Surface Organization and Cooperativity during Nonspecific Protein Adsorption Events

Despite many experimental studies on cooperative effects during protein adsorption events, this phenomenon is still poorly characterized and subject of much controversy. In this study, we address the topic of cooperativity using two distinct experimental approaches, namely, kinetic analysis and surface imaging, both based on supercritical angle fluorescence (SAF) microscopy. Several model systems comprising the two proteins BSA and fibrinogen, two different ionic strength conditions and varying pH environments were investigated. The combination of the experimental information obtained from kinetic analysis and from real-time in situ scan images unravel a clear correlation between cooperative adsorption and a heterogeneous protein layer build-up. We propose a mechanistic model of protein adsorption based on an overlap of classical Langmuir-type adsorption on unoccupied surface areas and an additional cooperative adsorption pathway near preadsorbed proteins which is consistent with the experimental observations. Moreover, the growth of two-dimensional surface clusters as an often assumed element of cooperativity could be excluded for the studied systems. The model includes the often observed phenomenon that the adsorption rate decelerates abruptly above a certain coverage limit. Furthermore, the observed evolution of the heterogeneous protein distribution on the surface is in good agreement with the proposed model.