Head group and chain length dependence of phospholipid self-assembly studied by spin-label electron spin resonance

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Abstract
The critical micelle concentrations (cmc''s) of a variety of spin-labeled phospholipids, 1-acyl-2-[4-(4,4-dimethyloxazolidine-N-oxyl)valery]-sn-glycero-3-phospho derivatives, have been determined by electron spin resonance (ESR) spctroscopy. The narrow, three-line ESR spectra of the rapidly tumbling monomers are clearly distinguished from the spin-spin broadended spectra of the micellar aggregates, allowing a direct determination of the concentrations of the two species. The influence of both the hydrocarbon chain length and the polar head group on the energetics of self-assembly has been studied. For phosphatidylcholine, 1n [cmc] decreases linearly with the length of the sn-1 chain. The gradient of this linear dependence corresponds to a free energy of transfer of the monomer from the aqueous phase to the micelle of .DELTA.Gtr = -1.1RT per CH2 group. The cmc''s of the 1-lauroyl derivatives of both phosphatidylcholine and phosphatidylglycerol have relatively shallow, biphasic temperature dependences with a minimum at approximately 20.degree.C. Both of these properties are characteristic of the hydrophobic effect, with the free energy of transfer being slightly less than that for the solubility of n-hydrocarbons in water, corresponding to the reduced configurational entropy of the lipid chains in the micellar state. The cmc''s of the 1-lauroyl derivatives of the phospholipids in 0.15 M NaCl, for their various charge states, are as follows: phosphatidic acid(2-), 0.77 mM; phosphatidic acid(1-), 0.13 mM; phosphatidylserine(1-), 0.24 mM; phosphatidylglycerol(1-), 0.17 mM; phosphatidylcholine, 0.10 mM; phosphatidylethanolamine, 0.05 mM. For the negatively charged phospholipids, the cmc''s decrease by 1-2 orders of magnitude on increasing the ionic strength from approximately 0 to 2.0 M NaCl, with a characteristic Debye-Huckel type of screening. For the zwitterionic lipids, the salt dependence is considerably less; the cmc''s for phosphatidylcholine and phosphatidylethanolamine decrease by a factor of approximately 4 on going from 0 to 2 M NaCl. At a constant ionic strength of 0.1, the cmc of the 1-lauroyl derivative of phosphatidic acid increases by almost an order of magnitude on titration from pH 5 to pH 9, with a pKa of 7.4 corresponding to the second deprotonation state. These results demonstrate that the polar head group can also have a very marked influence on the energetics of self-assembly and correspondingly on the kinetics of lipid transfer between vesicles. The range of the effects of the polar head group and salt on the free energy of transfer corresponds to those of changing the chain length by up to approximately four CH2 groups. The rate of monomer-micelle collisions has been measured from the exchange broadening of the monomer signals as a function of the micelle concentration, for the 1-lauroyl derivatives of both phosphatidylcholine and phosphatidylglycerol in 0.15 M NaCl at pH 7. The collision rates follow an Arrhenius law temperature dependence with activation energies of 4.0 kcal .cntdot. mol-1 and 5.3 kcal .cntdot. mol-1 for phosphatidylcholine and phosphatidylglycerol, respectively. The interpolated second-order rate constants at 25.degree.C are k+/.hivin.n = 9.4 .times. 107 and 4.4 .times. 107 L .cntdot. (mol of micelle)-1 .cntdot. s-1 for phosphatidylcholine and phosphatidylglycerol, respectively. The differences in rates correspond to the effects of the electrostatic surface potential for phosphatidylglycerol and also to possible differences in the micelle size .hivin.n. The overall rates and activation energies are consistent with a diffusion-controlled process.