Phospholipid bilayers on a polyion-alkylthiol layer pair: microprobe imaging, electrochemical properties and peptide association Ruxandra Vidu a , Liqin Zhang a , Alan J. Waring b , Robert I. Lehrer c , Marjorie L. Longo a , Pieter Stroeve a, * a Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA), Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USA b University of California at Los Angeles School of Medicine and Harbor-UCLA, Los Angeles, CA, 90095, USA c Department of Medicine and Molecular Biology Institute, UCLA, Los Angeles, CA 90095, USA Abstract Atomic force microscopy (AFM) images of successive layers and peptide insertion in supported mobile phospholipid bilayers on polyion/alkylthiol layer pairs are reported. The morphology of each layer observed in a step-by-step adsorption process is correlated with electrochemical (cyclic voltammetry (CV)) and surface plasmon spectroscopy (SPR) results. The insulating properties of long- chain and short-chain alkylthiol layers are associated with a continuous layer and domain formation, respectively. The multilayer surface morphology is quantitatively characterized using roughness measurements for a large set of data involving root-mean-square roughness (RMS). Since the AFM observation with atomic and molecular resolution requires very flat and wide terraces to monitor the adsorption process, it is very important that initial surface morphology and roughness is known. Peptide association with the lipid membrane was studied by high resolution AFM in liquid, where pore formation was demonstrated for protegrin transmembrane insertion. The association of peptides with supported lipid bilayer of various compositions is discussed in terms of mobility and membrane permeability to charge transfer through the formation of pores. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical properties; Ellipsometry; Peptide 1. Introduction Understanding and controlling surface morphology in model systems for biological membranes is of both fundamental and technological importance. Of particu- lar interest are defects or pores formed during the assembly process or formed as a result of protein adsorption to the assembled membrane. Such defects or pores can be advantageous (such as in the deliberate formation of ion channels) or disadvantageous when one desires an electrically sealed system. Usually, adsorption and structural properties of biological mem- branes are studied with various techniques, such as ellipsometry, surface plasmon spectroscopy (SPR), X- ray and neutron reflectometry, and quartz microbalance [1  /4]. Cyclic voltammetry, impedance spectroscopy and other electrochemical methods have been also used to study the electron transfer properties across lipid bilayer membranes used as experimental models of biomem- branes [5  /11]. However, it is desirable to have com- plementary in situ techniques for high-resolution topographical studies. In recent years, measurements of the thin-film surface and interface topographies have undergone significant progress with the introduction of scanning probe microscopes (SPM) [12,13]. The atomic force micro- scopy (AFM) has become the instrument of choice for surface characterization owing to its wide dynamic range of lateral measurements. The state of art AFMs generally have a lateral resolution of 10 nm and a vertical noise floor of about 0.05 nm, offering accurate measurements. Changes in surface topography caused by surface modifications through adsorption of layers are commonly investigated in situ by AFM [14]. However, the difficulty of imaging the soft surfaces of solid-supported membranes for biomimetic systems * Corresponding author. Tel./fax:  /1-530-752-8778 E-mail address: pstroeve@ucdavis.edu (P. Stroeve). Materials Science and Engineering B96 (2002) 199  /208 www.elsevier.com/locate/mseb 0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5107(02)00318-5