Science & Technology Multiscale Modeling of Lipid Bilayer Interactions with Solid Substrates David R. Heine, Aravind R. Rammohan, and Jitendra Balakrishnan October 23 rd, 2008 RPI High Performance Computing Conference

2Science & Technology Outline Background –structure of lipid bilayers –applications of supported lipid bilayers Modeling challenges Atomistic modeling Mesoscale modeling Experimental work Conclusions

3Science & Technology Lipids and Bilayers

4Science & Technology Technological Relevance of Supported Lipid Bilayers SLBs are important for various biotech applications –Biological research Model systems to study the properties of cell membranes Stable, immobilized base for research on membrane moieties Biosensors for the activity of various biological species Cell attachment surfaces –Pharmaceutical research Investigation of membrane receptor drug targets Membrane microarrays: High throughput screening for drug discovery –How does bilayer-substrate interaction affect bilayer behavior?

5Science & Technology Supported Lipid Bilayers at Corning Applications: Membrane-protein microarrays for pharmaceutical drug discovery Substrate texture is important in the adhesion and conformation of bilayers on the surface –Crucial for the biological functionality of bilayers Objective: Quantify the effect of substrate topography and chemical composition on bilayer conformation and dynamics

6Science & Technology Bilayer Length & Time Scales Bilayer dynamics vary over large length and time scales, suggesting a multiscale approach. Undulations: 4 Å – 0.25 mm Bilayer Thickness: 4 nm Area per lipid: 60 +/- 2 Å 2 Stokes Radius: 2.4 nm Length Scales Peristaltic Modes: 1-10 ns Undulatory Modes 0.1 ns – 0.1 ms Lateral Diffusion Time: 4 ps Bond Vibrations: fs Membrane Fusion: 1-10 s Time Scales

7Science & Technology Multiscale Approach Atomistic model –capture local structure and short term dynamics Mesoscale model –capture longer length and time scales –sufficient to look at interaction with rough surfaces

8Science & Technology Atomistic Model The bilayer is composed of 72 DPPC lipid molecules described in full atomistic detail using the CHARMM potential Water uses the flexible SPC model to allow for bond angle variations near the substrate The substrate is the [100] face of  - quartz with lateral dimensions of 49 x 49 Å described by the ClayFF potential lipid water substrate

9Science & Technology Simulation Technique System is periodic in x and y directions with a repulsive wall above the water surface in the z direction NVT ensemble must be used since pressure control is prohibited by the solid substrate Temperature is maintained at 323K with a Nose-Hoover thermostat Total energy and force on the bilayer are extracted during the simulation. Heine et al. Molecular Simulations, 2007, 33(4-5), pp Substrate Water Bilayer Water Lipids Upper leaflet Lower leaflet

10Science & Technology Simulation Technique System is periodic in x and y directions with a repulsive wall above the water surface in the z direction NVT ensemble must be used since pressure control is prohibited by the solid substrate Temperature is maintained at 323K with a Nose-Hoover thermostat Total energy and force on the bilayer are extracted during the simulation. Heine et al. Molecular Simulations, 2007, 33(4-5), pp

11Science & Technology Comparison with Experimental Measurements SFA Measurements Between Substrate and Bilayer Bilayer-Substrate Interaction Energy from Simulations Simulations show an energy minimum at a separation of 3 to 3.5 nm Experimental measurements show a repulsion starting around 4 nm and pullout at 3 nm separations courtesy J. Israelachvili, UCSB

12Science & Technology Bilayer structure near the substrate Lower monolayer is compressed in the vicinity of substrate Upper monolayer seems relatively unaffected

13Science & Technology Effect of substrate on lateral lipid diffusion Reduction in lateral diffusivity observed, compared to free bilayers –Bulk simulations match diffusivity of free bilayers Suppression of transverse fluctuations near substrate inhibit a key mechanism for lateral diffusion Experimental value For free bilayers Transverse lipid motion enables lateral diffusion Substrate reduces transverse motion & reduces diffusivity

14Science & Technology Atomistic Simulation Results MD simulations show bilayer-substrate equilibrium separation of 3 – 3.5 nm, in agreement with SFA experiments Lateral diffusion of the lipid head groups decreases as the bilayer approaches the substrate Suppression of transverse fluctuations may be responsible for reduced lateral diffusion

15Science & Technology Mesoscopic Model Membrane Substrate Continuum solvent Dissipative force –Formulation based on Newtonian solvent viscosity Random force –Formulation based on fluctuation-dissipation theorem Conservative force –Elastic stretching of bilayer –Bending modes of bilayer –Surface interactions –Other (electrostatic, etc.)

16Science & Technology Mesoscopic Modeling of Supported Lipid Bilayers Continuum representation to study large length and time scales –1  m 2, 1 ms Allows study of bilayer behavior on textured substrates Dynamic model that includes effect of solvent and environment All dimensions in nanometers z axis not to scale

17Science & Technology Mesoscopic Model Results Substrate topography contoursMembrane topography contours

18Science & Technology Mesoscopic Model Results Membrane Coating Membrane spanning Maximum Separation Minimum Separation

19Science & Technology Mesoscopic Model Results Allows study of bilayer on micron and microsecond scales Minimum surface roughness of 4-5 nm required for membrane spanning conformation Spanning configuration important for maintaining bilayer mobility

20Science & Technology AFM measurements Spreading of Bilayer on Synthetic Substrates AFM image & measurements courtesy Sergiy Minko, Clarkson University Ref: Nanoletters, 2008, 8(3),

21Science & Technology AFM measurements Smoothening of membrane on rough substrates AFM image & measurements courtesy Sergiy Minko, Clarkson University

22Science & Technology Model shows membrane coating up to about 4-5 nm AFM images show membrane coating 5 nm particles Lipid membrane conformation Numerical and Experimental Results AFM images courtesy Sergiy Minko, Clarkson U.Macroscopic model predictions Maximum Separation Minimum Separation ~ 5 nm SUBSTRATE BILAYER Roiter et al. Nanoletters 8, 941 (2008)

23Science & Technology Conclusions MD simulations show bilayer-substrate separation of 3 – 3.5 nm, in agreement with SFA experiments MD simulations show reduced lateral diffusion in lipids as the bilayer approaches the substrate Mesoscopic model shows membranes coat particles up to 4 – 5 nm in diameter, in agreement with AFM observations Larger surface features are needed to achieve separation between bilayer and substrate High-performance computing has opened up new approaches for understanding biomolecule-substrate interactions, which aids design There is still plenty of room to grow as these models are still restricted in terms of size, timescale, and complexity

24Science & Technology Acknowledgements Professor Sergiy Minko & his group at Clarkson U. Professor Jacob Israelachvili & his group at U. C. Santa Barbara

26Science & Technology Lipid Behavior on Nanoparticles Bilayer conforms to Nanoparticles < 1.2 nm Bilayer undergoes structural re- arrangement involving formation of holes between 1.2 – 22 nm Beyond 22 nm bilayer envelops the particle Ref: Nanoletters, 2008, 8(3),