Biuret complexes

As an example, the Ag(II)/Ag(I) couple, which has the relatively low A value of 44 kcal mol-1 in water, displays in TFA an oxidation peak in the region of 2.0-2.4 V (reduction takes place at potentials <0 V) showing that its E° is probably lower than 2.0 V (Eberson, 1980b). Similarly, Cu(III)/Cu(II) in the form of its Cu(III) (H 4 biuret) complex displays an oxidation peak with Ep x... 

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. 

Recently it has been found that formation of the biuret complex also suppresses mutarotation almost completely, while acidification (and consequent destruction of the complex) reactivates the mutarotation process (von Hippel and Wong, 1961). It was inferred from these results that chelation of the cupric ion across the peptide bond inhibits the development of the poly-L-proline Il-type helix by restraining adjacent residues in sterically unfavorable configurations. 

K6. Kekki, M., and Siltanen, P., Microdetermination of protein by determining the protein bound copper of the biuret complex with oxalyldihydrazide. Scand. J. Clin. Lab. Invest. 12, 235-238 (1960). 

The only non-octahedral complex of cobalt(ni) whose structure has been described during the period of this Report is that of the paramagnetic biuret complex KCo(3-propbi)2,2(l-propbiH2), where 3-propbi = HN-CO-NPr --CO NH and 1-propbi = Pr HN-CONH CO NH2. The crystals contain [Co(3-propbi)2] anions (13) in which the cobalt atom is in a square-planar co-ordination. The Co-N distances are equal at 1.88 A. ... 

Lunte and coworkers [162] recently showed the utility of on-column com-plexation using a Cu(II)-containing buffer for monitoring Leu-enkephalin metabolism in plasma. Using CE separation, biuret complexation, and electrochemical detection, a complete separation of Leu-enkephalin and its five metabolites was achieved. Applicability of this technique to plasma samples allowed two major... 

Chen J-G, Logman M, Weher SG (1999) Effect of peptide primary sequence [Pg.35]

In alkaline solution biuret, HN(CONH2)2 reacts with copper(II) sulfate to give a characteristic violet colour due to the formation of the complexes [Cu2(/l-OH)2(NHCONHCONH)4] (Fig. 28.6a) and [Cu(NHCONHCONH)2] . This is the basis of the biuret test in which an excess of NaOH solution is added to the unknown material together with a little CUSO4 soln a violet colour indicates the presence of a protein or other compound containing a peptide linkage. 

Cobalt(III) complexes containing mixed chelating ligands have been produced. Reaction of potassium bis[biuretocobaltate(III)], K2[Co(bi)2] with R2dtc or Rxant at 0° produces (313) the blue-violet [Co(bi)2(S—S)] ion (S—S = R2dtc or RXant). If the reaction is performed above 0° in the presence of water, the products are [Co(bi)2(S—S>2] and biuret. 

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

Biuret (bu) co-ordinates transition metals in both neutral and anionic forms. Compounds containing neutral bu include both bis- and tetrakis-complexes. The latter are limited to the larger cations of the group II [e.g., [Sr(bu)4]2+ [9] and lanthanide groups e.g., [Sm(bu)4]2+ [ 10,11]. Only the former are considered in detail in this review, together with bis-complexes containing anionic bu as they have the greater potential for formation of 1-D chains and 2-D sheets. 

When co-ordinating transition metal cations the neutral biuret molecule is ver-satile it adopts diverse conformations to form [M(bu)2Cl2] (M=Cu [12],Zn [13], Cd [14] or Hg [15]) complexes. With cadmium(II) andmercury(II),bu adopts a similar conformation to that in the free ligand, retaining its short intramolecu-... 

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.

This is a modification of the Lowry method involving a dye-binding step. The copper-protein complex that forms the basis of the biuret and the Lowry methods can be chelated by bicinchoninic acid to produce a very stable complex with a strong absorption maximum at 562 nm. 

The situation is even more complex since the N—H bonds of both urethane and urea linkages add to isocyanate groups to form allophanate and biuret linkages, respectively. These... 

Deprotonated amides can be very effective in stabilizing nickel(III). Complexes with biuret and substituted biurets have been reported.3083,3084... 

Very little IR or Raman spectroscopy of copper(III) complexes have been reported,297 but recently the spectra of [Cuu,11I(biuret)] have been examined by normal coordinate analysis,1349 and the biuret ligand undergoes little change on oxidation from copper(II) to copper(III), with a general increase in the frequency of the modes of vibration. 

When substances containing two or more peptide bonds react with the biuret reagent, alkaline copper sulfate, a purple complex is formed. The colored product is the result of coordination of peptide nitrogen atoms with... 

In view of the presence of active functional groups at suitable sites in the above ligands and their substituted derivatives, these tend to form coordination compounds/chelates with a large number of metals. Excellent reviews have been published on the complex compounds of biguanides151152 and guanylureas, 51 and biuret.153,154... 

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