Biuret reaction conditions

The nitrogen content of this mixture amounted in the case of normal urine to 0.68-1.27% of the total nitrogen, whereas the peptides precipitated under these circumstances constituted 16.1-20.9% of all substances appearing in urine and giving the biuret reaction. The peptide mixtures obtained in identical conditions from six samples of pathological urine (liver cirrhosis and leukemia) exhibited marked differences in the nitrogen and peptide content as compared with those isolated from normal urine. 

In the Biuret reaction, a purple colour develops when the protein is treated with alkaline copper sulphate. This reaction is dependent on peptide bonds and not on the side chains of individual amino-acids present. In the Folin-Ciocalteu reaction, the protein is treated with tungstate and molybdate under alkaline conditions and the formation of a complex such phenylalanine and tyrosine gives rise to a blue colour. Lowry developed one of the most widely used protein assays in which a combination of the above reactions is involved07, l8). 

Using controlled reaction conditions such as the temperature profile and rate and time of the addition of polyols, more uniform materials can be produced. The correct spacing of the hard segments required to produce the physical properties can be obtained. The controlled conditions will also help prevent the formation of undesirable side products such as allophanate, biuret, and trimers. These reactions will give branching of the polymer chains. 

Side reactions, e.g., formation of allophanate, biuret, isocyanurate, or catbodiimide linkages, may be formed, depending upon the reaction conditions. 

These compounds and their various derivatives prepared by Curtius and his pupils are white crystalline compounds, forlthe most part soluble with difficulty in cold water. Some of them give the biuret reaction, but others do not, in particular the less complex compounds where the influence of the acid radical inhibits the reaction, although the compounds possess the exact conditions, as determined by Schiff, for the positive exhibition of the reaction. 

Details of the TDI and MDI foam model systems have been previously published [2]. The models require the use of mono-functional reactants that are quantitatively analysed to correlate structure-activity relationships for various classes of catalysts. A realistic thermal profile is produced through the imposition of an external exotherm. Urethane, urea, allophanate and biuret reaction products are quantified by liquid chromatographic analysis of quenched reaction samples. The models effectively account for such nonideal conditions as reactant depletion at variable rates, temperature and concentration-dependent catalyst activity, and catalyst selectivity as a function of isocyanate distribution. 

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. 

Biuret itself may, depending on the experimental conditions and, in particular, on the concentrations, give three kinds of complexes with Cu + in alkaline medium. Recall also that globulins, albumins, peptones, polypeptides (provided the number of amino acids constituting them is suitable), and amino acids give the biuret reaction. Let s finally recall that this reaction, carried out in well-determined conditions, permits us to determine the quantity of proteins in the analyzed solution. 

Bisphenol-A benzoxazine reaction, under acidic conditions, 416-417 Bisphenol-A polyarylates, 77 synthesis of, 109-113 Bisphenol-A polysulfone, 327 nucleophilic synthesis of, 337 Bisphenolic monomer, 354 Biurets, 227... 

The Biuret method depends on the reaction of compounds containing several peptide bonds with Cu2+ under alkaline conditions to form a violet-coloured species. 

There are a few other chemical reactions on the wood surface that could make important contributions. One is that of moisture on the surface of wood to form an unstable carbamic acid group that quickly decomposes to form a primary amine with evolution of carbon dioxide. The primary amine formed has active hydrogens reactive to isocyanate. Other successive reactions ensue leading first to disub-stituted ureas and then to biurets. Furthermore, isocyanate reaction with urethane to form allophanates, and trimerization of isocyanates to form isocyanurate are also possible to variable extents, under the conditions of bonding. The different reactions are summarized in Scheme 2. 

The reactions of phosgene with ammonia lead, depending upon the conditions, to the formation of the useful materials urea, biuret (NH CONHCONH j), cyanuric acid (1,3,5-triazine-2,4,6-triol), and successive products of ammonolysis up to melamine HjNCN(NHj)NC(NHj)N (see Section 9.10.2.1) [718a]. Urea is usually manufactured from ammonia and carbon dioxide, but a useful fertilizer mixture can be produced by injecting ammonia into gaseous phosgene in the ratio of 4 1 respectively [1439]. Urea and ammonium... 

In the above methods, the separated albumin (or whole serum if total protein is to be determined) is reacted with excess copper in approximately 1 N alkali. A macromethod is also available which utilizes the capacity of albumin to react with copper(II) in a mole ratio of 1 1 (K32). This is an amperometric titration of blood serum (1 ml) in 0.1 M ammoniacal am-moniiun nitrate pH 9.2 with CuS04 (4.8 X 10 M). Under these conditions human y-globulin did not react, so that the fair agreement by this method with results obtained by a biuret method after sulfite fractionation (K32) is probably the result of a balance of errors. In view of the interest in the small fraction of albumin-boimd copper in hepatolenticular degeneration, and the paucity of information on the unique binding reaction presumably with N-terminal aspartyl residues, further investigation would be valuable. 

Temperatures of reaction can, of course, be important. At up to SC C the linear chain-forming reaction predominates but as higher temperatures (up to 150°C) are reached then biuret and isocyanurate formation become effective and branching occurs. At above 150°C some of the less stable links are affected and reversion or degradation can then take place. It must be stressed that the isocyanate reactions are highly exothermic, and under conditions where heat transfer is slow appreciable temperature rises can be experienced care is necessary to minimize their occurrence, otherwise deterioration in properties results. 

HNCO is stable in the gas phase at conditions relevant for SCR [9], but is highly reactive with respect to byproduct formation in the condensed state [36]. Figure 16.1 shows a reaction scheme for urea decomposition, including the two byproducts biuret and cyanuric acid which are formed first [37]. 

This mechanism, however, can be considered as a single-step reaction only if the heating is carried out fast. A slow heating of the urea will produce side reactions, due to the high reactivity of isocyanic acid, with the production of undesirable chemical products such as biuret, trimet, and cyanuric acid [25]. Usually, the reaction is supposed to occur without any side effect and is endothermic (+185.5 kJ at standard conditions). 

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