Influence of the long chain to short chain amphiphile ratio on lateral diffusion of PEG-lipid in magnetically aligned bilayers from pulse field gradient NMR
Ronald Soong 1 and Peter M. Macdonald 1*
1 University of Toronto at Mississauga
* To whom correspondence should be addressed. E-mail: pmacdona{at}utm.utoronto.ca.
Submitted on April 15, 2005
Revised on May 26, 2005
Accepted on 17 June 2005
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Abstract |
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Lateral diffusion measurements of PEG-lipid incorporated into magnetically aligned lipid bilayers, composed of dimyristoyl phosphatidylcholine (DMPC) plus dihexanoyl phosphatidylcholine (DHPC) plus 1 mol % (relative to DMPC) dimyristoyl phosphatidylethanolamine-N-[methoxy(polyethylene glycol)-2000] (DMPE-PEG 2000), were performed using stimulated echo (STE) pulsed field gradient (PFG) proton (1H) nuclear magnetic resonance (NMR) spectroscopy. The DMPE-PEG 2000 (1 mol %, 35 °C) lateral diffusion coefficient D varied directly with the mole fraction of DMPC, XDMPC = q/(1+q) where q = DMPC / DHPC molar ratio, decreasing progressively from D = 1.65 x 10-11 m2s-1 at q = 4.7 to D = 0.65 x 10-11 m2 s-1 at q = 2.5. Possible sources of this dependence, including orientational disorder, obstruction, and PEG-lipid sequestration, were simulated using, respectively, a diffusion-in-a-cone model, percolation theory, and a two phase PEG distribution model. Orientational disorder alone was not capable of reproducing the observations, but in combination with either obstruction or PEG-lipid two-phase distribution models did so satisfactorily. A combination of all three models yielded the most reasonable fit to the observed dependence of lateral diffusion on q. These same effects would be expected to influence lateral diffusion of any bilayer-associating species in such systems.
Key Words:
bicelles, lateral diffusion, obstruction, orientational order, phase distribution, stimulated echo
