurs at this stage, with formation of a stable enzyme-bound radical intermediate, whereas the enzyme from C. acidiurici remains in the oxidized state. Addition of CoA leads to the subsequent formation of acetyl-CoA from both enzymes, which reduction (two electrons ) of an unknown chromophore in the... [Pg.382]

Oxidation products depending on the hardener structure. For example, diamino diphenyl methane (4.53) undergoes the strongest yellowing of all amines, and produces an unknown chromophore absorbing at 615 nm, according to the mechanism ... [Pg.277]

The structural unit associated with an electronic transition m UV VIS spectroscopy IS called a chromophore Chemists often refer to model compounds to help interpret UV VIS spectra An appropriate model is a simple compound of known structure that mcor porates the chromophore suspected of being present m the sample Because remote sub stituents do not affect Xmax of the chromophore a strong similarity between the spectrum of the model compound and that of the unknown can serve to identify the kind of rr electron system present m the sample There is a substantial body of data concerning the UV VIS spectra of a great many chromophores as well as empirical correlations of sub stituent effects on k Such data are helpful when using UV VIS spectroscopy as a tool for structure determination... [Pg.567]

UV/VIS spectrophotometry can be used to determine many physico-chemical characteristics of compounds and thus can provide information as to the identity of a particular compound. Although UV/VIS spectra do not enable absolute identification of an unknown, they are frequently used to confirm the identity of a substance through comparison of the measured spectrum with a reference spectrum. However, UV spectrophotometry is not highly specific, and can obviously only be applied to polymer additives which are absorbers of UV radiation, i.e. contain chromophoric groups. Both UV and IR monitor functional entities rather than the entire molecular structure. A functional group s proximity to other electropositive or electronegative structures in a molecule affects the absorbance spectrum, allowing one to infer some details of molecular structure. [Pg.304]

Both the identification of a species and the determination of the kinetics of its formation or decay can be achieved with longer pathlength cells, such as that depicted in Figure 2.103. In kinetic experiments, however, there is the proviso that the experiment can be performed before natural convection currents interfere with the measurements i.e. the operator must be certain that the removal of a chromophore from the optical path is due to reaction and not due to convection currents. It should be noted that the strength of UV-visible spectroscopy does not lie primarily in the identification of unknown species as the information it provides is not of a molecularly specific nature. [Pg.205]

Charon and Szabo17 demonstrated that chromophore 4 may be produced to a significant extent from a 5-O-substituted derivative of KDO (which does not contain a free diol grouping at C-4-C-5). These authors synthesized 3-deoxy-5-0-methyl-2-octulosonic acid (7 configuration at C-5-C-7, arabino at C-4, unknown) by the Comforth reaction18,19 from 4-0-formyl-2-0-methyl-D-arabinose (see Scheme 4 and Section IV,1). Compound 7 gave a millimolar extinction coefficient of 13 in the TBA assay (as compared to 92 5 for KDO). Based on this result, Charon and Szabo17 formulated for the TBA reaction of 5-0-... [Pg.329]

Hathaway has attempted to assess the value of the electronic properties of polycrystalline mononuclear copper complexes with the [CUN4], [CuNj], and [CuN ] chromophores of unknown crystal structure, in predicting the stereochemical environment of the copper(ii) atom. The value of having B. J. Hathaway, J.C.5. Dalton, 1972, 1196. [Pg.331]

Last but not least, ageing and destruction processes can be monitored in polymers under application, and structural and quantitative analysis of unknown additives (stabilizers etc.) is possible in commercial polymers using UV-vis spectroscopy. Advantage can be taken here of the fact that the position of an electronic absorption in unsaturated systems depends only weakly on the surroimd-ing medium. Even though UV-vis spectroscopy is not very specific in the absorption band, it is highly sensitive and therefore much better than NMR or IR spectroscopy to detect small amounts of chromophors. [Pg.85]

As part of extensive studies lasting over 30 years on the structures of chromophores involved in nonenzymatic browning reactions, two intensely orange, previously unknown, compounds have been identified (2R,8aR)-l and ZS, 8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2//,7//,8a//-pyrano[2,3-3]pyran-3-one <1998CAR215>. Additional studies on the single Maillard reaction products of these compounds have also been reported <1998JFA3912>. [Pg.714]

Before NMR spectroscopy and mass spectrometry revolutionized the structural elucidation of organic molecules, UV spectroscopy was an important technique and was used to identify the key chromophore of an unknown molecule. The importance of UV is much diminished nowadays, but it still retains its place in certain applications, such as the determination of kinetic parameters, (the Michaelis constant) and A cat (the turnover rate of an enzyme, in molecules per second), for a number of enzymic reactions and in the analysis of pharmaceuticals. [Pg.19]


See other pages where Unknown chromophore is mentioned: [Pg.657]    [Pg.658]    [Pg.111]    [Pg.657]    [Pg.658]    [Pg.111]    [Pg.4]    [Pg.295]    [Pg.224]    [Pg.243]    [Pg.247]    [Pg.260]    [Pg.489]    [Pg.322]    [Pg.190]    [Pg.95]    [Pg.54]    [Pg.305]    [Pg.349]    [Pg.331]    [Pg.335]    [Pg.64]    [Pg.186]    [Pg.45]    [Pg.262]    [Pg.289]    [Pg.571]    [Pg.22]    [Pg.166]    [Pg.1342]    [Pg.561]    [Pg.581]    [Pg.606]    [Pg.611]    [Pg.612]    [Pg.616]    [Pg.627]    [Pg.633]   


SEARCH



Unknown

© 2024 chempedia.info