Thiobarbituric acid complexes with

Analytical Techniques. Sorbic acid and potassium sorbate are assayed titrimetricaHy (51). The quantitative analysis of sorbic acid in food or beverages, which may require solvent extraction or steam distillation (52,53), employs various techniques. The two classical methods are both spectrophotometric (54—56). In the ultraviolet method, the prepared sample is acidified and the sorbic acid is measured at 250 260 nm. In the colorimetric method, the sorbic acid in the prepared sample is oxidized and then reacts with thiobarbituric acid the complex is measured at - 530 nm. Chromatographic techniques are also used for the analysis of sorbic acid. High pressure Hquid chromatography with ultraviolet detection is used to separate and quantify sorbic acid from other ultraviolet-absorbing species (57—59). Sorbic acid in food extracts is deterrnined by gas chromatography with flame ionization detection (60—62). 

The thiobarbituric acid (TBA) test was proposed over 40 years ago and is now one of the most extensively used methods to detect oxidative deterioration of fat-containing foods (41). During lipid oxidation, malonaldehyde (MA), a minor component of fatty acids with 3 or more double bonds, is formed as a result of the degradation of polyunsaturated fatty acids. It is usually used as an indicator of the lipid oxidation process, both for the early appearance as oxidation occurs and for the sensitivity of the analytical method (42). In this assay, the MA is reacted with thiobarbituric acid (TBA) to form a pink MA-TBA complex that is measured spectrophotometrically at its absorption maximum at 530-535 nm (Figure 2) (9,43,44). The extent of oxidation is reported as the TBA value and is expressed as milligrams... 

Thiobarbituric Acid Value. The 2-thiobarbituric acid (TEA) test is a popular method for measuring sensory oxidation products. It is based on the formation of a colored complex between two molecules of TEA reagent with one molecule of malonaldehyde or TEA reactive substances (TEARS). This intensity of the pink chromogram is measured at 532 nm. 

Many other sulphur-organic reagents have found use in the determination of Ru, e.g., rubeanic acid [44] and unithiol [45]. 2-Thiobarbituric acid has been used in a method based on derivative spectrophotometry [46]. Dithizone (at 85 C) forms with Ru a complex extractable into CHCI3 [47]. 

There are few mechanistic details on the formation and structures of the DNA oxidation products in the Cr + Red + O2 + DNA systems. Reduction of Cr(VI) with GSH or ascorbate induced simultaneous formation of comparable amounts of single-strand breaks and abasic sites in isolated DNA (376, 377, 381). Malondialdehyde was detected (by reaction with thiobarbituric acid) as an oxidation product of DNA or 2-deoxyribose in the Cr + Red + O2 systems (Red = GSH or ascorbate) (379). These features correspond to DNA oxidation by hydrogen abstraction at C4 atom of the deoxyribose moiety (405,406). More detailed studies of DNA oxidation mechanisms were performed using a model Cr(V) complex, [CrVO(ehba)2]- (Section III.C.4). 

The kinetics of the transfer of the 2 -desoxyribosyI group from 2 -desoxythymidine to adenine is very complex. In each of twelve different concentrations of each substrate the DRT activity of V2 was measured with the thiobarbituric acid method and the substrate saturation curves plotted. 

One of the earliest reports of microwave-assisted organometallic synthesis involved the preparation of arylmercuric chlorides. Heating an ethanol solution of diphenyldiazene and 2-phenylpyridine with Hg(OAc>2 inside a sealed Teflon vessel led to the formation of the desired arylmercury complexes. Yields were a little lower than conventional approaches, but reaction times were at least 30-times shorter. This chemistry was extended 10 years later when a solvent-free method was developed. Arylmercuric chlorides together with benzoquinone, barbituric acid, or thiobarbituric acid were adsorbed on basic alumina before being exposed to microwave irradiation for 1-3 min to produce derivatives (YIF = 1.3-1.5). 

Complex carbohydrates frequently contain adsorbed lipids, and this is especially the case where tissue, cell and membrane preparations are desialylated. For this reason, two extractions of the supernatant or dialysate containing free sialic acids with one volume each of diethyl ether or n-hexane is advisable, prior to ion-exchange chromatography (Schauer et al. 1975, Schauer 1978). If this extraction is not performed, erroneous results due to non-sialic acid compounds may be obtained in the analytical procedures, especially in the periodic acid/thiobarbituric acid assay of sialic acids (Schauer et al. 1975). Furthermore, lipid extraction improves sialic acid resolution on the following column chromatographic steps. 

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