Spin label EPR study of the effects of monovalent cations, anions, and chaotropics on DPPC multilayers

The electron paramagnetic resonance (EPR) spectroscopy with the spin-labeling technique is used to investigate the effects of monovalent ions on multibilayer dispersions of dipalmitoylphosphatidylcholine (DPPC). Cations of chloride salt (Li+, Na+, K+ and Cs+) and anions of potassium salt (Br-, Cl- and NO3-) at the concentration of 1 M do not affect both the molecular order and the packing of the phospholipid acyl chains in the different phases compared to DPPC dispersions in buffer. Moreover, they leave unaffected the characteristics of the main transition, whereas the pre-transition temperature increases of about 2 degrees C in the presence of cations and changes in the order NO3- < Br- < buffer < Cl- in the presence of anions. The anions that exhibit pronounced chaotropic properties (I-, SCN-) result the most effective in perturbing the bilayer. In fact, DPPC dispersions in 1 M of these salt solutions do not show the pre-transition and have the main one shifted to lower temperature in the order: SCN- < 1- < buffer. Furthermore, the spin-label EPR results on the lipid chain dynamics indicate the presence of a flexibility gradient both in DPPC/buffer and in DPPC/chaotropic systems. However, the chaotropic anions influence the DPPC hydrocarbon chains in the gel phase in a manner such that interpenetration or interdigitation of the terminal methyl groups from opposing monolayers is likely to occur.