Determination of proteins at nanogram levels by their quenching effect on large particle scattering of colloidal silver chloride

A novel quantitative method for the determination of proteins in aqueous so lutions has been based on the quenching of the resonance scattering light o f colloidal silver chloride in the presence of proteins. The detection limi ts for eight kinds of proteins (BSA, HSA, egg albumin, human gamma-IgG,alph a-chymotrypsin, E. Coli. alpsase, myoglobin, a-casein) were at about 8 ng/m L; the linear ranges of the calibration curves were 10-400 ng/mL under opti mal conditions,except for human gamma-IgG (20-400 ng/mL), myoglobin (10-300 ng/mL), and a-casein (10-300 ng/mL). Three wavelengths (398 nm, 475 nm, 49 9 nm) were all suitable for the determination and any acidity from pH 3.0 t o pH 9.0 could be chosen. A few non-protein substances at high concentratio n levels interfered with this method, but this problem could simply be over come by diluting the samples before the assay. Mechanism studies showed tha t the quenching effect of proteins on the scattering light of colloidal sil ver chloride was mainly due to the coagulation of AgCl particles retarded b y protein. The method was employed for the determination of total protein i n human serum with sactifactory results.