The Interaction between Lipid Bilayers and Biological Membranes

1. Chapter 18 The Interaction between Lipid Bilayers and Biological Membranes 2002. 12. 5 전 희중, 이 민영, 이 원실

2. Introduction • Membrane & Phospholipid Bilayer Structure • Membrane • Lipid bilayer

3. Introduction • Forces Acting between Surfaces in Liquid Double-layer force • DLVO forces • van der Waals force(引力) • permanent or induced dipolemoment • electrostatic(double-layer) force(斥力) • surface charge density(potential) • Non-DLVO forces • solvation(hydration) force (斥力) • ionization:Na+[Na(H2O)n]+ • steric force (斥力) • thermal fluctuation • hydrophobic force (引力) • entropy effect:disordering van der Waals Force

4. Attractive van der Waals forces • van der Waals Forces • between two planar surfaces : • between amphiphilic structures : generally small • Hamaker constant for D < 3nm • Non-retarded: • Retarded: ionic screening of the zero-frequency

5. Electrostatic(double-layer) forces • Electrostatic(double-layer) Forces -The double-layer repulsion is much more sensitive to : the type and concentration of electrolytepresent, the pH, temperature and the surface charge density or potential.

6. Electrostatic(double-layer) forces • Type & Concentration of Electrolyte Effect - Measured repulsive forcesbetween the DSPG bilayersin various aqueous solutions at 22oC due to attractive ion correlation force CaCl2 : Surface charge ~ ion binding ---- : measured forces more attractive than expected from the DLVO theory DSPG : Distearoyl Phosphatidylglycerol

7. Electrostatic(double-layer) forces • Surface Charge(Potential) Effect • At higher monovalent electrolyte the double-layer repulsion generally • diminishes. • But between lipid bilayers it usually remains strong enough to keep the • surface apart even for low surface potentials and in high salt. • because of the relatively weak van der Waals attraction. • Theoretical DLVO interaction energy per unit area • between two amphiphilic surfaces in 0.15 M NaCl • weak secondary minimum • occurs at 4-6nm

8. Hydration forces • Hydration Forces (I) - Repulsive hydraton forces arise whenever water molecules bind strongly to hydrophillic surface groups - Between two solid crystalline surfaces the hydration force is oscillatory.Oscillations have a periodicity of the diameter of the water molecule(0.25nm)and reflect the ordering of water molecules into semidiscrete layers between the smooth, rigid surfaces.- Between bilayer surfaces : no such ordering into well-defined layers is possible ∵ 1) the head groups are rough on the scale of a water molecule 2) the surfaces are thermally mobile giving rise to a steric repulsion any oscillatory force becomes smeared out.

9. Hydration forces • Hydration Forces (II) - The range of the steric-hydration forces : between surfactant and lipid bilayers ~ 1-3nm - The overlapping of the mobile headgroups protruding out from two approaching bilayers producesa variety of repulsive steric or osmotic-type forces between them. Roughly exponentially repulsive - Forces between layersof di-hexadecyl phosphate in aqueous NaCl solutions • below 10-2 M : DLVO theory D>2nm : double-layer repulsion dominate D<2nm : van der Waals attraction domonateabove 10-2 M, high pH : repulsive hydration forces come in Hydration Force

10. Limitation of the hydration model • Hydration Model - The orgin of the monotonically repulsive hydration forces 1) Marcelja theory : the polarization induced by surfaces or surface groups on the water molecules adjacent to them.Molecular dynamic simulationsfailed to reproduce 2) Experimentally : more complex and less consistant- Pressure between egg lecithin bilayers in water, formamide, and PDO not unique to water !!

11. Limitation of the hydration model • Undulation Force Effect • Repulsion forces in water between uncharged monolaidin bilayers in the gel state and monocaprylin bilayers in the liquid-crystalline state, as measured by the Osmotic Pressure technique. • Temperature increasing • gel liquid crystalline : increase inthemagnitude • andrange of the repulsion Undulation Force

12. Limitation of the hydration model • Crystal/Gel/Fluid Lipid Bilayer(I)

13. Limitation of the hydration model • Crystal/Gel/Fluid Lipid Bilayer(II)

14. Limitation of the hydration model • Temperature Effect - Repulsion forces in water between various free phospholipid bilayers in water in the liquid-crystalline and fluid states, I.e., below and above the chain melting temperature, Tc, as measured by thee Osmotic Pressure technique. DPPC : DiphalmitoryPhosphatidylcholine

15. Steric forces • Four-type repulsive Steric forces (I) With the apparent failure of models based on purely electrostatic or solvation interactions to explain these forces, attention has recently focused on the role of thermal fluctuation interactions. Four-type repulsive Steric forces Undulation Identically long range Peristaltic Protrusion Headgroup overlap L : effective thickness of fluctuating headgroup region 2L : range of interaction

16. Steric forces • Four-type repulsive Steric forces (II) Theoretical plot of the four steric repulsion (double-chained lipid bilayers in the fluid state)

17. Hydrophobic forces • Hydrophobic forces (I) Attractive hydrophobic interaction (between hydrocarbon molecule or surface in water) At small D, much stronger than van der Waals attraction When bilayer are subjected to a stretched force, lateral expansion of bilayer Exposition of increased hydrophobic area to the aqueous phase Hydrophobic interaction Exposition of Hydrocarbon group to aqueous phase ∝ strongly attractive hydrophobic force Force between supported DLPC and DMPC layers in water

18. Hydrophobic forces • Hydrophobic forces (II) • Stress of bilayers and membranes Applying an electric field across bilayers and membranes Stressed by Osmotic swelling of cells or vesicles Introduce local stresses on the lipid via ion binding or packing mismatches with other membrane components • Temperature (T) Ex) Interaction between certain non-ionic micells and bilayers surfactant : polyoxyethylene headgroup

19. Specific Interactions • Specific Interaction (I) Electrostatic - some electrostatic interactions are unusually strong : ex) adhesion of phosphatidylserine bilayers in the presence of Ca2+ ions - certain divalent ions trigger conformational or chemical changes in proteins and membranes while others do not Bridging - bridging forces also arise in biological systems ex)polymer brides : connect myelin membranes prevent from moving too far from each other. Bridging

20. Specific Interactions • Specific Interaction (II) • Lock-and-key or ligand-receptor interactionsbiological interactions : immunological recognition (ligand-receptor interactions) cell-cell contactsbiotin : a ligandstreptavidine : a protein receptorbinding energy : 88kJ/mol (35kT per bond)by a change in thepHorsolution conditions locked unlocked

21. Interdependence of intermembrane and Intramembrane forces • Interdependence of Inter- & Intramembrane forces a0 (headgroup area)  a0가 증가함에 따라, bilayer사이에서 큰 Repulsive force를 수반 Large hydration of the lecithin headgroup : large surface area, a0≈ 0.7 nm2 large swelling in fully hydrated lecithin multilayrers Headgroup repulsion in phosphatidylethanolamin is much less : smaller headgroup area, a0≈ 0.5 nm2 much reduced swelling Low pH, Addition of divalent cation between two different bilayer  Charged lipid layer 형성 Addition of divalent cation Low pH condition Electrostatic headgroup repulsion 감소 Bilayer swelling 감소, Adhesion of vesicle 증가

22. Fusion • Vesicle Fusion (I) 1. Adhesion force가 강한 경우 Fusion이 생겨 완전히 다른 구조로 변환 2. 생물체에서도 특정 조건에서 이루어짐  Specific membrane or vesicle, specific time & space etc : Synaptic nerve transformation, Exocytosis(vesicle incorporation), Pinocytosis(vesicle shedding) 3. 반응속도가 굉장히 빠름 ( 0.1 ~ 1 ms 이내)  Fusion process 중의 intermediate stage trap이 불가능  Electron microscope 보다는 freezing 또는 fixing을 통한 분석 4. Drug carrier의 역할  vesicle 간의 adhesion & fusion  vesicle과 planar membrane 간의adhesion & fusion

23. Vesicle fusion cooling Pure stable Lipid vesicles Large, less curved vesicle T < Tc Tc T > Tc Chain 간 재정렬 Large, less curved bilayer로의 packing Stress가 작은 상태 Fusion • Vesicle Fusion (II) i) T < Tc (chain melting temp.) T < Tc에서 작은 vesicle의 Curved bilayer의 stress 증가 Intravesicle 또는 Internal Packing force의 변화에서 기인 ii) Divalent cation 첨가 CaCl2 (1~10mM) 첨가 CaCl2 Electrostatic repulsion 감소, Short range attraction 증가 Vesicle Fusion에 의한 lipid packing stress 감소 Biological vesicle 또는 membrane에서는 Ca 농도가 1~10uM에서 fusion이 개시됨 Negative charged lipid vesicle

24. Fusion • Fusion Mechanism Driving force of vesicle fusion Ionic binding  Ionic change or osmotic change Hydrophobic interaction Fusion Mechanism Fig. 18. 12 Probable molecular events taking place during the adhesion, fusion and fission of bilayers and membranes