title: Critical temperature for unilamellar vesicle formation in dimyristoylphosphatidylcholine dispersions from specific heat measurements.
abstract: Using a heat conduction calorimeter with very high resolution (+/- 0.00005 J/degrees C.cm3), we have measured the specific heat CpL between 25 and 35 degrees C of dimyristoylphosphatidylcholine (DMPC) in aqueous dispersions. Previous studies of the temperature dependence of the chemical potential of DMPC in the L alpha phase (lamellar, liquid crystalline) indicated that a dispersion consisting only of unilamellar vesicles forms spontaneously at a critical temperature T* of 29.0 degrees C. Our present measurements show an anomaly in CpL between 28.70 and 29.50 degrees C: the curve for CpL versus T first decreases and then exhibits an inflection point at 28.96 degrees C before it flattens. This anomaly is attributed to the transformation from multilamellar dispersion to unilamellar vesicles at T* = 28.96 degrees C. Two independent properties of the CpL data also indicate T* is a critical point for the formation of unilamellar vesicles: (a) the time to reach equilibrium upon changing temperature increased dramatically between 28.7 and 28.96 degrees C, increasing as (T* - T)-1; at T > T* the dramatic "slowing-down" phenomenon was not observed. This slowing-down near T* is a general characteristic of critical phenomena. (b) The free energy change for the multilamellar-unilamellar transformation was obtained from the CpL-T data over this temperature interval and found to be 3.2 J/mol or 0.016 ergs/cm2 of bilayer, in agreement with other estimates of the interaction energy between neutral bilayers. We conclude with a discussion of the implications for membrane bilayer stability of these newly identified dynamic properties of the transformation.
authors.affiliation: Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: N L
authors.initials: NL
authors.lastname: Gershfeld
authors.forename: C P
authors.initials: CP
authors.lastname: Mudd
authors.forename: K
authors.initials: K
authors.lastname: Tajima
authors.forename: R L
authors.initials: RL
authors.lastname: Berger
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D002151:Calorimetry; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry*; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D046911:Macromolecular Substances; D008968:Molecular Conformation; D013696:Temperature; D013816:Thermodynamics; D014867:Water
chemical_list: D008051:Lipid Bilayers; D046911:Macromolecular Substances; D014867:Water; D004134:Dimyristoylphosphatidylcholine