Calcium-Induced Phospholipid Ordering Depends on Surface Pressure

Abstract

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (approximately 5 mN/m), the presence on Ca2+ results in the unexpected appearance of ordered domains. For pressures between approximately 5 and approximately 25 mN/m, Ca2+ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca2+ with the lipids is evident from Ca2+-induced changes in the signals corresponding to area changes of 4 A2/lipid-precisely the surface area of a Ca2+ ion, with evidence for a change in lipid Ca2+ complexation at high pressures.

Observation of buried water molecules in phospholipid membranes by surface sum-frequency generation spectroscopy

“…8 Briefly, the visible beam ͑VIS͒ ͑800 nm, ϳ30 J / pulse, spectral bandwidth of 25 cm −1 ͒ is overlapped at the sample position with ϳ5 J / pulse, 150 fs infrared ͑IR͒ beam. The IR wavelength is continuously tuned from 3570 to 5110 nm, such that both the O-D and the C-H stretch vibrational region are covered.…”

Sum-frequency vibrational spectroscopic studies of Langmuir monolayers

“…Sovago et al observed that Ca 2þ ions changed the alkyl chain ordering of zwitterionic DPPC and anionic DMPS phospholipids monolayers [93]. The Ca 2þ ions appeared to induce more ordered structure in the monolayers through their adsorption on the negatively charged phosphate groups, which supposedly decreased the Coulomb repulsion between molecules and enhanced condensation of the monolayer.…”

Microelectrophoretic investigation of the interactions between liposomal membranes formed from a phosphatidylcholine-phosphatidylglycerol mixture and monovalent ions

“…Ion adsorption on the lipid membrane surface has been explored through various techniques, including nuclear magnetic resonance [8,9], Langmuir method [10][11][12] microelectrophoresis [13][14][15] and computational methods, such as molecular dynamics simulations [16,17]. In particular, microelectrophoresis can be used to determine membranes surface charge density values.…”