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Interactions between lipid membranes

www.iupui.edu/~lab59. Interactions between lipid membranes. Horia I. Petrache. Department of Physics. Indiana University Purdue University Indianapolis, USA. Support:.

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Interactions between lipid membranes

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  1. www.iupui.edu/~lab59 Interactions between lipid membranes Horia I. Petrache Department of Physics Indiana University Purdue University Indianapolis, USA Support: IUPUI Biomembrane Signature Center IUPUI Integrated Nanosystems Development Institute Alpha 1 Foundation NIH Generous student volunteering

  2. You can contribute with: • More (better) theory • Applications

  3. Lipid molecules have two parts 5- 7 Å dipolar head 15- 25 Å oily tails

  4. Lipids aggregate and form bilayers (membranes) ~ 40 Å Visible by X-ray depending on electron density.

  5. Electron densities at T = 300 K liquid water lipid Zero net density contrast but...

  6. Electron densities at T = 300 K lipid headgroup lipid tails compared to 0.333 e/Å3 for water => can see them!

  7. X-ray scattering from unoriented lipid membranes

  8. X-ray scattering from oriented lipid membranes Biophys. J. 2005, J. Lipid Research, 2006

  9. X-ray scattering from multilayers (1D randomly oriented lattice) Scattered beam(s) 2q2 Bragg rings seen on the detector Incident beam 2q1 MLV sample Bragg’s Law

  10. Bragg’s Law With D = 60 Å, l = 1.54 Å, and h = 1, obtain (small angle) q = 0.74o => Need a small x-ray machine angle

  11. detector sample chamber x-ray source (tube)

  12. Fixed anode BrukerNanostar U, 40 kV x 30 mA. Wavelength = 1.54 Å (Cu source) Sample-to-detector distances: 0.15 m, 0.6 m, and 1 m Lattice spacings: 8 Å to 900 Å

  13. Electron density of a typical lipid bilayer 0.333 e/Å3 Note: broad distributions (no sharp lipid-water interface)

  14. Higher spatial resolution from oriented samples (DLPC: a lipid we like) J. Lipid Research 2006

  15. Electron microscopy of lipids in water Cryo-EM, DganitDanino, Technion, Israel

  16. Equilibrium distance means attractive force + repulsive force = 0 F2 F1 D-spacing => Any measured change in distance means a change in membrane forces.

  17. => Can control spacing by hydration/dehydration (osmotic stress) + water => + more water =>

  18. ... or by adding ions/electrolytes + electrolyte =>

  19. 1 Molar = a pair of ions for each 55 water molecules. 100 mM = 10 times less ions or 10 times more water. Debye screening lengths for electrostatic interactions in solution: 10 Å in 100 mM monovalent ions 3 Å in 1M

  20. Example: D-spacing increases in KBr DLPC/water 20mM KBr 40mM 60mM 80mM 100mM 200mM 400mM 600mM q (Å-1)

  21. Example: D-spacing increases in KBr DLPC/water 20mM KBr 40mM 60mM 80mM 100mM 200mM 400mM 600mM q (Å-1)

  22. Equilibrium distances depend on polarizabilities (as expected) . Numbers indicate polarizability ratios Szymanski, Petrache, J. Chem. Phys. 2011

  23. ...but need to explain a curiously large difference between the effects of KBr and KCl KBr Water spacing KCl

  24. Looks like electrostatics but distances are large 2lD lD screening length

  25. Attractive interactions between lipid bilayers van der Waals With Hamakerparameter H ~ 1-2 kBT Hamaker, Parsegian, Ninham, Weiss,...

  26. Repulsion #1 (lipids don’t want to give water away) hydration repulsion Empirical exponential form with two adjustable parameters: Ph~ 1000 – 3000 atm l ~ 2 – 3 A Rand, Parsegian, Marcelja, Ruckenstein, ...

  27. Repulsion #2 (membranes bend and undulate) shape fluctuation KC=bending moduluss = fluctuation amplitude Helfrich, de-Gennes, Caillé

  28. Repulsion #3 (electric charges exist) electrostatics: some analytical forms, mostly numerical calculations Poisson-Boltzmann, Debye-Huckel, Gouy-Chapman, Andelman, ... Main parameters: membrane surface charge Debye screening length (of the electrolyte)

  29. Additivity/separability model of membrane interactions + elec shape fluctuation hydration vdW Fitting parameters: Ph, l, H, KC Also need s(DW) Parsegian, Nagle, Petrache

  30. Long story short: s(DW)from X-ray line shape analysis (DOPC and DOPS are two popular lipids) Petrache et al., Phys. Rev. E 1998

  31. Osmotic pressure It can be measured with an osmometer.

  32. Reduce inter-membrane spacing by using osmolytes (e.g. polyethylene glycol, PEG) Rand and Parsegian, 1979 PEG Lipid

  33. Example of interaction analysis giving Ph, l, H, KC (no electrostatics) hydration fluctuations vdW Zero pressure di(14:0)PC (DMPC) at 35oC

  34. Practical method: use well calibrated reference lipid to investigate salt/electrolyte effects on membrane interactions Main results: Screening of vdW interactions Electrostatic charging due to affinity of polarizable ions to lipids Some interesting complications at the water/lipid interface Koerner et al., Biophys. J. 2011 Danino et al. Biophys. J. 2009 Rostovtseva et al. Biophys. J. 2008 Petrache et al., PNAS 2006 Kimchi et al., J. Am. Chem. Soc. 2005

  35. Fit with ~50% vdWreduction (no elec.) Fluid DLPC at 30oC 1M salts water KCl KBr Water spacing (Å) J. Lipid Res. 2006

  36. Detect Br-binding from data in 100 mM salt Binding constant

  37. Obtain vdW strength (H) vs. salt concentration Water spacing Br Cl Expect (according to Ninham, Parsegian)

  38. Functional form OK but needs empirical correction Petrache et al., PNAS 2006

  39. Detect electrostatic charging due to zwitterions Koerner et al., Biophys. J. 2011 Common pH buffers Our calibrated lipid

  40. Zwitterions (e.g. MOPS buffer) swell multilayers really well (Koerner et al., BJ 2011)

  41. Expect reduction of vdW attraction of membranes weaker vdW

  42. ...and electrostatic charging (at total 200 mM concentration)

  43. Measure charging by competition with calibrated KBr neutral point: 75% MOPS, 25% KBr % MOPS replacing KBr (at total 200 mM concentration)

  44. Lipid multilayers are found around nerve axons source: Public domain (Wiki)

  45. Lipid multilayers are found around nerve axons source: Public domain (Wiki)

  46. Conclusions [1]X-ray scattering measurements on well calibrated membrane systems provide experimental parameters for vdWand electrostatics. Experiments show larger screening length (reduced screening power of salt ions) than predicted theoretically. [2] Can detect weak electrostatic interactions by competition measurements (e.g. MOPS vs. KBr). [3] Water, mobile charges, and membrane fluctuations complicate calculations of interactions. Huge room for improvement.

  47. Visit us at www.iupui.edu/~lab59 Acknowledgements Megan KoernerZwitterions Ryan LybargerBuffers, mixtures Jason WalsmanE. coli (adaptation to ionic sol.) Torri Roark Lithium salts Johnnie Wright Exclusion measurements Luis Palacio, Matt JusticeX-ray

  48. Acknowledgements (cont.) John Nagle (Carnegie Mellon Univ., USA) Stephanie Tristram-Nagle (Carnegie Mellon Univ., USA) Daniel Harries (Hebrew Univ., Israel) Luc Belloni (Saclay, France) Thomas Zemb (formerly at Saclay, France) Adrian Parsegian (Univ. of Massachusetts, formerly at NIH) Rudi Podgornik (University of Ljubljana, Slovenia) Tanya Rostovtseva (NIH, USA) Philip Gurnev (NIH, USA)

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