Experimental and computational approaches to the lipid L₃ sponge mesophase

Abstract

Among a multitude of studies on lipid polymorphism over the last half century, the sponge mesophase remains relatively little-studied, and poorly
understood.
The sponge (L3) mesophase consists of a lipid bilayer separating the system into
two bicontinuous water channels. In that sense, it is closely related to the so-called
cubic phases, which possess cubic translational symmetry. Unlike the cubic phases,
however, the sponge mesophase is aperidoic, its underlying surface often described
as being ‘random’. While cubic phases have been the subject of much study due
to their potential applications for membrane protein crystallisation, there is increasing evidence to suggest that this occurs via the sponge mesophase, creating a
significant gap in our understanding of the process, and how to maximise chances
of success. This thesis therefore seeks to further our understanding of the dynamics of the sponge mesophase through a mixture of experimental (X-ray scattering)
and computational (molecular dynamics) approaches.
Chapters 1 and 2 introduce the subject of lipid polymorphism, the sponge mesophase,
and outlines the theoretical basis of the techniques used to study lipid systems in
this work.
In chapter 3, we conduct a systematic type-doping study of the sponge mesophase
with common co-crystallisation amphiphiles. We show that the sponge mesophase
is significantly less adaptable than the cubic phase, with significant implications
for the engineering design of membrane protein crystallisation media.
In chapter 4, we develop computational techniques with which to study lipid mesophases
using molecular dynamics. Chapter 5 then utilises the these tools to study the cubic to sponge transition using coarse grained molecular dynamics simulations,
showing that the system undergoes the transition on very rapid timescales, and is
particularly driven by a combination of a reduction in elastic moduli and increase
in interfacial area.
In chapter 6, we study the effect of an interface on the sponge mesophase by combining acoustically levitated droplets with X-ray scattering studies. While the
study is overall inconclusive, we present a series of analysis methods which could
be utilised to resolve the subject in future.
Chapter 7 summarises the key results of the thesis, and presents an outlook on the
future of the topic of lipid sponge mesophases.

Date of Award	2 Dec 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Annela M Seddon (Supervisor)