(115a) Phase Behavior And Permeability Of Lipid Bilayer Membranes In The Presence Of C60 Fullerene

As potential drug delivery vehicles, therapeutic agents, or environmental pollutants, the interaction between carbon fullerenes and cells will be modulated by the lipid cell membrane. Fullerenes are hydrophobic and, depending on their surface chemistry and aggregation behavior, are capable of accumulating within the lipid membrane acyl region. To characterize fullerene/cell membrane interactions, the phase behavior and permeability of dipalmitoylphosphatidylcholine (DPPC) bilayers containing cholesterol (0 to 20 mol%) are being examined as a function of C₆₀ concentration. DPPC/cholesterol/C₆₀ bilayer vesicles are formed with DPPC:C₆₀ molecular ratios as low as 10 by co-dissolving the components in chloroform/carbon disulfide mixtures, drying under nitrogen, and hydrating with phosphate buffered saline. Changes in bilayer phase behavior are examined using a combination of fluorescence anisotropy (FA), calorimetry, and microscopy techniques. Preliminary results from FA, which measures the rotational motion of a membrane-bound probe molecule (in this case 1,3,5-diphenylhexatriene), suggest that C₆₀ does not mix freely within a DPPC bilayer as individual molecules, but rather it aggregates within the bilayer to form C₆₀-rich membrane domains. Hence, strong C₆₀ intermolecular interactions favor aggregation despite being suspended in a hydrophobic acyl phase. This behavior would significantly affect local lipid ordering, bilayer stability, and transmembrane permeability.