Microcalorimetric study on the phase behaviour of S-layer coated liposomes

Isolated S-layer subunits from Bacillus coagulans E38-66/v1 were recrystallized on positively charged, unilamellar liposomes composed of dipalmitoylphosphatidylcholine, cholesterol and hexadecylamine. The thermotropic phase behaviour of S-layer coated and uncoated liposomes was characterized by differential scanning microcalorimetry indicating for both preparations a broad transition around 50 degrees C due to the chain-melting from a liquid-ordered gel-like to a liquid-ordered fluid phase as described for phosphatidylcholine/cholesterol mixtures. The slightly higher phase transition temperature for the S-layer coated liposomes was explained by increased intermolecular order. Cross-linking the S-layer subunits covalently to hexadecylamine with glutaraldehyde induced phase separation within the liposomes. Based on deconvolution of the normalized excess heat capacity functions it was proposed that the different lipid domains arise from phospholipids representing different degrees of mobility.