Low density lipoprotein interaction with amino acid-modified self assembled monolayers on surface plasmon resonance surfaces Original Research Article

Using a 1-mercaptundeconoic acid assembled on Au, surface plasmon resonance (SPR) was used to investigate specificity of low density lipoprotein (LDL) interaction with the surface. The 1-mercaptundeconoic acid was dispersed in a mixed 1-mercaptoundecanol: 1-mercapto-octyl-hexa(ethylene glycol) (80:20) self assembled monolayer (SAM) showing an LDL adsorption dependent on 1-mercaptundeconoic acid. The 1-mercaptundeconoic acid SAM was modified by coupling amino acids via the N-terminal. Glycine modification gave a surface that resisted LDL adsorption. Amino acids with –COOH side chains favoured LDL adsorption, whereas lysine encourages oxidised LDL (oxLDL) adsorption. Comparison with LDL and macrophage scavenger receptors indicated some useful amino acid combinations that were shown to have a high affinity for native but not oxLDL (aspartate–glutamate) or vice versa (lysine–lysine). Sequences with affinity for oxidised but not native LDL were either net positively charged or contained thiol-groups: a correlation with known oxLDL scavengers did not show complete homology here, but indicated some similarities. The ‘best’ (GlyCystineSerAspGlu and GlyLysLys–OH) surfaces were selected from the library for determination of native and oxLDL, respectively and detection limits of 1 μg ml−1 estimated in both cases. The ratio of the adsorption on these surfaces was shown to give a robust estimate of the degree of LDL oxidation as related to the relative electrophoretic mobility (REM).