Evidence for transbilayer, tail-to-tail cholesterol dimers in dipalmitoylglycerophosphocholine liposomes.
The behavior of multilamellar liposomes of 2,3-dipalmitoyl-sn-glycero-1-phosphocholine (DPPC) was studied by differential scanning calorimetry (DSC) in the presence of < or = 5 mol % of the amphiphilic solutes methyl oleate, cholesterol, pregnenolone, and dehydroandrosterone. The DSC thermograms indicate that the solutes are miscible only with the liquid-disordered (Id) phase, and not with the solid-ordered (so) phase. The slopes of the Tm vs solute concentration curves confirm this conclusion: It appears that the so-1d phase transition of DPPC, which corresponds to the melting of the phospholipid chains, can be treated as a simple melting process and, thus, could be used as a cryoscopic system. In that case, its melting point depression constant, Kf, can be calculated a priori from the experimentally measured heat of fusion per gram of DPPC, lf, and the temperature of the phase transition of pure DPPC, T(o), by the equation Kf = RTo2/(1000lf) = 12.3 +/- 0.9 K g M-1 cm3. With methyl oleate as the solute, the Tm vs methyl oleate concentration plot is linear, and from the slope we calculate Kf = 12.9 +/- 0.8 K g M-1 cm3. Thus, methyl oleate appears to form an ideal cryoscopic system with dipalmitoyllecithin liposomes: It is fully miscible with the 1d phase but is apparently insoluble in the s(o) phase. Pregnenolone and dehydroandrosterone also form ideal cryoscopic systems with dipalmitoyllecithin liposomes: The Tm vs solute concentration plots are linear and yield the correct MWs for these solutes.(ABSTRACT TRUNCATED AT 250 WORDS)
Harris
J S
JS
Department of Biochemistry, Upjohn Laboratories, Kalamazoo, Michigan 49001.
Epps
D E
DE
Davio
S R
SR
Kézdy
F J
FJ
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D002784:Cholesterol / Q000737:chemistry*; D008081:Liposomes; D013816:Thermodynamics
D008081:Liposomes; D015060:1,2-Dipalmitoylphosphatidylcholine; D002784:Cholesterol