Two sides of the coin. Part 2. Colloid and surface science meets real biointerfaces.

Part 1 revisited developments in lipid and surfactant self assembly over the past 40 years [1]. New concepts emerged. Here we explore how these developments can be used to make sense of and bring order to a range of complex biological phenomena. Together with Part 1, this contribution is a fundamental revision of intuition at the boundaries of Colloid Science and Biological interfaces from a perspective of nearly 50 years. We offer new insights on a unified treatment of self assembly of lipids, surfactants and proteins in the light of developments presented in Part 1. These were in the enabling disciplines in molecular forces, hydration, oil and electrolyte specificity; and in the role of non Euclidean geometries-across the whole gammut of physical, colloid and surface chemistry, biophysics and membrane biology and medicine. It is where the early founders of the cell theory of biology and the physiologists expected advances to occur as D'Arcy Thompson predicted us 100 years ago.

Two sides of the coin. Part 1. Lipid and surfactant self-assembly revisited.

Hofmeister, specific ion effects, hydration and van der Waals forces at and between interfaces are factors that determine curvature and microstructure in self assembled aggregates of surfactants and lipids; and in microemulsions. Lipid and surfactant head group interactions and between aggregates vary enormously and are highly specific. They act on the hydrophilic side of a bilayer, micelle or other self assembled aggregate. It is only over the last three decades that the origin of Hofmeister effects has become generally understood. Knowledge of their systematics now provides much flexibility in designing nanostructured fluids. The other side of the coin involves equally specific forces. These (opposing) forces work on the hydrophobic side of amphiphilic interfaces. They are due to the interaction of hydrocarbons and other "oils" with hydrophobic tails of surfactants and lipids. The specificity of oleophilic solutes in microemulsions and lipid membranes provides a counterpoint to Hofmeister effects and hydration. Together with global packing constraints these effects determine microstructure. Another factor that has hardly been recognised is the role of dissolved gas. This introduces further, qualitative changes in forces that prescribe microstructure. The systematics of these effects and their interplay are elucidated. Awareness of these competing factors facilitates formulation of self assembled nanostructured fluids. New and predictable geometries that emerge naturally provide insights into a variety of biological phenomena like anaesthetic and pheromone action and transmission of the nervous impulse (see Part 2).

Periodic minimal surface organizations of the lipid bilayer at the lung surface and in cubic cytomembrane assemblies.

The existence of infinite periodic lipid bilayer structures in biological systems was first demonstrated in cell membrane assemblies. Such periodicity is only possible in symmetric bilayers, and their occurrence is discussed here in relation to the asymmetry of cell membranes in vivo. A periodic membrane conformation in the prolamellar body of plants corresponds to a dormant state without photosynthesis. A similar reversible formation of a dormant state has also been observed in the mitochondria of the amoeba Chaos. In these cases the energy production has become insufficient to maintain the membrane asymmetry. Formation of membranes that are symmetric over the bilayer is proposed to be a principal mechanism behind formation of cubic membrane systems. Another type of bicontinuous minimal surface structure is considered to form the alveolar lining of mammals at normal breathing conditions. The CLP surface corresponds to such a tetragonal surface phase. It is also a symmetric bilayer and in a state of zero energy expenditure. Structural alternatives of the bilayer conformation in this latter system are also discussed here.

The bilayer melting transition in lung surfactant bilayers: the role of cholesterol.

Aqueous dispersions of a porcine lung surfactant (PLS) extract with and without cholesterol supplementation were analyzed by X-ray scattering. Lamellar liquid-crystalline and gel-type bilayer phases are formed, as in pure phosphatidylcholine (PC)-cholesterol systems. This PLS extract, developed for clinical applications, has a cholesterol content of less than 1% (w/w). Above the limit of swelling, the bilayer structure shows a melting (main) transition during heating at about 34 degrees C. When 13 mol% cholesterol was added to PLS, so that the cholesterol content of natural lung surfactant was reached, the X-ray scattering pattern showed pronounced changes. The main transition temperature was reduced to the range 20-25 degrees C, whereas according to earlier studies of disaturated PC-cholesterol bilayers in water the main transition remains almost constant when the amount of solubilized cholesterol is increased. Furthermore, the changes in scattering pattern at passing this transition in PLS-cholesterol samples were much smaller than at the same transition in PLS samples. These effects of cholesterol solubilization can be related to phase segregation within the bilayers, known from pure PC-cholesterol systems. One phase, solubilizing about 8 mol% cholesterol, exhibits a melting transition, whereas the other bilayer phase, with a liquid-crystalline disordered conformation, has a cholesterol content in the range 20-30 mol% and this phase shows no thermal transition. The relative amount of bilayer lipids that is transformed at the main transition in the PLS-cholesterol sample is therefore only half compared to that in PLS samples. The reduction in transition temperature in the segregated bilayer of lung surfactant lipids is probably an effect of enrichment of disaturated PC species in the phase, which is poor in cholesterol. This work indicates that cholesterol in lung surfactant regulates the crystallization behavior.

Enhanced efficacy of porcine lung surfactant extract by utilization of its aqueous swelling dynamics.

This study investigates the interactions between a porcine lung surfactant (PLS) extract and distilled water, saline solution or Ringer solution. The phases which coexist in equilibrium with water or electrolyte solutions were analysed by X-ray diffraction and cryo transmission electron microscopy (cryo-TEM). A lamellar phase with a structure unit consisting of double bilayers was observed in water, whereas lamellar phases with the usual bilayer structure unit were formed in saline and in Ringer solutions. At 25 degrees C the presence of a 4.2-A peak in the X-ray diffraction wide-angle region of these three maximally swollen phases showed that most of the hydrocarbon chains were organized in a crystalline packing. At 42 degrees C the chains in all three phases were melted which, in combination with the low-angle diffraction, shows that they were liquid-crystalline. Polyhedral-like vesicles and spherically shaped multilamellar vesicles were observed in cryo-TEM. The bilayer unit structures were consistent with the periodicity seen by X-ray diffraction. The dynamic swelling behaviour was followed in the polarizing microscope. A remarkable growth of birefringent networks was seen at the air interface of samples swollen in Ringer solution and saline solution. No such interfacial growth phenomena were observed during swelling in water without electrolytes. Then, these dynamics were analysed in relation to time-dependent pulmonary administration of the surfactant extract in rats. Variation in the time of administration (20 and 60 min) after mixing the extract with saline or Ringer solution showed clear differences in physiological effects. At pulmonary administration when the swelling behaviour in vitro showed a maximum in dynamics, the arterial oxygenation was superior to that of administration at a time after a steady-state had been reached. This means that the clinical performance of mammalian lung surfactant extracts can be significantly improved by taking the time-dependent aqueous swelling of the extract into account.