The Role of Cardiolipin and pH in the Self-Assembly of Bacterial Membrane Mimics

Abstract

Cardiolipin is a unique four-tailed pH-responsive lipid found in the mitochondria of animals and in the membranes of bacteria. It has been found to play a role in altering numerous physicochemical properties of bacterial membrane mimics including membrane fluidity, mechanical stability, and thermotropic behaviour, and it is also unique in that its two headgroups have different pKa values (~2.8 and 7.5-9.5), allowing cardiolipin to be either uncharged, or carry one or two negative charges. However, despite these properties there is still relatively little research on its specific function and role in shaping the self-assembly and phase behaviour of bacterial membranes and the response of these membranes to their environment. In light of the rising need for novel strategies for targeting bacteria due to antibiotic resistance, cardiolipin is an important target for investigation in order to gain a deeper understanding of the bacterial membrane and how it can be disrupted.
This project aimed to investigate the effects of cardiolipin and pH on the properties of bacterial membrane mimics, specifically the phase behaviour of mesophases and the stability of liposomes, two models for bacterial membranes. High-pressure small-angle x-ray scattering (HP-SAXS) was used to construct pressure temperature phase diagrams of mesophases consisting of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) in a 78:22 ratio, and DOPE, DOPG and 18:1 cardiolipin in a 70:20:10 ratio, mimicking the membrane of E. coli., at pH 1.3, 5.1 and 12.1. Both cardiolipin and pH were found to have an impact on the phase behaviour of the system, with the HII phase forming under ambient conditions at pH 1.3 and the Lα phase forming at pH 5.1 and 12.1. Cardiolipin was found to have an effect on the behaviour of the system, increasing the sensitivity of the coherence length L to temperature, increasing the d-spacing at all temperatures studied at pH 5.1, and increasing the dependence of d-spacing on pressure at pH 5.1. DOPE/DOPG and DOPE/DOPG/CL liposomes were also used to model the E. coli membrane at pH 5.1 and 12.1 and, where cardiolipin and pH were also found to have an effect on the properties of the liposomes. The cardiolipin-containing liposomes did not undergo any significant change in average diameter D or zeta potential ζ between pH 5.1 and 12.1, while the DOPE/DOPG liposomes had both a lower D and ζ at pH 12.1 than at pH 5.1, pointing to a definite role of cardiolipin in mediating the response of the lipids to pH.

Date of Award	26 Nov 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Wuge H Briscoe (Supervisor)