Effects of Sodium Salts of Lyotropic Anions on Low-Temperature, Ordered Lipid Monolayers

Abstract

Electrolytes are known to impart considerable disorder to lipid assemblies, including monolayers at the air-water interface, bilayers and vesicles. In the present work we have investigated the disordering effect of sodium salts of monovalent anions that span the lyotropic series on the monolayers of 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC) at 12 ºC. Pressure-area isotherms, Brewster-Angle Microscopy (BAM), grazing-incidence Xray diffraction (GIXD), and infrared absorption-reflection spectroscopy (IRRAS) were used to investigate in complementary ways the salt effects on lipid monolayers. At 12 ºC these effects were found to be quite dramatic, a major finding being that the liquid-expanded phase, which is not present at this temperature on a pure water subphase, reappears and dominates in the presence of electrolytes. Salts expand the monolayer, destroy the ordered phase that exists at zero pressure, and affect the ordering of the lipid chains and their tilt angle in the liquid condensed phase. Finally, very chaotropic anions force DPPC lipids to adopt an untilted conformation in the condensed phase, an unprecedented finding for non-mixed Langmuir monolayers of this phospholipid. A distinctly different behavior of very chaotropic anions from that of normal chaotropic ones thus emerges. The effect of the former is not just a limited perturbation of the lipid assembly, but a major disruption of the structure, which arises from competition between the lipids and the ions for interfacial sites.

Effect of cation enrichment on dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface

“…A total expansion of the monolayer occurs on the 2.0 M NaCl solution, where a larger MMA relative to water occurs at all surface pressures. The phase behavior of DPPC on NaCl solutions has been studied extensively, and a shift to higher MMAs in the LE and LE-LC coexistence phases has also been previously observed for moderately concentrated (P0.1 M) solutions [15,[38][39][40][41][42][43]. The extent of the MMA shift seems concentration-dependent, as it increases by 3-6 Å 2 /molecule for concentrated solutions (P0.5 M) [15,40,41], and is shifted 1-2 Å 2 /molecule for 0.1 M solutions [38,39].…”

Impact of anions on the surface organisation of lipid monolayers at the air–water interface

“…The difference in the sequence of ammonium salts between low and high concentrations implies that anions affect the ordering of the lipid chain and their tilt angle in the LC phase. [55] The expansions of the monolayers at fixed surface pressure follow the Hofmeister sequence at very low ammonium salt concentrations.…”

The Chaotropic Effect as an Assembly Motif in Chemistry

“…[164] In contrast, some of the interactions of organic ions (AsPh 4 + and BPh 4 À )w ith proteins are presumably accounted for in terms of their hydrophobic character. [47,51] Numerous examples for unspecific interactions of chaotropic anions with biological membranes are also known; [7,162,[168][169][170][171][172][173][174] in particular,t heir membrane-disrupting properties are noteworthy. [171,172] Gabel and co-workers investigated the interaction between dodecaborate cluster dianions and lipid bilayers; [34][35][36]175] at high salt concentrations they observed changes in the morphology of liposomes as well as bilayer leakage (see Figure 8f or an example).…”

“…Numerous examples for unspecific interactions of chaotropic anions with biological membranes are also known;7, 162, 168, 169, 170, 171, 172, 173, 174 in particular, their membrane‐disrupting properties are noteworthy 171, 172. Gabel and co‐workers investigated the interaction between dodecaborate cluster dianions and lipid bilayers;34, 35, 36, 175 at high salt concentrations they observed changes in the morphology of liposomes as well as bilayer leakage (see Figure 8 for an example).…”