Permeability of liposomal membranes to water: Results from the magnetic field dependence of T1 of solvent protons in suspensions of vesicles with entrapped paramagnetic ions
✍ Scribed by Seymour H. Koenig; Quet F. Ahkong; Rodney D. Brown III; Michael Lafleur; Marga Spiller; Evan Unger; Colin Tilcock
- Publisher
- John Wiley and Sons
- Year
- 1992
- Tongue
- English
- Weight
- 770 KB
- Volume
- 23
- Category
- Article
- ISSN
- 0740-3194
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✦ Synopsis
Abstract
The diffusive permeability to water molecules, P~d~, of lipid vesicles with entrapped paramagnetic solute ions can be determined rapidly from analysis of the magnetic field dependence (nuclear magnetic relaxation dispersion, or NMRD profile) of T~1~ of exterior solvent water protons. Such data yield τ the mean lifetime of solvent molecules inside the vesicles, from τ = (fT~lpara~) – T~1ves~, where f is the volume fraction of entrapped water, T~1Para~ is the observed T~1~ corrected for buffer background, and T~1Ves~ is the relaxation time of water protons in the entrapped solution. For small spherical unilamellar vesicles of inner radius R, P~d~ = R/3τ. f can be obtained accurately from knowledge of both the concentration of Gd(DTPA)^2−^ in the solution in which the vesicles were formed and the average concentration of ions in the final sample. At low temperatures, in the limit of slow exchange, T~1Para~ becomes independent of field and τ = f T~1Para~; the observation of a field independent profile is a control that confirms that no paramagnetic material is external to the vesicles. We have measured T~1Para~, using a field‐cycling relaxometer, for suspensions of POPC (1‐palmitoyl‐2‐oleoyl‐lecithin) vesicles with 100–500 mM entrapped Gd(DTPA)^2−^ and membrane concentrations of cholesterol ranging from 0 to 40 mol %. These profiles, which span the field range 0.01–50 MHz proton Larmor frequency, were taken at 5, 15, 25, and 35°C. Concentrations of Gd(DTPA)^2−^ were determined independently by both ICP analyses and NMRD methods. Values for P~d~ for vesicles with 100 mM Gd(DTPA)^2−^ and outer diameters 100 nm ± 20%, as determined by quasielastic light scattering, are 63, 47, 24, 16, and 8.7 × 10^−4^cm s^−1^, at 25°C, for cholesterol concentrations of 0, 10, 20, 30, and 40%, respectively. The corresponding activation enthalpies are 14, 14, 14, 17, and 17 kcal/M. Comparison with ^2^H NMR studies of deuterated POPC vesicles with no cholesterol at 20°C, and with 10% at 40°C, which yielded the same order parameter for the palmitoyl acyl chains, gives no indication of a correlation between order parameter and permeability.