[Thesis]. Manchester, UK: The University of Manchester; 2018.
Among the great breakthroughs in nanoscience and nanotechnology is the emergence
of synthetic polymers that demonstrate biological activity and thus can be exploited
for biomedical applications, extending from agents in therapeutics to drug
delivery and tissue engineering. A key factor in the fabrication of such polymeric
materials is the ability to tune and control their properties. To this end, an insight
into the mode of interactions with biological systems is imperative. Computer simulations
have proved to be a valuable tool that can compliment experiments and provide
-otherwise inaccessible- information.
In the context of this thesis, different aspects of the polymeric biological activity
investigated by studying two polymeric materials suitable for different types of applications,
aiming to clarify yet undisclosed mechanisms that govern the polymersĂ˘
behaviour either in solution or in conjunction with model lipid membranes.
The first part of the thesis is dedicated to a nonionic amphiphilic copolymer known
Pluronic L64 that is considered as a candidate for the design of novel hybrid polymer-lipid
vesicles that will act as carriers for drugs or genes. The hybrid bilayers are subjected
to mechanical stress and their properties are compared to those of pure lipid
bilayers. The simulations showed that the hybrid membranes can sustain increased
surface tension prior to rupture, are stiffer, thicker and the polymers can induce
lipid tail packing and also reduce the lipid mobility, rendering the membranes more
ordered and less fluid. At high values of lateral pressure, which leads to pore formation,
the copolymer chains decelerate the pore growth. The examination of the defect
formation mechanism reveals that the hydrophilic PEO segment plays the most vital
role. The same systems were also observed in varying temperatures and the impact of
the inserted polymers on the phase behaviour was investigated. The data suggested
that the polymers change the nature of the phase transition from a discontinuous to
a continuous one. The hybrid membranes transform between the ordered and the
disordered phase in a continuous manner and not at a critical melting temperature.
Interestingly, the effect of polymers is different at the low and high temperature
as proved by the analysis of the mechanical, structural and dynamic membrane
The second part is focused on the study of polyhexamethylene biguanide (PHMB), a
biguanide-based polyelectrolyte, that possesses remarkable biocidal properties. Even
though PHMBĂ˘s activity is known, the specific mode of action against bacterial membranes
is still puzzling. Our work revealed that the polyelectrolyte assumes a counterintuitive
behaviour in aqueous solution tending to self-organise into ordered compact
structures, despite the repulsive electrostatic interactions of its positively charged
The formed nano-objects are thermodynamically stable, as was confirmed by
free energy calculations and could be linked to PHMBĂ˘s antibacterial mechanism.
These findings pave the way for further computational and experimental exploration
of these fascinating and promising materials that could lead to the design of novel
smart biologically active nanoparticles.