Stability and structures of liquid crystalline phases formed by branched-chain phospholipid diastereomers

Diastereomers of 1-alkyl-2-acylglycerophosphocholines containing 2-methyl-b ranched fatty acids at the C2 position of the glycerol are synthesized and separated, varying the acyl-chain systematically (m = 13, 15, 17). The phys icochemical properties are studied in monolayers at the air-water interface by pressure area isotherms and grazing incidence X-ray diffraction (GIXD) measurements. The phase behavior is examined by a combination of differenti al scanning calorimetry and time-resolved X-ray diffraction experiments, In monolayers at the air-water interface, the introduction of a methyl-branch disturbs the lateral packing drastically, resulting in phases with strongl y tilted chains and an undefined tilt azimuth. The monolayer structures of the diastereomeric fractions cannot be distinguished. The comparison of the lyotropic mesophases of the aqueous dispersions shows comparable structura l parameters. The temperature dependency of the d spacings is nonlinear and is discussed in terms of a critical swelling of the bilayer. The present e xperiments show that the two racemic pairs of the resolved diastereomers (S S + RR/SR + RS) exhibit pronounced differences in the thermodynamic paramet ers of phase transitions both in two-dimensional (monolayers at the air/wat er interface) and three-dimensional systems (aqueous dispersions). However, the influence of the chiral carbon atoms is not sufficient to change the s tructure of the phases despite being able to affect their stability.