LATERAL DIFFUSION-COEFFICIENTS OF PHOSPHOLIPIDS IN SPHERICAL BILAYERS ON A SOLID SUPPORT MEASURED BY H-2-NUCLEAR-MAGNETIC-RESONANCE RELAXATION

An alternative nuclear-magnetic-resonance (NMR) method for the measurement of the lateral diffusion coefficient D of phospholipids along the plane of a spherical bilayer on a solid support is presented. D values are determined at various temperatures for palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayer on a spherical silica support of 640 nm diameter. The method is based upon the measurement of the quadrupolar transverse relaxation times obtained by the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence for a series of different pulse spacing times. It takes advantage of the fact that the CPMG sequence can progressively filter out contributions to the transverse relaxation arising from motions like lateral diffusion, which are slow on the NMR time scale. The combination of this sequence with a new membrane model system, single bilayers on a spherical support of well-defined diameter, enables the determination of D. The D values which we obtain for POPC supported bilayers [D=(2.1+/-0.7) X 10(-12) m2/s at 10-degrees-C, D=(4.0+/-0.8) X 10(-12) m2/s at 30-degrees-C, and D=(7.0+/-1.0) X 10(-12) m2/s at 50-degrees-C] are in good agreement with those obtained by other methods like fluorescence recovery after photobleaching (FRAP) and pulsed-field-gradient NMR (PFG-NMR). The discrepancies between the obtained D values and those reported recently from quasielastic neutron-scattering studies are discussed in terms of the different characteristic lengths and time scales over which the methods are sensitive. This NMR method is superior to FRAP and electron spin resonance methods since it requires no bulky labels attached to the phospholipids and it measures the average diffusion of all molecules (and not only that of the probes) in the bilayer. Moreover, the characteristic length scale of the CPMG method is significantly shorter than for FRAP or PFG-NMR. These advantages give the present method potential for the study of demixing and domain formation processes near phase transitions in model membrane systems.