SURFACE-PLASMON RESONANCE IMAGING MEASUREMENTS OF ELECTROSTATIC BIOPOLYMER ADSORPTION ONTO CHEMICALLY-MODIFIED GOLD SURFACES

A combination of in situ and ex situ surface plasmon resonance (SPR) i maging experiments is used to characterize the differential electrosta tic adsorption of proteins and synthetic polypeptides onto photopatter ned monolayers at gold surfaces. The nonspecific electrostatic adsorpt ion of proteins onto negatively charged self-assembled monolayers (SAM s) of 11-mercaptoundecanoic acid (MUA) is found to depend on the prote in pI, solution ionic strength, and solution pH. The pH dependence of the electrostatic adsorption of the protein avidin onto a MUA SAM indi cates that a full monolayer adsorbs at a solution pH greater than 5.0, and an ''effective pK(a)'' of 3.6 is determined for the avidin adsorp tion, This effective pK(a) is a combination of the pK(a) of the MUA mo nolayer and the ion pairing adsorption coefficient for the avidin. Add itional SPR imaging experiments show that the electrostatic adsorption of the synthetic polypeptide poly-L-lysine (PL) onto a MUA SAM varies with molecular weight, forming a full PL monolayer for polypeptides w ith more than 67 lysine residues.