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Chapter 2

Quantitative Membrane Electrostatics with AFM

2.1 Introduction

This chapter presents an overview of the work done by others, on quantitative membrane
electrostatics using AFM [1]. The work discussed herein forms the foundation for the
original work done by us in Chapter 3. In Chapter 1, we have discussed the importance
of membrane electrostatics. Many probes and techniques have been developed to measure
the electrostatic potentials of lipid membranes. Ofthese, atomic force microscope (AFM)
is a highly noninvasive probe. The AFM is able to image in fluid with nanometer-scale
topographical resolution [27, 28]. With its ability to image in fluid, AFM is useful for
biological applications because it can image biological structures under near-native
conditions. The AFM can also hold the tip over a specified position and measure force as
a function of tip-sample separation. This force-curve analysis has been applied to
molecular recognition interactions [29-32], protein unfolding [33], and nonspecific
hydrophobic, hydration, van der Waals, and electrostatic interactions [34, 35]. At low
electrolyte concentrations (0.5-5 mM) and tip-sample separations greater than a few
nanometers, one can reach a regime where electrostatics dominates the long-range tip
sample interaction.

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