Biuret reactions

Formation of Biuret., and Biuret reaction. Place 0 2 g. of urea in a dry test-tube, heat very gently just above the m.p. and note the production of ammonia. After 1-2 minutes the liquid suddenly solidifies with the formation of biuret ... 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

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 frequently used biuret reaction of proteins results in the formation of the following reddish-violet complex in alkaline medium [4, 2S, 26] ... 

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. 

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. 

Figure 11.9 Biuret reaction. The coordination complex formed in alkaline solution between cupric ions and the nucleophilic nitrogen atoms in four molecules of biuret.

Figure 11.10 Absorption spectrum of the protein-copper complex of the biuret reaction.

Procedure 11.1 Quantitation of protein using the Biuret reaction ... 

A reaction of protein with the Cu(II) ion, which forms a violet Cu chelate, is well known as a biuret reaction. Studies of this reaction65 show that Cu chelate formation was hindered for polypeptides which assumed a helical or -structure. 

The biuret reaction is generally found to be negative after five hours. 

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). 

The terminal amino and carboxyl groups react in the same way as the corresponding amino acids (acylation, animation, esterification, etc.). Some of the reactions of the amino groups (with ninhydrin, orthophthaldehyde, fluorescamine, etc.) are used for detection purposes, as will be discussed later. Peptides also react in ways that free amino acids do not, like the classic biuret reaction, which consists of the formation of a colored complex with a transition metal (Cu, Ni, etc.)... 

Human hair (Note i) is freed from foreign matter (Note 2), washed well with a lukewarm solution of soap (Note 3), then twice with cold distilled water, and dried. This washing removes the natural oils from the hair (Note 4). Two kilos of the dry, washed hair is pushed into a 12-I. round-bottom Pyrex flask, and 4 1. of 20 per cent hydrochloric acid (Note 5) is added. An air-cooled reflux condenser, consisting of a wide glass tube, is attached to the flask. The hair is hydrolyzed by heating on the steam bath (Note 6) until the biuret reaction is entirely negative this requires one hundred twenty to one hundred forty-four hours. 

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. 

The biuret reaction (M16), the Folin and Ciocalteu reagent (K22), ultraviolet adsorption (H2) and polarographic determinations (Fig. 25) have also been proposed (B2, H16). 

In the familiar biuret reaction the copper ion2 forms the center of a violet-red, anionic complex ... 

Biuret Reaction. The particular capabilities of the stop-ped-flow vidicon system have been used to help explain some unusual behavior in the biuret reaction. The biuret reaction is the basis for the standard clinical chemistry technique for determining the total protein content of human blood serum (23). It involves a complexation reaction, in alkaline solution, between the cupric ion (Cu2+) and the peptide bonds of the protein. In the standard biuret technique, the reaction is allowed to go to completion and then the absorbance of the copper-protein complex is measured at 55 nm. This technique tends to be rather slow since the reaction, although very rapid over the first few seconds, does not go to completion for at least 20 to 30 minutes. 

Figure 7. Absorption spectra at various times during the biuret reaction with...

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