In the Biuret method for total protein, what is formed when peptide bonds react with copper II sulfate under alkaline conditions?

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Multiple Choice

In the Biuret method for total protein, what is formed when peptide bonds react with copper II sulfate under alkaline conditions?

When peptide bonds are present, copper(II) ions in the alkaline Biuret reagent coordinate with the deprotonated amide nitrogens and carbonyl oxygens to form a copper–peptide complex. This complex absorbs light to give a violet (purple) color, and the intensity grows with more peptide bonds (i.e., more total protein). So the key outcome is a violet complex formed in solution, not a precipitate. The violet color is distinctive of the Biuret reaction, setting it apart from other possible color changes or precipitates.

When peptide bonds are present, copper(II) ions in the alkaline Biuret reagent coordinate with the deprotonated amide nitrogens and carbonyl oxygens to form a copper–peptide complex. This complex absorbs light to give a violet (purple) color, and the intensity grows with more peptide bonds (i.e., more total protein). So the key outcome is a violet complex formed in solution, not a precipitate. The violet color is distinctive of the Biuret reaction, setting it apart from other possible color changes or precipitates.

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