The adsorption of prothrombin to phosphatidylserine multilayers quantitated by ellipsometry

We investigated by means of an automated ellipsometer the adsorption of prothrombin from a buffer solution by multilayers of 14:0/14:0- and 18:1/18:1-phosphatidylserine (PS) stacked on chromium slides. In this instrument thickness and refractive index of the adsorbed phospholipid and proteins are monitored continuously. Two equations are derived to relate the mass of stacked phospholipids and the mass of protein adsorbed to the thickness and refractive index. These equations are based upon the Lorentz-Lorenz relation among the molar refractivities, refractive indices, and the densities of binary mixtures. Experimental validation of these equations is performed by measuring stacked multilayers of known mass of phosphatidylserine and the adsorption of [125I] albumin and [3H]prothrombin on these multilayers. Using these equations we measured the dissociation constants Kd and the number of binding sites nb of prothrombin. Values of Kd = 0.15 x 10(-8) M and nb = 122 molecules of PS/molecule of prothrombin were observed for di C14:0 PS and values of Kd = 0.45 x 10(-8) M and nb = 54 molecules of PS/molecule of prothrombin for di C18:1 PS. These data compare well to data obtained by other methods available in the literature.