Critical Behaviour in DOPC/DPPC/Cholesterol Mixtures: Static 2H NMR Line Shapes Near the Critical Point

Static ²H NMR spectroscopy is used to study the critical behavior of mixtures of 1,2-dioleoyl-phosphatidylcholine/1,2-dipalmitoyl-phosphatidylcholine (DPPC)/cholesterol in molar proportion 37.5:37.5:25 using either chain perdeuterated DPPC-d₆₂ or chain methyl deuterated DPPC-d₆. The temperature dependence of the first moment of the ²H spectrum of the sample made with DPPC-d₆₂ and of the quadrupolar splittings of the chain-methyl-labeled DPPC-d₆ sample are directly related to the temperature dependence of the critical order parameter η, which scales as [(T_c?T)/T_c]^β_c near the critical temperature. Analysis of the data reveals that for the chain perdeuterated sample, the value of T_c is 301.51 ± 0.1 K, and that of the critical exponent, β_c = 0.391 ± 0.02. The line shape analysis of the methyl labeled (d₆) sample gives T_c = 303.74 ± 0.07 K and β_c = 0.338 ± 0.009. These values obtained for β_c are in good agreement with the predictions of a three-dimensional Ising model. The difference in critical temperature between the two samples having nominally the same molar composition arises because of the lowering of the phase transition temperature that occurs due to the perdeuteration of the DPPC.