Kovacs’ reagent

Test for indole Indole is a component of the amino acid tryptophan, which can he broken down by the bacterial enzyme tryptophanase. When tryptophan is broken down, the presence of indole can be detected through the use of Kovacs reagent. Kovacs reagent, which is yellow, reacts with indole and produces a red colour on the surface of the test tube. Kovacs reagent is prepared by dissolving 10 g of p-aminobenzaldehyde in 150 mL of isoamylalcohol and then slowly adding 50 mL of concentrated HCl. 

This test consists of incubating cells in a medium containing the amino acid tryptophan and measuring the production of indole. A culture 2-3 days old is added to Kovacs reagent (dimethylaminobenzaldehyde in HCl) and a pink/red colour gives a positive reaction. E. coli gives a positive result, and the test is frequently used to distinguish it from Klebsiella. 

Tube (b) is tested for indole by the addition of Kovacs reagent (page 62). E. coli rapidly gives a positive reaction. 

Lauryl tryptose mannitol broth with tryptophan (presumptive for E. coli) Completed (Optional) Tests 44 C. 5°C for 24r-48 h Gas production in Durham tube and formation of a red ring after addition of Kovacs reagent [67]... 

Kovacs-Hadady and Kiss [27] studied the chromatographic characteristics of thia-zolidinecarboxylic acid derivatives, formed by reaction of (i>) and (L)-penicillamine with various substituted benzaldehydes and heterocyclic aldehydes in order to evaluate the aldehydes as derivatizing agents for separation of the penicillamine enantiomers. The TLC method of Martens et al. [28] was used. Transformation to thiazolidine carboxylic acids with benzaldehyde and substituted benzaldehydes was not complete, so formaldehyde is still the preferred reagent for separation of the enantiomers. 

Analysis by TLC. Confirmation of the identity of the halogenated compounds in the soil samples was achieved by resolution of selected extracts on TLC plates, followed by gas chromatography of the separate compounds eluted from the plates. The thin layer chromatography system of Kovacs (6) was used, and the halogenated compounds were detected with the silver nitrate-2-phenoxyethanol reagent. Comparison with standards chromatographed on the same plates permitted an estimate of the pesticide concentrations. 

L. Kovacs (2009) Cyanogen bromide. An update. In e-EROS Encyclopecba of Reagents for Organic Synthesis, 2nd ed., L. A. Paquette, D. Crich, P. L. Fuchs, G. Molander Eds., John Wiley and Sons, Chichester. http //onlinelibrary.wiley.eom/doi/10.1002/047084289X.rc269.pub2/ abstract (last access May 19,2014). 

Detection reagents for biochemical reactions Indole reagent (according to Kovacs) (Recipe 30) Composition... 

Kovacs [52] has suggested a further improvement of the fluorescein-silver nitrate reagent (No. 24), namely, replacing the fluorescein with the ethyl ester of tetrabromophenolphthalein. Amounts as low as 0.05 jxg insecticide and therefore traces of 0.02—0.05 ppm in extracts can then be detected. The TLC-procedure is consequently 20 times more sensitive than similar methods using PC. Stanley [95] uses iodine vapour, platinum chloride, silver nitrate solution (Rgt. No. 232), fluorescein (Rgt. No. 116), methylumbelhferone (Rgt. No. 163) and ammonium molybdate-perchloric acid (Rgt. No. 166) for detection. 

Kovacs-Hadady, K., and Varga, T. (1995). A systematic study of impregnation of thin layers with different ion-pairing reagents and by different methods. J. Planar Chromatogr.—Mod. TLC 8 292—299. 

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