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

Diastereomers of 1-alkyl-2-acylglycerophosphocholines containing 2-methyl-branched fatty acids at the C2 position of the glycerol are synthesized and separated, varying the acyl-chain systematically (m = 13, 15, 17). The physicochemical 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 differential 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 strongly 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 structural parameters. The temperature dependency of the d spacings is nonlinear and is discussed in terms of a critical swelling of the bilayer. The present experiments show that the two racemic pairs of the resolved diastereomers (SS + RR/SR + RS) exhibit pronounced differences in the thermodynamic parameters of phase transitions both in two-dimensional (monolayers at the air/water interface) and three-dimensional systems (aqueous dispersions). However, the influence of the chiral carbon atoms is not sufficient to change the structure of the phases despite being able to affect their stability.