The properties of brain galactocerebroside monolayers

Abstract

Using a Langmuir film balance we have compared the properties of films of the brain galactocerebrosides at 37°C. There are two types of cerebroside in brain, those with an α-hydroxy substituent on the acyl chain (HFA) and those without (NFA). At equivalent pressures the areas of both cerebroside films are significantly less than the areas of films of the brain glycerolipids, the choline and ethanolamine phosphatides. The isotherm of NFA galactocerebrosides has two discontinuities, one at low and one at high film pressure, while the isotherm of HFA galactocerebrosides is a smooth curve at all film pressures. Below the high-pressure transition the area of the NFA film is significantly larger than the area of the HFA film. When compressed beyond the high-pressure transition there is a marked hysteresis between compression and expansion isotherms of the NFA galactocerebrosides. The pressures of both films continue to rise steeply when they are compressed into areas which are too small for them to exist as simple monolayers. We conclude that under compression cerebroside films form bilayer structures; that bilayer formation starts at low pressure and occurs progressively as the HFA cerebroside monolayer is compressed, but occurs more abruptly in the NFA cerebroside monolayer at the high-pressure-transition region of the isotherm. A study of pure cerebrosides with a single defined acyl chain shows that there is a correlation between the relative volumes of the hydrophobic and hydrophilic parts of the molecule and the ease of bilayer formation. The larger the relative volume of the hydrophilic group the more readily the cerebroside forms a bilayer film. Other brain lipids added to cerebroside monolayers have sharply differing effects on their areas. The areas of films containing cholesterol are less than the areas calculated by adding the areas of the pure components multiplied by their mole fractions. On the other hand, the area of phosphatidylcholine-containing films is much larger than calculated.