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 (ΔG‡), area of activation (Δa‡), and entropy of activation (ΔS‡), was obtained. The free energy barrier for flip-flop of DSPC was determined to be ΔG‡ = 105 ± 2 kJ/mol at 40 °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 on the activation thermodynamics of flip-flop was also investigated. Decreasing the alkyl chain length led to a decrease in ΔG‡ due primarily to an increase in ΔS‡. The values obtained here are compared to previous studies investigating flip-flop by vesicle based methods.