Tuning conformations of fibrinogen monolayers on latex particles by pH of adsorption

Adsorption of fibrinogen on polystyrene latex particles was studied using the micro-electrophoretic and the concentration depletion methods. Measurements were carried out for pH 3.5, 7.4 and ionic strength of 0.15 M, NaCl. Electrophoretic mobility of latex was determined as a function of the amount of adsorbed fibrinogen, expressed as a surface concentration. A monotonic increase in the electrophoretic mobility (zeta potential) of the latex was observed, indicating a significant adsorption of fibrinogen on latex for pH equal to 3.5 and 7.4, respectively. The anomalous adsorption in the latter case was explained in terms of the heterogeneous charge distribution on the fibrinogen molecule. The stability of fibrinogen monolayers formed on latex was also determined in pH cycling between 3.5 and 9.7. These measurements revealed that fibrinogen adsorption was irreversible, governed by the two main adsorption mechanisms: (i) the unoriented (random) mechanism prevailing for pH = 3.5 where adsorbing molecules significantly penetrate the latex particle core and (ii) the side-on adsorption mechanism prevailing for pH equal to 7.4. In both cases, variations in the zeta potential with the fibrinogen coverage were adequately described in terms of the electrokinetic model, previously formulated for particle adsorption on planar substrates. Based on these experimental data, an efficient procedure of preparing fibrinogen monolayers on latex particles of controlled conformations and coverage was envisaged.