Probing DMPG vesicle surface with a cationic aqueous soluble spin label

A small, highly aqueous soluble, deuterated, cationic spin label, 4-trimeth ylammonium-2,2,6,6-tetramethylpiperidine-d(17)-1-oxyl iodide (dCAT1), was u sed to directly monitor the negatively charged DMPG vesicle surface in orde r to test a recent suggestion (Riske et al., Chem. Phys. Lipids, 89 (1997) 31-44) that alterations in the surface potential accompanied apparent phase transitions observed by light scattering. The temperature dependence of th e label partition between the lipid surface and the aqueous medium indicate d an increase in the surface potential at the gel to liquid-crystal transit ion, supporting the previous suggestion. Results at the phase transition oc curring at a higher temperature were less definitive. Although some change in the dCAT1 ESR spectra was observed, the interpretation of the phenomena is still rather unclear. DMPG surface potentials were estimated from the dC AT1 partition ratios (surface label moles/total label moles), using a simpl e two-sites model, where the electrostatic potential is zero everywhere but at the vesicle surface, and the interaction between the spin label and the membrane surface is chiefly electrostatic. The Gouy-Chapman-Stern model pr edicts surface potentials similar to those observed, although the measured decrease in the surface potential with ionic strength is somewhat steeper t han that predicted by the model. (C) 1999 Elsevier Science B.V. All rights reserved.