Determination of diffusion constant of antimicrobial peptide nisin interacting with Langmuir monolayers of DPPC and DPPG molecules

We present a method of determining the diffusion constant of peptides interacting with lipid films. The method involves studying the interactions of diffusing peptides with the lipid Langmuir monolayer at the air-water interface. Due to the interaction between peptides and lipid molecules in the monolayer at the air-water interface, the surface pressure of the monolayer changes as a function of time which can be measured from the surface manometry technique. A model based on the diffusion of particles under the influence of a constant force is developed to obtain an analytical expression for change in the surface pressure of Langmuir monolayer as a function of time. The expression was found to fit well with the experimental data. The average hydrodynamic radius and the translational diffusion constant of the nisin molecules are calculated from the fit parameters for the different subphase pH solutions. The diffusion constant was found to vary with the pH of the media.