title: Detection of coexisting fluid phospholipid phases by equilibrium Ca2+ binding: peptide-poor L alpha and peptide-rich HII phase coexistence in gramicidin A'/phospholipid dispersions.
abstract: The isothermal phase behavior of three gramicidin A'/phospholipid mixtures was investigated by an equilibrium Ca(2+)-binding technique. The phospholipid component was 1,2-dioleoyl-sn-glycero-3-phosphoserine (DOPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS), or POPS/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at a constant mole ratio of 1/4. The bulk aqueous free Ca2+ concentration, [Ca2+]*f, in equilibrium with one or two gramicidin A'/phospholipid fluid phases and a small amount of the Ca (phosphatidylserine)2 gel phase, was measured as a function of composition at 20 degrees C by use of chromophoric high-affinity Ca2+ chelators. The coexistence of two gramicidin A'/phospholipid fluid phases was detected by an invariance in [Ca2+]*f over the range of compositions throughout which the two phases coexist. The compositions of the two coexisting phases are determined by the compositions at which the invariance in [Ca2+]*f begins and ends. With each of the gramicidin A'/phospholipid mixtures, we estimate that the composition of the gramicidin-poor phase is 0.03-0.04 mole fraction gramicidin A' and the composition of the gramicidin-rich phase is 0.13-0.14 mole fraction gramicidin A'. Characterization of these phases by low-angle X-ray diffraction revealed that, in each case, the gramicidin-poor phase is an L alpha phase and the gramicidin-rich phase is an HII phase. The isothermal phase behavior of gramicidin A'/POPC mixtures at approximately 23 degrees C, as determined by low-angle X-ray diffraction, was found to be similar to that of the other gramicidin A'/phospholipid mixtures.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, New York 14853.
authors.forename: A R
authors.initials: AR
authors.lastname: Dibble
authors.forename: G W
authors.initials: GW
authors.lastname: Feigenson
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000737:chemistry; D006096:Gramicidin / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D010713:Phosphatidylcholines / Q000737:chemistry / Q000378:metabolism*; D010718:Phosphatidylserines / Q000737:chemistry / Q000378:metabolism*; D010761:Phosphorus Radioisotopes; D012015:Reference Standards; D014961:X-Ray Diffraction
chemical_list: D002614:Chelating Agents; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D010761:Phosphorus Radioisotopes; D006096:Gramicidin; C050294:1-palmitoyl-2-oleoylglycero-3-phosphoserine; C042511:1,2-dioleoylphosphatidylserine; D002118:Calcium; C028694:1-palmitoyl-2-oleoylphosphatidylcholine