The Influence of Calcium Traces in Ultrapure Water on the Lateral Organization in Tetramyristoyl Cardiolipin Monolayers

Abstract

Cardiolipin (CL) plays an important role in administering the structural organization of biological membranes and therefore helps maintaining the integer membrane functionality. CL has a dimeric structure consisting of four acyl chains and two phosphate groups. With its unusual structure, the phospholipid is responsible for curvature formation in CL containing biological membranes. The acidic properties of the phosphate head groups are still not sufficiently investigated since there are controversial results in literature. The main aim of this work was to gain deeper insights into the influence of the pH on the ionization degree of CL. During the experiments, it became clear that the used ultrapure water contained traces of calcium. These unexpected calcium ions had a major impact on the behavior of CL monolayers. Therefore, the focus was put on the analysis of CL layers without and with EDTA in the subphase used to complex divalent calcium ions. For the analyses, traditional surface pressure (π) – molecular area (A) isotherm experiments combined with total reflection x‐ray fluorescence (TRXF) and grazing incidence x‐ray diffraction (GIXD) have been used.

Enhanced chain packing achieved via putative headgroup ion-triplet formation in binary anionic lipid/cationic surfactant mixed monolayers

“…A https://doi.org/10.1016/j.chemphyslip.2019.104827 typical reference for cations is behenylsulfate (BS) containing one charge. However, in the present case we encountered the problem that the used ultrapure water contained traces of calcium (Sturm et al, 2019). These unexpected divalent calcium ions competed with the monovalent cesium for interaction with the negatively charged DPPG.…”

Interfacial Dynamics in Lipid Membranes: The Effects of Headgroup Structures

“…Traditionally, the different packing and organization of PC and PE lipids result from a balance of three closely related effects: (1) PE has less steric bulk in the headgroup region than PC, and as a result, PE lipids are described as having an intrinsic curvature; (2) lacking the three methyl groups, PE lipids have less overall bulk than PC lipids, and as a result, packing density is increased; and (3) direct lipid–lipid H-bonding in PE further decreases the area-per-lipid. The importance of these factors is supported by a variety of findings, including measurements of area-per-lipid, , phase transition temperatures, , and characterization of lipid packing geometries. ,, Despite their presence across biological membranes and inherent structural effects, PE lipids have been largely understudied relative to PC lipids , due to their difficulty forming unsaturated planar membrane structures, which are used in numerous biophysical techniques including sum-frequency generation spectroscopy, neutron and X-ray reflectometry, NMR spectroscopy, , electrochemical sensors, and fluorescence microscopy . While planar membranes with up to 90 mol % PE can be prepared, PE concentrations above 50 mol % can lead to the formation of defects, which limit the reproducibility of experiments …”

“…The importance of these factors is supported by a variety of findings, including measurements of area-perlipid, 53,54 phase transition temperatures, 55,56 and characterization of lipid packing geometries. 49,51,57 Despite their presence across biological membranes and inherent structural effects, PE lipids have been largely understudied relative to PC lipids 46,47 due to their difficulty forming unsaturated planar membrane structures, which are used in numerous biophysical techniques including sum-frequency generation spectroscopy, 21 neutron and X-ray reflectometry, 58 NMR spectroscopy, 59,60 electrochemical sensors, 61 and fluorescence microscopy. 62 While planar membranes with up to 90 mol % PE can be prepared, PE concentrations above 50 mol % can lead to the formation of defects, which limit the reproducibility of experiments.…”

Phase Diagram for a Lysyl‐Phosphatidylglycerol Analogue in Biomimetic Mixed Monolayers with Phosphatidylglycerol: Insights into the Tunable Properties of Bacterial Membranes

“…In earlier work, we encountered the problem that the ultrapure water used in the subphase contained traces of calcium. To avoid the competition of divalent calcium ions with the monovalent cesium ions for interaction with the negatively charged lipid phosphate groups, 50 μM EDTA was added. At pH 7.4, DPPG is assumed to be fully ionized whereas DP3adLPG can be either zwitterionic or positively charged, in the same way as the native LPG (Figure C) …”

“…To make comparisons between the charge states of the different DPPG/DP3adLPG mixed monolayers, TRXF measurements were performed at a surface pressure of 30 mN ⋅ m −1 (Figure ), the physiological lateral pressure in membranes, when all of the different compositions were in the condensed phase (Figure A). Figure A and B show the L α and L β bands of the subphase Cs + and the K α signal of Br − , both of which exhibited high intensities, allowing quantitative evaluation of the integrated values (Figure C).…”