Ultrasonic Absorption by Liposomes Near the Phase Transition as a Function of Diameter (Size)
Maynard, Valerie Marie
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Permalink
https://hdl.handle.net/2142/70711
Description
Title
Ultrasonic Absorption by Liposomes Near the Phase Transition as a Function of Diameter (Size)
Author(s)
Maynard, Valerie Marie
Issue Date
1984
Department of Study
Physiology and Biophysics
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, General
Abstract
The specific ultrasonic absorption coefficient per wavelength as a function of temperature in the vicinity of the phase transition of liposomes, composed of a 4:1 mixture of DPPC and DPPG, of different sizes was determined using an acoustic interferometer. Small unilamellar vesicles, SUV (average diameter 20 nm) and large multilamellar vesicles, MLV, (average diameter 500 nm) yielded results similar to those in the literature, viz., a peak with the following properties: for the SUV, an amplitude of 0.32 ml/g occurring at 39(DEGREES)C and a width at half the amplitude of 8(DEGREES)C, and for the MLV, an amplitude of 0.60 ml/g occurring at 42.5(DEGREES)C, and a width at half the amplitude of 1.3(DEGREES)C.
This study of seven intermediate sizes alleviates a paucity of data for these more useful liposomes, yielding information on size dependencies of the temperatures at which the peaks occur, the peak amplitudes, the areas under the peaks, and the widths at half the amplitudes. Maximum absorption occurs at the membrane phase transition temperature. All liposome sizes, except the SUV, exhibit approximately the same transition temperature as do the largest MLV. The amplitudes of the peaks exhibit a general increase with size with two exceptions, viz., the SUV and the vesicles with average diameters of 90 to 100nm. The areas under the peak were used to determine the enthalpy changes due to the transition. They are independent of size except for the SUV, whose value is twice that of the other sizes. The van't Hoff enthalpies, determined from the sharpness of the peaks, exhibit a general increase with size, the dependence weakening as the size increases. The ratios of the van't Hoff enthalpies to the transition enthalpies indicate that intermolecular cooperativity during the transition increases with size to a diameter of 100 to 150 nm, above which it is independent of vesicle size. The fractional changes in membrane volume were calculated and found to be approximately the same for all sizes, which, along with enthalpy results, indicate a large internal change for the SUV, compared to all other vesicles.
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