Thiochromous acid

Chromic sulphide is a broAvnish-black, lustrous amorphous powder, of density 3-538 at 14° C. When heated in air it gives sulphur dioxide and chromium sesquioxide, Avhile in hydrogen it yields chromous sulphide. It is attacked and oxidised by nitric acid, aqua regia, and fused potassium nitrate. Compounds of chromic sulphide with sulphides of other metals haA-e been described. Regarding chromic sulphide as the thioanhydride of thiochromous acid, H2Cr2S4, these compounds may be described as thiochromites. 

By the action of hydrochloric acid on these substances in absence of air, thiochromous acid, H2Cr2S4, and thiodichromous acid, H2Cr4S are respectively obtained. ... 

Tetrahydrocannabinol (= THC)-11-carboxylic acid 290 Tetrahydrocortiso 221 Tetrahydrocortisone 221 Tetrahydrosteroids 222 Tetrazolium salts, reduction 61 Thalidomide 45 -, hydrolysis products 45 Thiabendazole 307, 308 Thiamine 235, 236, 397 Thickening agents 179 Thin-layer chromatography, advantage 5 -, numbers of publications per year 6 Thiobarbiturates 45,66 Thiocarbamide derivatives 322 Thiocarbamides, N -ary I-N -benzenesulfo-nyl- 248,249 Thiochrome 395... 

Extraction is commonly carried out by hydrolysis in boiling acid such as chloridric acid or sulfuric acid. To release thiamine bonded to phosphate enzyme, hydrolysis with phosphatase, alone or together with claradiastase or takadiastase, is carried out. After the enzymatic digestion, an acid treatment is applied in order to precipitate the protein and denaturate the enzymes. Ndaw et al. [603] proved that for extraction of vitamins Bj, B, and Bg, acid hydrolysis is always superfluous if the activity of the enzymes chosen is sufficiently high. SPE or column chromatography may be used in further purification, mainly to remove excess of derivatization reagents used to convert thiamine to a highly flnorescent thiochrome derivatives. lEC may be used in purification step, as well. 

Bioassay methods include yeast fermentation polyneuritic rate of cure in rat bacterial metabolism. Physicochemical methods include thiochrome fluorescence polarography chromatography absorption in neutral and acid solutions,... 

Thiamine shows a pH-dependent UV absorbance range of 230-270 nm. However, its UV absorbance is prone to interference by other endogenous UV absorbers in foods, such as nucleic acids (67,68). In a recent interlaboratory comparison of thiamine methods (42), the results obtained from an HPLC method using UV absorbance detection were rejected due to the presence of peaks that interfered with thiamine. In the interests of increased sensitivity and selectivity, the thiamine vitamers are generally converted to their thiochrome derivatives by alkaline oxidation and determined fluorimetrically (42,70). The thiochrome derivatives of thiamine and its phosphate esters all fluoresce at nearly identical excitation (365-375 nm) and emission (425-435 nm) maxima at pH over 8. The thiochrome derivatives are all relatively stable in alkaline solution at pH greater than 9 and room temperature. 

The reaction mixture was composed of 50 p.L of membrane preparation, 10 mM Hepes-Tris buffer (pH 6.8), 1.5 mM MgQ2, 1.5 mM EGTA, and 0.1 mM thiamine triphosphate in a total volume of 100 fiL. After 15 minutes of incubation at 25°C, the reaction was stopped by addition of 500 fiL of 6% trichloroacetic acid. The supemate obtained by centrifugation was extracted with 4 volumes of diethyl ether. The thiamine derivatives were transformed into fluorescent thiochromes by the addition of 50 fiL of oxidant [4.3 mJW K3Fe(CN)6 in 15% NaOH] to 80 fiL of sample. Thiamine diphosphatase activity was minimized by using magnesium as the metal and EGTA to chelate calcium. 

The second general method into the thiochrom-4-one ring involves the interaction of a thiophenol with compounds possessing a carbon atom bonded by at least two electron withdrawing groups, usually in the presence of a dehydrating agent (phosphorus pentoxide or poly-phosphoric acid). The first application of this was by Simonis and Elias and is illustrated in Eq. (19). The most successful active methylene substrates have been /3-ketoesters, i )3-cyanoketones, ... 

Vitamin Bi, vitamin B2, and nicotinic acid, all of which frequently occur together in foods, were separated by TLC and fluorimetrically determined by using a commercially available fiber optic-based instrument. A fluorescent tracer (fluoresceinamine, isomer II) was used to label the nicotinic acid. Vitamin B1 was converted to fluorescent thiochrome by oxidizing with potassium ferricyanide solution in aqueous sodium hydroxide. These vitamins were separated by HPTLC on silica gel using methanol-water (70 30 vol/vol) as mobile phase. Under these conditions, the Rf values of the vitamin Bi, vitamin B2, and nicotinic acid derivatives were 0.73, 0.86, and 0.91, respectively. 

Chemical ionization mass spectrometric detection has been explored for the detection of methyl hydroperoxides However, fluorometry has dominated the current detection schemes for the organic peroxides. Typically, a nonfluorescent substrate is oxidized by the peroxide to generate a fluorescent product. These methods are sufficiently sensitive for accurate measurement of the peroxides in the low ppt by volume. For example, the peroxidase-catalyzed dimerization of p-hydroxyphenylacetic acid (POPHA) occurs in the presence of a peroxy group at elevated pH. The formation of the fluorescent dimer, detected by excitation at 310 nm and emission at 405 nm, is proportional to the concentration of the peroxide. The most common peroxidase catalyst used for this reaction is horseradish peroxidase (HRP). Cost and stability issues with the use of HRP led to the use of other catalysts, such as metalloporphyrins or phthalocyanine complexes. Another fluorescent reaction scheme involves the oxidation of the nonfluorescent thiamine (vitamin Bi) to the fluorescent thiochrome by the peroxide group. This reaction is catalyzed by bovine hematin. This reaction is 25-fold more sensitive for H2O2 than for the organic peroxides. 

Vitamins are the foodstuff components most often quantified using fluorimetric means. There are several official fluorimetric methods for the determination of three water-soluble vitamins vitamin Bi (thiamine) (AOAC 942.23 and 957.17), B2 (rib-oflavine) (AOAC 970.65 and 981.15), and C (ascorbic acid) (AOAC 984.26). Thiamine is determined by oxidation to fluorescent thiochrome with alkaline hexacyanoferrate(III) or an alternative oxidant (Figure 1). The method is quite simple, reproducible, and selective and provides good recoveries. Many LC methods for thiamine determination in foods have... 

To investigate thiamine metabolism in mammals, thiamine (R values 0.16, 0.04, and 0.03), urinary excretion of thiamine metabolites [thiochirome (Rf values 0.31, 0.28, and 0.33), thiazole (Rf values 0.85, 0.79, and 0.81), and 2-methyl-4-amino-pyrimidinecarboxylic acid (Rf values 0.42, 0.21, and 0.26)], and related compounds [pyrimidinesulfonic acid (Rf values 0.48, 0.39, and 0.46), a-hydroxyethylthiamine (Rf values 0.23, 0.09, and 0.06), 7/ -methylnicotinamide (Rf values 0.31, 0.06, and 0.05)] were analyzed and identified by TLC on silica gel with acetonitrile-water (40 10 vol/vol) adjusted to a pH of 2.54, 4.03, and 7.85 with formic acid as solvents, respectively. Although A -methylnicotinamide and thiochrome could not be separated in single-phase chromatography at pH 2.54, a second phase at right angle, with a pH 4.03 solvent, separated these quite clearly without affecting the resolution of the other compounds. ... 

Thiamine in meat and meat products was determined by the HPLC system with precolumn derivatization. The thiochrome derived from thiamine in the sample filtrate, treated with acid phosphatase, was extracted with isobutyl alcohol, chromatographed with chloroform-methanol (90 10, v/v), and then determined fluorometrically (17). The recovery values ranged between 84.4% and 94.2%, and the detection limit for thiamine was 0.05 ng (0.15 pmol). 

The majority of breakfast cereals in the United States are fortified with PN, and additional PN is also added to infant formula products to ensure adequate vitamin Be supply to the infant. Gregory (100) reported an isocratic HPLC method for the determination of PN in breakfast cereals (Table 5). Other investigators attempted simultaneous determination of PN and other vitamins used in food fortification. Wehling and Wetzel used ion pair HPLC to separate pyridoxine, riboflavin and thiamine from each other after acid extraction of the vitamins from cereals (101). Using a dual fluorescence detector setup, pyridoxine and riboflavin were monitored by the first detector. After the column eluate had passed the first detector, an alkaline ferricyanide solution was introduced, resulting in the formation of a fluorescent thiochrome derivative of thiamine, which was detected by the second fluorescence detector. A similar method for simultaneous determination of pyridoxine and riboflavin in infant formula products has also been described (102). 

Vitamin Bi (thiamin) is related to beriberi, a disease associated with a deficiency of this vitamin. In fact, thiamin is a coenzyme in different biochemical reactions. Pork, legumes, as well as liver and kidney products are regarded as excellent sources of this vitamin. Thiamin, as well as other water-soluble vitamins, is frequently found bound to proteins or carbohydrates or even phosphorylated. Therefore, prior to their analysis, a sample treatment to release the free forms of the vitamin is common. A t)q)ical extraction protocol for water-soluble vitamins includes autoclaving the sample with hydrochloric acid for the acid hydrolysis of the vitamin followed by an adjustment in the pH to values around 4.0—4.5, adequate for an enzymatic treatment. This vitamin can be, subsequently, separated by ion-pair RP chromatography and detected with a fluorescence detector after postcolumn oxidation to thiochrome. MS detection through electrospray ionization is also used, although the separation pH should be adjusted to maximize the ionization of the vitamin. 

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