title: A design and synthesis of model membrane systems for probing lipid-polyphenol interactions
Creators: name: Coones, Ryan T.
Publisher: name: University of Reading
Year: 2023

description: Modelling cellular membranes is crucial to understand how their structure affects function. Further, to investigate membrane-target interactions, accurate biological membrane models are essential. Understanding membrane interactions in this way has applications for in vivo processes, pharmaceutics and medicine, nutrition, and agriculture among others. This thesis presents research findings for two particular kinds of model membranes, bacterial and epithelial, and their interactions with a series of polyphenolic compounds. A distinction is made within about the nature of the “models”, where some models referred to are physical in nature, and others are computational or mathematical models. The epithelial model becomes focused on the human gastrointestinal epithelium. Here the lipid composition of this novel model epithelial membrane presented is the most complex and accurate model published to date. This work emphasises the importance of developing more complex model membranes for biological studies. This research aims to bridge the gap between their study and the ability to use surface sensitive techniques to measure membranes and their interactions. Similarly, the benefits of polyphenolic compounds are identified, and some polyphenols whose mechanism of action is relatively poorly understood are investigated in order to move towards developing structure-activity relationships. Analysis of model membranes through development and interaction with polyphenolic compounds is achieved through complementary surface sensitive techniques that allow lipid bilayer formation and interaction with polyphenols to be measured. Development of the lipid composition of the model bacterial and epithelial membranes takes place through analysis of single lipid components and investigates the effects of mixing lipids within model membranes through calorimetric and surface pressure measurements. Determination of polyphenol presence at a membrane interface, real-time measurement of mass changes for membrane formation and polyphenol interaction, and most crucially structural resolution of interactions in the nanometer regime are achieved using state of the art supported and floating lipid membranes. Mechanisms by which interaction of polyphenols with model biological lipid membranes is explored, with a comparison between bacterial and epithelial membranes highlighted to show variation in polyphenol interaction with membranes of differing phospholipid composition. The effects of lipid composition of membranes is studied using epithelial membranes of iteratively more complex and accurate composition, with membrane composition informed by a thorough meta-analysis of epithelial membrane lipid headgroup composition. We are able to show two different modes of action of polyphenols within membranes depending on the type and lipid composition of the membrane where nuances of polyphenol interaction are rationalised in terms of the molecular properties of the polyphenol under investigation. Bacterial model membranes, both physically and computationally, show strong and persistent interactions under flow with (-)-epigallocatechin gallate and Tellimagrandin-II, when characterised by neutron reflectometry. In the case of Tellimagrandin-II, apparent membrane lysis is observed with multilamellar membrane stacking occurring in the sample cell. By contrast, the epithelial membrane shows binding to the surface of, and intercalation into the tail region, of the membrane. The differences between these two modes of interaction have important implications where the dietary, pharmaceutical, and antimicrobial properties of these compounds is concerned.