Free Energy and Entropy of Activation for Phospholipid Flip-Flop in Planar Supported Lipid Bilayers

Basic transition state theory is used to describe the activation thermodynamics for phospholipid flip-flop in planar-supported lipid bilayers (PSLBs) prepared by the Langmuir-Blodgett/Langmuir-Schaeffer method. The kinetics of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) flip-flop were determined as a function of temperature and lateral surface pressure using sum-frequency vibrational spectroscopy (SFVS). From the temperature and lateral pressure dependent DSPC flip-flop kinetics, a complete description of the activation thermodynamics for flip-flop in the gel state, including free energy of activation (Delta G(double dagger)), area of activation (Delta a(double dagger)), and entropy of activation (Delta S-double dagger), was obtained. The free energy barrier for flip-flop of DSPC was determined to be Delta G(double dagger) = 105 +/- 2 kJ/mol at 40 degrees C at a deposition surface pressure of 30 mN/m. The free energy barrier was found to consist of large opposing entropic and enthalpic contributions. The influence of alkyl chain length oil the activation thermodynamics of flip-flop was also investigated. Decreasing the alkyl chain length led to a decrease in Delta G(double dagger) due primarily to an increase in Delta S-double dagger. The values obtained here are compared to previous studies investigating flip-flop by vesicle based methods.