Physical (in) stability of liposomes upon chemical hydrolysis: the role of lysophospholipids and fatty acids.

As a consequence of chemical hydrolysis of liposomal phospholipids the organization of the lipid assembly can change from a lamellar into a micellar system. Different approaches provided evidence for this conversion: 31P-NMR analysis, turbidity measurements and ultracentrifugation experiments. Two conditions have to be met before this conversion can take place: (1) the liposomes must pass through a gel-to-liquid crystalline phase-transition during a heating or cooling run, and (2) the degree of chemical hydrolysis must exceed a critical hydrolysis percentage (or the phospholipid bilayer must contain critical amounts of lysophospholipid and fatty acid). As monitored by turbidity measurements, this critical level of hydrolysis and the relative change depended on the chain length and on the head group of the liposomal phospholipids. It does not depend on concentration, pH, storage temperature or on size of the liposomes within the experimental range. Addition of cholesterol to bilayers composed of dipalmitoylphosphatidylcholine prevents the lamellar to micellar transformation. Fluorescence anisotropy measurements of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene in 0.18-microns dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylglycerol (10:1)-liposomes indicated that behavior of the probe below and above the phase-transition temperature was not affected by chemical hydrolysis, or even by formation of micelles. However, the phase-transition temperature range broadened and shifted towards higher temperatures upon hydrolysis.