SPECTROPHOTOMETRIC METHOD FOR THE SIMULTANEOUS DETERMINATION OF PROTEINS AND AMINO-ACIDS WITH PARA-BENZOQUINONE

A spectrophotometric method for the simultaneous determination of proteins and total amino acids in mixtures is described, based on the observation that the products of the reaction between p-benzoquinone (PBQ) and proteins show an absorbance band at about 350 nm, whereas for most of the essential amino acids the absorbance band is at about 440 nm. Beer's law was obeyed by the products of the reaction between PBQ and 1:1 or 2:1 (w/w) mixtures of proteins and amino acids for a wide range of concentrations. The products of the reaction were stable for 2 h at either 5 or 30-degrees-C; shifts of the wavelength of maximum absorbance were not observed for up to 48 h after starting the reaction. The molar and specific absorptivities for several amino acids and proteins, respectively, were determined. The detection limit observed with the PBQ method was compared with those obtained with other methods. A recovery of about 100% was observed for both BSA and total amino acids when the PBQ method was applied to an amino acid-protein mixture resembling cerebrospinal fluid.