Laser raman studies of molecular interactions with phosphatidylcholine multilayers. II. Effects of mono- and divalent ions on bilayer structure

Laser Raman spectroscopy was applied to characterize structural behavior of dipalmitoyl phosphatidylcholine multibilayer systems in the presence of several cations (K⁺, Na⁺, Cs⁺, Rb⁺, Ca²⁺, Mg²⁺, Cd²⁺, Ba²⁺) and anions (Cl⁻, Br⁻, I⁻, NO₃⁻, SO₃²⁻, SO₄²⁻, S₂O₃²⁻, S₂O₈²⁻). To evaluate the Raman-spectroscopical data quantitatively, characteristic intensity ratios, lateral and trans order parameters were used and compared. It was shown that the different trans order parameters are rather sensitive to ion-polar head group interactions and thus, they cannot give unequivocal information on the trans-gauche isomerization of hydrocarbon chains of phospholipids. The observed effects of ions on Raman spectra of phospholipid multilayers could not be explained simply on the basis of electrostatic interactions. The possible involvement of other factors (changes in polarizability, hydrogen bonds, etc.) is also discussed. It was demonstrated that the order parameters defined in different ways may result in different effectiveness sequences of ions. Of monopositive ions Na⁺ was found to be the most effective to influence the bilayer structure. For dipositive ions, of which Ca²⁺ proved to be the most effective, concentration-dependent effectiveness sequences were obtained. A plausible interpretation and some consequences of the concentration-dependent two-step binding of divalent cations were also outlined. Bilayered phospholipid structures turned out to be more responsive to anions than to most cations investigated. Interdependent actions of cations and anions, as well as the possible relevance of the charge distribution on anions are postulated.