Biophysical Interactions of Membrane Anionic Phospholipids with pH, Calcium and Auxins

A series of phospholipid monolayers—phosphatidylinositol, phosphatidylserine and phosphatidyl 4, 5-bisphosphate, possessing an increasingly negative headgroup potential, were spread and compressed over aqueous sub-phases and, under varying conditions, assayed for changes in surface parameters. These included surface tension and degree of molecular compression expressed by surface pressure/molecular area isotherms. Experimentation employing Wilhelmy-Du Nouy tensio-metry and the Langmuir-trough compression procedure indicated that with increasing electrostatic Ca2+ cross-linking, this being a direct function of Ca2+ concentration in the aqueous sub-phase, there was a concomitant increase in surface tension, i.e. monolayer condensation. This effect was greater with the more negatively charged phospholipids and was more pronounced at pH 7–0 than at pH 4–5. Application of the auxins indoleacetic acid, indolebutyric acid and naphthaleneacetic acid (which presumably act as proton donors which may dislodge cross-linking Ca2+ ) causes a significant decrease in surface tension when applied at physiological Ca2+ concentrations. As evidenced by surface pressure/molecular area isotherms, Ca2+ increases monolayer rigidification and decreases phosphatidyl 4, 5-bisphosphate molecular area. The auxins tested possess an opposite and de-ngidifying effect as indicated by increase of the phospholipid’s molecular area and by the observed considerable lowering of monolayer collapse pressures. The surfactive effectivity of the auxins increased with decreasing sub-phase Ca2+ concentration in the 10−4 to 10−6mol dm−3 range. These data collectively suggest that surface-associated biophysical changes should be taken into account during interpretation of transmembrane hormonal signal transduction.