title: Ca2+-induced lateral phase separation in phosphatidic acid/phosphatidylcholine monolayers as revealed by fluorescence microscopy.
abstract: Phase separation in mixed monolayers of phosphatidylcholine (PC) and pyrene-labeled phosphatidic acid (PA) was observed by fluorescence microscopy on an air/water interface as a function of subphase Ca2+ concentration and lateral packing pressure of the film. Below 45 mN m-1 and in the absence of Ca2+ no indications of phase immiscibility were observed. Addition of 1 mM Ca2+ caused extensive phase separation, which was evident immediately after spreading of the film. Further increase in Ca2+ concentration up to 30 mM increased the pyrene excimer intensity of the separated phosphatidic acid enriched domains. In the presence of Ca2+ (1-30 mM) and at surface pressures below 10 mN m-1 phase separation was always evident. However, as surface pressure exceeded 10 mN m-1, mixing of PC and PA occurred. Upon decompression of the film, phase separation reappeared at surface pressures close to 10 mN m-1. The surface textures of the film before and after the compression and subsequent relaxation were different. Inclusion of 30 mol% cholesterol increased the number and decreased the size of the PA domains. In films containing 50 mol% cholesterol no phase separation could be detected at the resolution available.
authors.affiliation: Department of Membrane Physics, KSV Research Laboratories, Helsinki, Finland.
authors.forename: K K
authors.initials: KK
authors.lastname: Eklund
authors.forename: J
authors.initials: J
authors.lastname: Vuorinen
authors.forename: J
authors.initials: J
authors.lastname: Mikkola
authors.forename: J A
authors.initials: JA
authors.lastname: Virtanen
authors.forename: P K
authors.initials: PK
authors.lastname: Kinnunen
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine; D002118:Calcium; D008081:Liposomes; D008954:Models, Biological; D008968:Molecular Conformation; D010712:Phosphatidic Acids; D011721:Pyrenes; D013050:Spectrometry, Fluorescence
chemical_list: D008081:Liposomes; D010712:Phosphatidic Acids; D011721:Pyrenes; C056467:1-palmitoyl-2-(6-(pyren-1-yl)hexanoyl)-sn-glycero-3-phosphatidic acid; D015060:1,2-Dipalmitoylphosphatidylcholine; D002118:Calcium