Description_x000D_ Principle_x000D_ Colorimetric, Endpoint.assay. Ohnishi and Barr′s modification of micro Lowry method. According to procedure, dilute biuret reagent reacts with peptide bonds to yield a purple-blue complex, the color of which is intensified by the addition of phenol reagent. Absorbance, read at 550-750 nm, is used to determine results from a standard curve.