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

A spectrophotometric method for the simultaneous determination of prot eins and total amino acids in mixtures is described, based on the obse rvation that the products of the reaction between p-benzoquinone (PBQ) and proteins show an absorbance band at about 350 nm, whereas for mos t 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 rang e 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 absor bance were not observed for up to 48 h after starting the reaction. Th e molar and specific absorptivities for several amino acids and protei ns, respectively, were determined. The detection limit observed with t he PBQ method was compared with those obtained with other methods. A r ecovery of about 100% was observed for both BSA and total amino acids when the PBQ method was applied to an amino acid-protein mixture resem bling cerebrospinal fluid.