Adsorption of bovine serum albumin onto mica surfaces studied by a direct weighing technique

Adsorption of bovine serum albumin (BSA) from aqueous solution to muscovite mica has been investigated through the measurements of two quantities: (1) the adsorbance, i.e. adsorbed mass per unit area measured as a function of incubation time by means of a Mettler UM3 ultramicrobalance; and (2) the contact angle of water droplets resting on the adsorbed BSA layer to know the degree of surface covering. The protein concentration explored was 0.002 and 0.01 mg ml(-1) in aqueous solution at pH 5.8. The temperature was kept constant at 25 degreesC. The following results have been obtained in the present study. (1) The initial rate of adsorption is more rapid than generally accepted. The surface of mica is completely covered by adsorbed BSA molecules within 10 min even for a dilute solution at a concentration 0.002 mg ml(-1). (2) At a stage where the surface covering is completed, the adsorbance levels off at a steady value, which is approximately equal to mass per unit area of side-on monolayer. It concludes from this result that BSA molecules are adsorbed side-on directly onto mica surface to form a side-on monolayer. (3) If the adsorption is prolonged, the adsorbance exceeds the mass per unit area of side-on monolayer owing to the adsorption of BSA molecules onto the side-on monolayer already formed. This fact indicates the occurrence of multilayer adsorption of BSA. (C) 2002 Elsevier Science B.V. All rights reserved.