CHEMICAL-EXCHANGE BETWEEN LAMELLAR AND NONLAMELLAR LIPID PHASES - A ONE-DIMENSIONAL AND 2-DIMENSIONAL P-31-NMR STUDY

One- and two-dimensional P-31-exchange NMR has been used to investigate chemical exchange between coexisting lamellar (L(alpha)) and non-lamellar (hexagonal H(II) and cubic I2) lipid phases. Samples of DOPE, DOPE/DOPC (9:1 and 7:3), DOPE/ cholesterol sulfate (9:1), DOPC/monoolein (MO) (3:7 and 1:1), and DOPC/DOPE/cholesterol (1:1:2) were macroscopically oriented on glass plates and studied at the 0-degrees orientation (angle between the bilayer normal and the external magnetic field), where the L(alpha), H(II), and I2 resonances are resolved. A reversible L(alpha) to H(II) transition was observed for all of the samples except for the DOPC/MO mixtures, which displayed a reversible L(alpha) to I2 transition. Near-equilibrium mixtures of L(alpha) and either H(II) or I2 were obtained after prolonged incubation at a given temperature. Two-dimensional exchange experiments were performed on DOPE at 9-14-degrees-C for mixing times ranging from 500 ms to 2 s. For all samples, one-dimensional exchange experiments were performed for mixing times ranging from 100 ms to 4 s, at temperatures ranging from 3-degrees-C to 73-degrees-C. No evidence of lipid exchange between lamellar and non-lamellar phases was observed, indicating that if such a process occurs it is either very slow on the seconds' timescale, or involves an undetectable quantity of lipid. The results place constraints on the stability or kinetic behaviour of proposed transition intermediates (Siegel, D.P. (1986) Biophys. J. 49, 1155-1170).