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

Marina Sturm,Gerald Brezesinski,Olof Gutowski

doi:10.1002/cphc.201900126

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.

Highlights

The anionic phospholipid cardiolipin (CL) has been found in bacterial and mitochondrial membranes.[1]
total reflection x-ray fluorescence (TRXF) was the method of choice to determine the ionization degree of the phosphate head groups based on the concentration of the corresponding counterions in the electrical double layer (EDL) near a charged surface
The results presented in this work show that the ultrapure water used for the experiments contained traces of calcium

Summary

Introduction

The anionic phospholipid cardiolipin (CL) has been found in bacterial and mitochondrial membranes.[1] There, it fulfills a crucial function in maintaining the integer functionality of biological membranes as it takes part in regulating their structural organization.[2] The reasons for that important feature of CL are based in its dimeric structure.[3] With four alkyl chains and a relatively small head group, the phospholipid features a conical shape which induces negative curvature formation in CL containing membranes.[4,5,6,7] Modifying the packing of the lipid can lead to structural changes and instabilities in the shape of the membrane.[1,8,9] By means of controlled environmental alterations, these changes can be investigated

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