A way of analyzing the decrease insurface tension upon adsorption of surfactant molecules to the interface isthrough the Gibbs' adsorption equation. This theory considers the dividingsurface area in which only the excess of the surfactant is taken into account.However, the thickness of the interfacial region between water and air isestimated to be between 6-8 Å thick, where the water density ranges from 0 (airphase) to 1 gr/cm3 (pure water surface). According to Damodaran'sapproximation (1), water activity in this interfacial region (where the densityof water is around 0.17 gr/cm3) plays a fundamental role on theemergence of the variable surface pressure. Surface pressure can then beexpressed as a function of surface water activity. Interestingly,the dependence of surface pressure vs water activity of this region is quitesimilar to the liquid-expanded phospholipid monolayer π vs Aisotherm (1). LAURDAN fluorescence parameters has been shown to be sensitive tothe extent of water dipolar relaxation in lipid interfaces displaying differentphases (2). In particular, the steady-state fluorescence parameter known asgeneralized polarization (GP) function has been characterized and applied tostudy phase transitions in biomembranes (2). GP linearly correlates with thelipid order parameter measured by NMR (3) and in a liquid-expanded lipidinterface the GP function depends on the lipid packing (surface pressure)(4). Since both water activity and GP depend on surface pressure, we propose tocombine these variables and correlate water activity with Laurdan GP.As the activity of PLA2s depends on lateral packing (5), our correlation allowsus to quantitatively correlate enzymatic activity with water activity inthe surface. As complex glycosphingolipids impose greater hydration (6)(lower GP) and inhibit the activity of both PLA2 and PLC (7) inisobaric form, we propose that this inhibition is regulated by changes inthe interfacial water activity.
