Calorimetric and X-ray diffraction data are reported for two series of saturated mixed-chain phosphatidylcholines (PCs), 18:0/n:0-PC and n:0/18:0-PC, where the sn-1 and sn-2 fatty acyl chains on the glycerol backbone are systematically varied by two methylene groups from 18:0 to 10:0 (n = 18, 16, 14, 12, or 10). Fully hydrated PCs were annealed at -4 degrees C and their multilamellar dispersions characterized by differential scanning calorimetry and X-ray diffraction. All mixed-chain PCs form low-temperature "crystalline" bilayer phases following low-temperature incubation, except 18:0/10:0-PC. The subtransition temperature (Ts) shifts toward the main (chain melting) transition temperature (Tm) as the sn-1 or sn-2 fatty acyl chain is reduced in length; for the shorter chain PCs (18:0/12:0-PC, 12:0/18:0-PC, and 10:0/18:0-PC), Ts is 1-2 degrees C greater than Tm, and the subtransition enthalpy (delta Hs) is much greater than for the longer acyl chain PCs. Tm decreases with acyl chain length for both series of PCs except 18:0/10:0-PC, while for the positional isomers, n:0/18:0-PC and 18:0/n:0-PC, Tm is higher for the isomer with the longer acyl chain in the sn-2 position of the glycerol backbone. The conversion from the crystalline bilayer Lc phase to the liquid-crystalline L alpha phase with melted hydrocarbon chains occurs through a series of phase changes which are chain length dependent. For example, 18:0/18:0-PC undergoes the phase changes Lc----L beta'----P beta'----L alpha, while the shorter chain PC, 10:0/18:0-PC, is directly transformed from the Lc phase to the L alpha phase. However, normalized enthalpy and entropy data suggest that the overall thermodynamic change, Lc----L alpha, is essentially chain length independent. On cooling, the conversion to the Lc phases occurs via bilayer gel phases, L beta', for the longer chain PCs or through triple-chain interdigitated bilayer gel phases, L beta, for the shorter chain PC 18:0/12:0-PC and possibly 10:0/18:0-PC. Molecular models indicate that the bilayer gel phases for the more asymmetric PC series, 18:0/n:0-PC, must undergo progressive interdigitation with chain length reduction to maintain maximum chain-chain interaction. The L beta phase of 18:0/10:0-PC is the most stable structure for this PC below Tm. The formation and stability of the triple-chain structures can be rationalized from molecular models.

The structure and properties of the ether-linked 1,2-dihexadecylphosphatidylcholine (DHPC) have been examined as a function of hydration. By differential scanning calorimetry, DHPC exhibits an endothermic (chain melting) transition with the transition temperature (limiting value, 44.2 degrees C) and enthalpy (limiting value, delta H = 8.0 kcal/mol) being hydration dependent. For hydration values greater than 30 wt % water, DHPC exhibits a pretransition at approximately 36 degrees C (delta H = 1.1 kcal/mol) and a subtransition at approximately 5 degrees C (delta H = 0.2 kcal/mol). By X-ray diffraction, at 22 degrees C DHPC exhibits a normal bilayer gel structure with the bilayer periodicity increasing from 58.0 to 62.5 A over the hydration range 9.5-25.4% water. At 30-32% water, two coexisting gel phases are observed with d = 63-64 A and d = 44-45 A; at higher hydration, only the latter phase is present, reaching a limiting d = 47.0 A at 37.5% water. Two different gel phases clearly exist at low and high hydrations. Electron density profiles at low hydration (9.5-25.4%) show a bilayer thickness dp-p = 46 A, whereas at greater than 32% water the bilayer thickness is markedly reduced, dp-p = 30 A. These and other structural parameters indicate a hydration-dependent gel----gel structural transition between a normal bilayer (two chains per polar group) and the chain-interdigitated bilayer (four chains per polar group) arrangement described previously for DHPC [Ruocco, M. J., Siminovitch, D. J., & Griffin, R. G. (1985) Biochemistry 24, 2406-2411].(ABSTRACT TRUNCATED AT 250 WORDS)

A combination of surface monolayer, scanning calorimetry, 31P NMR, and spin-label ESR techniques has been used to monitor the interactions of monovalent (NH4+, Na+, and Li+) and divalent (Ca2+) cations with phosphatidylserines (PS) differing in their levels of chain unsaturation. Comparisons are made between the disaturated dimyristoyl-, dipalmitoyl-, and dihexadecyl-PS (DMPS, DPPS, and DHPS), saturated cis-monounsaturated palmitoyloleoyl-PS (POPS) (and bovine brain PS), di-trans-monounsaturated dielaidoyl-PS (DEPS), and di-cis-monounsaturated dioleoyl-PS (DOPS). Na+ and NH4+ cations interact weakly with all PS monolayers and bilayers without significant changes in molecular conformation, chain packing, or headgroup dynamics and without dependence on chain composition. In contrast, considering these structural and dynamic parameters, Li+ shows a gradation in its interaction with PS (DMPS greater than POPS approximately bovine brain PS greater than DOPS), suggesting that Li+-PS interactions depend on the interfacial properties of the PS molecules (e.g., surface area). Finally, Ca2+ interacts strongly with all PS monolayers and bilayers, without obvious chain selectivity. Thus, ion binding to PS depends not only on the properties of the cation (Na+ vs Li+ vs Ca2+) but also on the molecular details of the PS membrane surface.

Exposure of the skin to surfactant-based products can result in irritation. To control this effect researchers are probing mechanisms of surfactant action. In vitro studies show that mixing surfactants often results in less denaturation (swelling) of stratum corneum. We have explored the in vivo human irritation response (using a 21-day cumulative irritation test) to two of these surfactants – sodium lauryl sulfate (SLS) and (C₁₂–C₁₄) alkyl, 7-ethoxy sulfate (AEOS-7EO). Results demonstrate that addition of AEOS-7EO to a constant dose of SLS results in a significant reduction in erythema, hence producing a milder system. The reason for the synergism is unclear, but may relate to experimentally determined alterations in the micellar solution properties of the SLS upon addition of AEOS-7EO.

Solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray powder diffraction were used to investigate the mechanism of trehalose (TRE) stabilization of lipid bilayers. Calorimetric investigation of dry TRE-stabilized bilayers reveals a first-order phase transition (L kappa----L lambda) at temperatures similar to the L beta'----(P beta')----L alpha transition of hydrated lipid bilayers. X-ray diffraction studies show that dry mixtures of TRE and 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) have a lamellar structure with excess crystalline TRE being present. The L kappa phase shows typical gel-phase X-ray diffraction patterns. In contrast, the L lambda-phase diffraction patterns indicate disordered hydrocarbon chains. 2H NMR of specifically 2H chain-labeled DPPC confirmed that the acyl chains are disordered in the L lambda phase over their entire lengths. 2H spectra of the choline headgroup show hindered molecular motions as compared to dry DPPC alone, and 13C spectra of the sn-2-carbonyl show rigid lattice powder patterns indicating very little motion at the headgroup and interfacial regions. Thus, the sugar interacts extensively with the hydrophilic regions of the lipid, from the choline and the phosphate moieties in the headgroup to the glycerol and carbonyls in the interfacial region. We postulate that the sugar and the lipid form an extensive hydrogen-bonded network with the sugar acting as a spacer to expand the distance between lipids in the bilayer. The fluidity of the hydrophobic region in the L lambda phase together with the bilayer stabilization at the headgroup contributes to membrane viability in anhydrobiotic organisms.