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Chapter 3

Mobile Lipid Charge Regulation in Membranes

3.1 Introduction

Biomembranes are usually in a fluid state in which individual membrane components are
free to move in lateral directions within the plane of the membrane, whereas their vertical
movements are highly restricted. Since the lipids are mobile, the membrane can respond
to interacting macromolecules by locally changing its composition. For example,
consider the adsorption of a negatively charged DNA molecule onto a membrane
containing a fraction of cationic lipids initially randomly dispersed among nonionic
lipids. Upon adsorption, mobile lipid charge regulation is thought to occur, resulting in
cationic lipids diffusing towards the interaction zone and neutral lipids diffusing away,
thereby minimizing the electrostatic interaction free energy. DNA-Cationic lipid
complexes have been observed experimentally [56, 57] and are candidates for nonviral
gene-therapy and for vehicles to transport DNA into cells [58-62]. These types of systems
have been studied extensively [63-66]. In particular, it has been shown that spatial
inhomogeneities in the membrane surface-charge density, in response to interactions with
the DNA, can have a significant effect on the phase behavior and stability of DNA-
cationic lipid complexes [67]. In addition, numerous other recent theoretical studies on
the adsorption of charged macromolecules other than DNA can be found in the literature
[68-70]. Recently, Netz et. al. [26, 71] have extended the Poisson-Boltzmann theory of
membrane electrostatics to include mobile lipid charge regulation and have applied it to

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