Peptides, detection biuret reaction

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.)... 

The biuret reaction [alkaline Cu(II)] and the arginine test [102] are used to detect peptides in nucleic acid preparations. Two other colour reactions permit either type of nucleic acid to be detected in the presence of the other one is due to Dische and the other is the phloroglucinol test of v. Etjler and Hahn. 

Weber et al. [111,147,149-156] have extensively studied the biuret reaction as a means of detecting peptides separated by HPLC. They identified a number of operating parameters that influence sensitivity, including post-column temperature, reaction time, buffer composition, Cu(II) concentration, peptide... 

The biuret reaction is best suited for detection of peptides three amino acid residues and longer but, nonetheless, a number of dipeptides are detectable at reasonable potentials [156]. Although N-terminal dipeptide amides require a high pH (pH 12) for Cu(II) complexation, they are detected at relatively low potentials (-+0.50 V vs. Ag/AgCl). In comparison, C-terminal dipeptide amides exhibit a... 

The LC-EC method is attractive for peptides because of the high sensitivity possible. The concentration detection limit is comparable to the low pM detection limits achieved by RIA [42,43] but with the added advantage of simultaneous detection of multiple analytes. The main limitation of the biuret approach is the uncertainty of detection limits for a given peptide. Exploration of other peptides has revealed that detection limits can vary by over an order of magnitude [3]. The detection limit variability may be due to several factors, including variable loss of samples due to adsorption, differences in electron transfer rates for the different peptides at C-fiber surfaces and the -tO.8 V potentials that were used, and differences in the yields of the derivatization reaction. 

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