Effects of Fullerenes on Phospholipid Membranes: A Langmuir Monolayer Study

We investigate two-component Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC)/C-60 by recording surface pressure/area (pi/A) and surface potential/area (Delta V/A) isotherms and by direct Brewster angle microscopy (BAM) imaging. Atomic force microscopy (AFM) is employed to study morphologies of the mixed monolayers transferred to a solid substrate by the Langmuir-Blodgett technique. C-60 is shown to have little influence on isotherms of the DPPC/C-60 monolayers even at a molar fraction as high as X-C60=0.3. The elastic modulus (C-s(-1)) versus pi curves of the DPPC/C-60 monolayers almost over-lay each other, as well as that of pure DPPC, that is, the elasticities of pure DPPC monolayers and DPPC/C-60 monolayers are remarkably similar. AFM studies reveal that fullerene flocs form at low surface pressures (pi <= 15 mN m(-1)), are gradually disaggregated and dispersed in the DPPC monolayer with increasing surface pressure up to 35 mN m(-1) and are then progressively squeezed out to form protruded islands as the surface pressure increases up to 65 mN m(-1). Our work provides experimental support to the computational result that C, can dissolve in lipid bilayers without significantly compromising their mechanical properties, a finding which has important implications for the toxicity and development of drug vehicles from fullerene materials.