Exclusion of bovine serum albumin from the air/water interface by sodium myristate

Abstract

Mixtures of sodium myristate (SM), of 1 or 2 mM, and bovine serum albumin (BSA), from 100 to 10 000 ppm, were studied at the air/water interface using tensiometry, ellipsometry, and infrared reflection-absorption spectroscopy (IRRAS). The equilibrium surface tension of the mixtures ranged from 23 to 40 mN/m, which are values between those of the pure SM (20–25 mN/m) and BSA (50 mN/m), showing that lower tensions are favored and suggesting that SM was primarily on the surface for the concentrations studied. For the mixtures containing 1000 ppm BSA or less, the ellipsometric angles were the same as those of SM alone with surface densities ranging from 3.2 to 4.5 mol/m 2×10 −6. For mixtures with 10 000 ppm BSA, larger surface densities were determined as a result of the presence of myristic acid from the protonation of SM due to the pH decrease caused by the BSA addition. IRRAS results are consistent with the ellipsometry results, showing that at the lower BSA concentrations only SM was present at the surface, and at the higher BSA concentrations SM was present along with myristic acid. The results demonstrate that at the concentrations studied, SM can exclude BSA completely from the air/water interface.