Copyright (c) 2009 All rights reserved. http://works.bepress.com/beth_cunningham Recent documents in Beth Cunningham en-us Sun, 31 May 2009 03:44:25 PDT 3600 http://works.bepress.com/beth_cunningham/9 http://works.bepress.com/beth_cunningham/9 Sun, 12 Oct 2008 18:46:45 PDT The miscibility of 1,3-dioleoylglycerol (DOG) with 1 -stearoyl-2-oleoylphosphatidylcholine (SOPC) and 1- palmitoyl-2-oleoylphosphatidylcholin(eP OPC) dispersed in excess buffer was characterized by physical and enzymatic methods. Thermograms for all SOPC-DOG mixtures exhibit a transition at 5.3 OC. Above 0.25 mole fraction of DOG, metastability is observed; after the first scan, a second peak appears at 23.4 OC which corresponds to the chain melting of pure DOG. This suggests that a complex or preferred packing array is formed which has a DOG mole fraction of 0.25 (X,). Bilayer morphology is maintained in the metastable state up to 0.8 mole fraction of DOG. Above 0.8, a novel, nonlamellar phase is formed. Fluorescence polarization of 1,6-diphenylhexatriene shows that, relative to SOPC alone, there is little change in the order of the acyl chains up to X, followed by a large decrease above X,. Similar results were obtained using POPC. Miscibility was also studied in lipid films at the argon-buffer interface. Isothermal phase diagrams for the mixtures at 15 and 24 "C exhibited phosphatidylcholine-DOG complex formation, a region of phosphatidylcholine and complex coexistence, and a region of complex and DOG miscibility. The mole fractions of DOG in the complex (X,) range from 0.24 to 0.27. Porcine pancreatic phospholipase A2 and pancreatic lipase plus colipase were used as probes of the surface in both the monolayer and bilayer systems. In both systems and with both enzymes, substrate hydrolysis increased abruptly with increasing DOG. Overall, the formation of a complex at the same mole fraction of DOG in bilayers as in monolayers and the similar regulation of lipolytic enzyme activity in both systems suggest that the structures of DOG-phosphatidylcholine monolayers and bilayers are governed by a similar lipid-lipid interaction. Beth Cunningham http://works.bepress.com/beth_cunningham/8 http://works.bepress.com/beth_cunningham/8 Sun, 12 Oct 2008 18:23:22 PDT We have used X-ray diffraction and the osmotic force technique to investigate the effect of monovalent salts on the interactive forces between dipalmitoylphosphatidylcholine (DPPC) bilayers, distearoylphosphatidylcholine (DSPC) bilayers, dilauroylphosphatidylcholine (DLPC) bilayers, and dioleoylphosphatidylcholine (DOPC) bilayers. The Hamaker constant of the van der Waals force and the decay equation of the hydration force were determined by both the analysis technique of W. Tamura-Lis, L. J. Lis, and J. M. Collins (J. Colloid Interface Sci. 114, 214 (1986)) and that of D. M. LeNeveu, R. P. Rand, V. P. Parsegian, and D. Gingell (Biophys. J. 18, 209 (1977)). The majority of monovalent cations had little effect on the net force between phosphatidylcholine bilayers. However, the presence of monovalent anions caused an increase in the attractive force between DPPC bilayers. The electrostatic force between DPPC bilayers in 1 M KBr was analyzed and a binding ratio of one ion per 20 lipid headgroups at maximum hydration was inferred. Beth Cunningham http://works.bepress.com/beth_cunningham/7 http://works.bepress.com/beth_cunningham/7 Sun, 12 Oct 2008 18:14:51 PDT Phase transitions in a 4:1 molar ratio mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been investigated using time resolved X-ray diffraction at the Daresbury Synchrotron Laboratory, United Kingdom. Phospholipids subjected to temperature scans of 10°C/min or temperature jumps of approx 5°C/s were found to undergo order-disorder phase transitions at previously reported temperatures. A fully hydrated sample with a 4:1 molar ratio of DPPC:DPPE produced a single phase transition, characteristic of the gel to liquid crystal bilayer transition, at approx 40°C when heated at a scan rate of 10°C/min. Upon cooling at 10°C/min, Lβ′-phase stability occurred at 35°C. The Lα to Lβ, transition was characterized by the coexistence of broad acyl chain peaks diagnostic for each phase with the eventual formation of a sharp peak at 0.41 nm. The transition from the Lα to the Lβ′ form in the lipid mixture requires that this miscibility be unaffected by the rate of the cooling process and proceeds by the nucleation of small domains of Lβ′-phase lipids and the eventual growth to larger domains before the transition is complete. Under rapid heating at approx 5°C/s the gel to liquid crystal bilayer phase transition of this mixture was observed to require approx 7.5 s after 40°C was observed. Beth Cunningham http://works.bepress.com/beth_cunningham/6 http://works.bepress.com/beth_cunningham/6 Sun, 12 Oct 2008 18:03:30 PDT The effect of different monovalent cations on the packing arrangements and phase behavior of dipalmitoylphosphatidylcholine (DPPC) in excess water and on the transition of the lamellar phase from the gel to liquid crystalline configuration has been investigated using differential scanning calorimetry and synchrotron radiation at the Daresbury Laboratory. Bilayers of DPPC in 1M KCI or water appear to undergo the same transition sequences without any change in the kinetics of phase induction. It was also observed that bilayers of DPPC in the presence of 1M LiCl produced no small angle x-ray scattering peaks characteristic of the P,, phase. Beth Cunningham http://works.bepress.com/beth_cunningham/5 http://works.bepress.com/beth_cunningham/5 Sun, 12 Oct 2008 17:53:40 PDT The apparent Mn2+ binding constant for l-α-dipalmitoylphosphatidylcholine (DPPC) bilayers dispersed in monovalent salt and MnCl2 dispersions was determined as a function temperature using electron paramagnetic resonance (ERP). Reproducibility in the data sets requires the use of a standard salt solution and dual cavity techniques. Changes in the binding constant at different phase states and temperatures were observed and correlated to the influence of monovalent salts on the thermal properties of DPPC. The turning points (i.e. changes in slope) in the curves of the apparent Mn2+ binding constant versus temperature can be understood in terms of differences in ion binding to headgroups with different bilayer surface areas. The influence of Li+ and SCN− on Mn2+ binding is viewed as a function of their presence in the ionic media in contact with the bilayer rather than as a competitive event. Other monovalent ions studied appear to have little effect on the measured apparent Mn2+ binding constants for DPPC headgroups. Beth Cunningham http://works.bepress.com/beth_cunningham/4 http://works.bepress.com/beth_cunningham/4 Sun, 12 Oct 2008 17:45:25 PDT Phase transitions in 1:1 molar mixtures of DLPE:DPPC, DMPE:DPPC, DPPE:DPPC, and DSPE: DPPC have been examined using time-resolved X-ray diffraction techniques. A model of the gel-to-liquid crystal phase transition observed upon heating for these lipid mixtures in which the mixing of domains of gel and liquid crystal phase lipids is the rate-limiting step can be formulated. Upon cooling, it is inferred that small domains of gel state lipid are nucleated from the Lα phase. The miscibility of components and domains is also related to the types of proposed phase diagrams. Beth Cunningham http://works.bepress.com/beth_cunningham/3 http://works.bepress.com/beth_cunningham/3 Sun, 12 Oct 2008 17:35:31 PDT Abstract The effect of monovalent anions on the packing structures and transitions of the lamellar phases of dipalmitoylphosphatidylcholine bilayers has been investigated at the Daresbury Synchrotron Laboratory (U.K.). Fully hydrated DPPC bilayers in the presence of 1M KBr swell past the usual repeat spacing observed in water by approximately 20 and in the presence of 1M KSCN form an interdigitated gel phase. The transition temperatures, determined by changes in the bilayer parameters in the small and wide angle scattering profiles, occur at temperatures previously observed. Our observations also indicate that the changes in the bilayer unit cell produced by the presence of Br or CNS- do not change the nature of the phase transition. Beth Cunningham http://works.bepress.com/beth_cunningham/2 http://works.bepress.com/beth_cunningham/2 Sun, 12 Oct 2008 17:20:58 PDT We have probed the character of the observed phase separation in mixtures of phosphatidylcholines (PC) and/or phosphatidylethanolamines (PE) in the presence of CaCl2 solutions. Egg yolk phosphatidylethanolamine (EYPE) and a 1:1 molar ratio of dioleoylphosphatidylcholine/dioleoylphosphatidylethanolamine (DOPC/DOPE) were observed to undergo phase separation in CaCl2 solutions, as was previously observed for egg yolk phosphatidylcholine (EYPC) (L.J. Lis et al. Biochemistry, 20 (1981) 1771-1777). However, the mixed chain lipid, palmitoyloleoyl-PC, yielded only a single phase in water or CaCl2 solution. We hypothesize that two lipid species are necessary for the observed phase separation to occur, but that the separation itself is not a function of the individual lipid species, but of the mixture. Beth Cunningham http://works.bepress.com/beth_cunningham/1 http://works.bepress.com/beth_cunningham/1 Sun, 12 Oct 2008 17:16:56 PDT The effects of various monovalent cations and anions on the bilayer packing and structure of dipalmitoylphosphatidylcholine were studied using X-ray diffraction and differential scanning calorimetry. It was observed from the X-ray diffraction studies that monovalent salts, in general, have no effect on bilayer packing. The results of DSC studies on metal chloride systems are consistent with the interpretation that cations in general and Li+ in particular bind to DPPC bilayers. The effect of potassium salts on pre- and main-transition temperatures suggest that anions, such as Acetate−, also significantly bind to DPPC head groups. Beth Cunningham