Effects of vitrified and nonvitrified sugars on phosphatidylcholine fluid-to-gel phase transitions

DSC was used to study the ability of glass-forming sugars to affect the gel-to-fluid phase transition temperature, T-m, of several phosphatidylcholines during dehydration. In the absence of sugars, T-m increased as the lipid dried. Sugars diminished this increase, an effect we explain using the osmotic and Volumetric properties of sugars. Sugars vitrifying around fluid phase lipids lowered T-m below the transition temperature of the fully hydrated lipid, T-o. The extent to which T-m was lowered below T-o ranged from 12 degrees to 57 degrees, depending on the lipids' acyl chain composition. Sugars vitrifying around gel phase lipids raised T-m during the first heating scan in the calorimeter, then lowered it below T-o in subsequent scans of the sample. Ultrasound measurements of the mechanical properties of a typical sugar-glass indicate that it is sufficiently rigid to hinder the lipid gel-to-fluid transition. The effects of vitrification on T-m are explained using the two-dimensional Clausius-Clapeyron equation to model the mechanical stress in the lipid bilayer imposed by the glassy matrix. Dextran and polyvinylpyrrolidone (PVP) also vitrified but did not depress T-m during drying. Hydration data suggest that the large molecular volumes of these polymers caused their exclusion from the interbilayer space during drying.