Biuret base/reaction

The biuret reaction is positive with the greater number of the polypeptides excluding the dipeptides. In the case of the glycine compounds it occurs first with the tetrapeptide, but it occurs with other tripeptides. It is distinctly intenser as the length of the polypeptide chain increases, and the colour is also more intense when the carboxyl group is esterified this is especially noticeable in the case of triglycyl-glycine and its ester, the so-called biuret base. The same occurs when... 

For the assay of ephedrine in the total alkaloids a colorimetric method based on the biuret reaction was used by Feng and Read and is described in detail by Feng. Krishna and Chose separated ephedrine and iji-ephedrine by treating the dry mixed hydrochlorides with dry chloroform in which the ephedrine salt is virtually insoluble and the -ephedrine salt soluble. ... 

The practically more convenient and less complicated Balikov method (Bl, B3) based upon the biuret reaction also exhibits poor specifity toward peptides, and being devoid of the appropriate system of reference standards, it does not permit any absolute evaluation of the amount of these compounds in urine. 

With an excess isocyanate in the above systems, allophanate and biuret reactions take place (Eqs (2.25) and (2.26)), resulting in further cross-linking. When increased rigidity and high-temperature performance are desired, further crosslinking may be accomplished via isocyanurate formation (Eq. (2.29)). Base catalysts such as alkoxides, quaternary ammonium or phosphonium, etc., promote this reaction. Aromatic isocyanates give iso-cyanurates far more easily than aliphatic ones. 

Bicinchoninic Acid (BCA) Assay. The bicin-choninic acid assay for proteins is based on the same reactions as the Folin-Ciocalteau assay. Proteins are again reacted with alkaline cupric ions to form the biuret complex, and these ions are reduced to cuprous ions by the aromatic amino acids in the proteins. In this case, however, the Cu1+ ions form a complex with bicinchoninic acid (Fig. II-6), which has an intense absorbance maximum at 562 nm. This assay shows the same variation from protein to protein as the Folin-Ciocalteau assay, but is more convenient experimentally and can be made somewhat more sensitive. 

Several s pec tropho tome trie methods are available for the determination of protein in solution. The biuret method is based on the reaction of Cu with peptides in alkaline solution to yield a purple complex that has an absorption... 

R-N=C=0 + R N-f—NR"— H A (urea) RN-S-NHR" 100°C Strong bases. Metal compounds. Biuret reaction. Moderate rate. 

There are three prime methods for quantification of proteins based on derivatization reactions the biuret, Lowry, and Bradford methods. 

The initial step to prepare polyurethane polymers for solution wet or dry spinning includes reaction of 1000-3500 molecular weight macroglycol with a diisocyanate at molar ratios of between about 1 1.4 and 1 2.0. Reaction conditions must be carefully selected and controlled to minimize side reactions, eg, allophanate and biuret formation, which can result in trifunctional branched chains and ultimately to insoluble cross-linked poljuners. For the prepolymer reaction, poly(tetramethylene ether) glycol [25190-06-1] and bis(4-isocyanatophenyl) methane [101-68-8] are currently the most commonly used macroglycol and diisocyanate. Several types of polyester-based macroglycols are included in spandex producers product lines, but with the exception of Dorlastan, made by Bayer AG in Germany, the polyester-based products represent only a minor part of their spandex fiber production. 

As mentioned in Section 8.4.3.1., the majority of hydroxy groups in a polyether triol are secondary groups and are comparatively unreactive towards isocyanates. It is therefore necessary to select a catalyst which favours the formation of urethane links relatively more than the formation of gas by the reaction of isocyanate and water. Tin compounds (e.g., stannous octoate and dibutyltin dilaurate) are particularly effective in this respect (cf.. Table 14.3) and are very widely used. In addition to the primary isocyanate-polyol and isocyanate-water reactions, several secondary reactions occur during the preparation of foam. As shown in Section 14.4, the final product may contain allophanate, biuret, isocyanurate and uretidione links. It will be appreciated that in a polymeric system, which is based on a diisocyanate, all of these links (except uretidione) represent points of branching or cross-linking. These secondary reactions are particularly favoured by tertiary amines (e.g., triethylenediamine and 4-dimethylaminopyridine) and these catalysts therefore contribute to the final cross-linking of the foam and hence to the achievement of, for example, a low compression set. Mixtures of tin compounds and tertiary amines are more... 

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