CHARACTERIZATION OF POLY-L-LYSINE ADSORPTION ONTO ALKANETHIOL-MODIFIED GOLD SURFACES WITH POLARIZATION-MODULATION FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND SURFACE-PLASMON RESONANCE MEASUREMENTS

The electrostatic adsorption of poly-L-lysine molecules onto a vapor-d eposited gold film modified with a carboxylic acid-terminated alkaneth iol monolayer is monitored with the spectroscopic techniques of polari zation-modulation Fourier transform infrared (PM-FTIR) spectroscopy an d surface plasmon resonance (SPR). The PIM-FTIR spectrum of a monolaye r of poly-L-lysine (PL) adsorbed onto a self-assembled monolayer of 11 -mercaptoundecanoic acid (MUA) indicates that the lysine residues and the MUA carboxylic acid moieties form ammonium - carboxylate ion pairs which electrostatically bind the polypeptide to the surface. The PL m olecules can be desorbed from the surface by rinsing with a buffer sol ution at a pH that destroys the ion pairing (pH < 6.5 or pH > 12). Mea surements of the shift in the SPR angle upon adsorption were used to d etermine the thicknesses of the adsorbed monolayers; the MUA and PL mo nolayers were found to be 17.0 and 10.5 Angstrom thick, respectively. These thickness results suggest that the poly-L-lysine monolayer adsor bs onto the packed MUA surface in an extended conformation with the PL backbone lying parallel to the surface. Subsequent exposure of the PL monolayer to a solution of iron phthalocyanine tetracarboxylic acid ( FePc) resulted in the adsorption of a third layer onto the surface. Th e ability of the adsorbed PL molecules to interact with FePc indicates the presence of free lysine residues available for interaction with m olecules other than the MUA monolayer.