We propose to build an atomic force microscope with a conducting tip to identify sites where electroactive molecules span an otherwise insulating lipid membrane. The microscope will be capable of detecting currents of a few femtoamps, and will operate under environmental and temperature (80-300K) control. Recent work by Mazur and Hipps indicates that the current-voltage characteristics at conductive sites are related to the redox chemistry of the electroactive molecules and we propose to study this phenomenon using well-characterized molecular assemblies of porphyrin-linked alkanethiols with the goal of distinguishing individual organic molecules. Operation at low temperatures will be used to control ion currents (such as unwanted surface conduction in a water layer) and to stiffen soft samples (leading to higher AFM resolution). Our ultimate goal is to develop a new spatial-mapping tool for studying electronic and ionic processes across cell membranes. The first biological sample we will study will be the well characterized (and highly ordered) arrays of the photosynthetic reaction center from Rhodopseudomonas viridis. We hope to be able to identify the cytochrome bcl complexes and determine their spatial distribution in the membrane.
