Human low density lipoprotein and human serum albumin adsorption onto model surfaces studied by total internal reflection fluorescence and scanning force microscopy

The adsorption of low density lipoprotein (LDL) and human serum albumin (HSA) to model surfaces of different hydrophobicities has been studied using two, surface-sensitive, real-time, in situ techniques: total internal reflection fluorescence (TIRF) and scanning force microscopy (SFM). The model surfaces used were: (1) hydrophilic negatively charged silica (TIRF) and mica (SFM) surfaces, (2) hydrophobic octadecyldimethylsilyl-(ODS)-modified silica (TIRF) and ODS-modified oxidized silicon (SFM) surfaces and (3) amphiphilic ODS-silica gradient surfaces (TIRF). The kinetics of fluorescein isothiocyanate-LDL adsorption onto the ODS-silica gradient surface from FITC-LDL solution and from a solution mixture of LDL and HSA showed that a transport-limited process on the clean silica changed into an adsorption-limited process with increasing surface coverage of ODS chains. SFM analysis of the in situ adsorption of LDL on hydrophilic mica demonstrated a steady increase in surface coverage with time which was somewhat lower than determined by TIRF for FITC-LDL adsorption on silica. The adsorption behavior of a binary mixture of HSA and LDL suggested that lateral interactions between HSA and LDL affect the adsorption process. The diameter of LDL adsorbed on mica and ODS-modified silicon has been determined using SFM to be approximately 55 nm. Tetrameric LDL aggregates were observed on all of the surfaces in addition to some dimers and trimers. Imaging LDL and HSA adsorption on clean oxidized silicon surfaces using 'contact mode' SFM techniques was hindered by probe manipulation of the proteins.