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Properties of Surfactants That Govern Their Functions and Applications on Lipid Membranes

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Properties of Surfactants That Govern Their Functions and Applications on Lipid Membranes Properties of surfactants that govern their functions and applications on lipid membranes by Mozhgan Nazari A thesis submitted in conformity with the requirements for the degree of Doctorate of Philosophy Pharmaceutical Sciences University of Toronto © Copyright by Mozhgan Nazari 2012 Properties of surfactants that govern their functions and applications on lipid membranes Mozhgan Nazari Doctorate of Philosophy Department of Pharmaceutical Sciences University of Toronto 2012 Abstract This work aims at a better understanding of the parameters and properties of surfactants that govern their specific biological functions and their suitability for applications related to biological membranes. First, the hypothesis is tested that membrane lysis starts when a surfactant has built up a critical level of curvature strain, that is detected as a chain disordering. This could be confirmed for many surfactants but some, mostly biosurfactants act by a locally focused, heterogeneous perturbation, which explains their enhanced antibiotic activity and selectivity. A model and protocol were established to obtain a volumetric characterization of micelles by pressure perturbation calorimetry. For some classic detergents studied so far, their chain packing was found to agree with that of bulk hydrocarbon. Finally, the effective charge and consequent membrane permeability were assessed by zeta potential measurements. ii Acknowledgements I would like to gratefully and sincerely thank Dr. Heiko Heerklotz for his guidance, understanding, patience, and most importantly, his friendship during my graduate studies at University of Toronto. His mentorship was paramount in providing a well-rounded experience and utmost devotion to my professional growth and academic training. He encouraged me to not only grow as an experimentalist and a scientist but also as an independent thinker. For everything you’ve done for me, Dr. Heerklotz, I thank you. I would also like to thank all of the members of the Heerklotz research group. They have always been willing and generous in sharing both their time and expertise. Through their kindness and friendship, my colleagues have made spending time in the lab an enjoyable process. I am also thankful to the members of my supervisory committee, Dr. Rob Macgregor, Dr. Tigran Chalikian, and Dr. Christine Allen, for their input, valuable discussions and accessibility. I thank you for providing me guidance throughout my development as a researcher. I would like to express deep gratitude to my parents, Houshang and Esmat, for their faith in me and allowing me to be as ambitious as I wanted. It was under their watchful eye that I gained so much drive and an ability to tackle challenges head on. Finally, I would like to thank my husband, Mehdi, my daughters, Donna and Deena, for their support, encouragement, quiet patience and unconditional love, which were undeniably the bedrock upon which the past five years of my life have been built. iii Table of Contents Acknowledgements .................................................................................................................................................. iii Table of Contents ..................................................................................................................................................... iv List of Abbreviations ............................................................................................................................................... viii List of Figures ............................................................................................................................................................ xi List of Tables ........................................................................................................................................................... xiv CHAPTER (1) - Introduction .......................................................................................................................................... 1 1.1 Lipid Membranes ........................................................................................................................................ 1 1.1.1 Structures, properties and functions ................................................................................................. 1 1.1.2 Hydrophobic effect and hydration force: Two phenomena being responsible for the formation and stabilization of lipid bilayer vesicles and cellular membranes ..................................................................... 2 1.1.3 The mobility of lipid molecules within a lipid bilayer: gel phase versus fluid phase......................... 3 1.1.4 Impacts of phospholipid acyl chain length and saturation on gel and fluid phases .......................... 3 1.1.5 The role of cholesterol in cell membranes ......................................................................................... 4 1.1.6 Lipid compositions in eukaryotic cell membranes ............................................................................. 5 1.1.7 Lipids in prokaryotic cell membranes ................................................................................................ 6 1.2 Surfactants .................................................................................................................................................. 7 1.2.1 Definition ........................................................................................................................................... 7 1.2.2 The Critical Micelle Concentration ..................................................................................................... 7 1.2.3 Micelle Size and Structure ................................................................................................................. 8 1.2.4 Dynamic structure of micelles ........................................................................................................... 9 1.2.5 Volumetric properties of micelles .................................................................................................... 11 1.2.6 Synthetic detergents and their applications .................................................................................... 11 1.2.7 Biosurfactants .................................................................................................................................. 12 1.3 Interaction of surfactants with lipid membranes ..................................................................................... 18 1.3.1 The three-stage model ..................................................................................................................... 18 1.3.2 Surfactant partitioning into the lipid bilayer .................................................................................... 19 1.3.3 Membrane asymmetry and permeation .......................................................................................... 20 1.3.4 Curvature strain ............................................................................................................................... 21 1.3.5 Membrane solubilization ................................................................................................................. 24 1.4 Objectives ................................................................................................................................................. 27 1.4.1 Critical curvature strain in membranes and the mode of membrane solubilization ....................... 27 1.4.2 Developing a new method to measure volume and expansivity changes of detergent micelle formation 27 1.4.3 Improving our interpretation of volumetric parameters to characterize the internal packing and hydration properties of micelles, macromolecules, and solutes ...................................................................... 27 iv 1.4.4 Establishing zeta potential measurements for the study of the effective charge of membrane- bound molecules and their trans-bilayer distribution ....................................................................................... 28 CHAPTER (2) - Basic Concepts of the Applied Methods ............................................................................................. 29 2.1 Fluorescent Probes ................................................................................................................................... 29 2.2 Time-resolved Fluorescence ..................................................................................................................... 30 2.3 Anisotropy ................................................................................................................................................ 30 2.4 Isothermal Titration Calorimetry (ITC) ...................................................................................................... 31 2.5 Pressure Perturbation Calorimetry (PPC) ................................................................................................. 33 2.6 Dynamic Light Scattering (DLS) ................................................................................................................. 34 2.7 Zeta Potential............................................................................................................................................ 34 CHAPTER (3) - Classifying Surfactants with Respect to Their Effect on Lipid Membrane Order ................................ 37 3.1 Abstract ...................................................................................................................................................
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