V-Shaped Crystalline Structures of Di-n-alkyl Esters of Phosphoric Acid

We prepared crystals of di-n-alkyl esters of phosphoric acid with chain lengths of n 10, 12, 14, 16, and 18. These were characterized by single-crystal X-ray analysis and differential scanning calorimetry (DSC). It was found that the alkyl chains are in an extended all-trans conformation and aligned close to perpendicular, forming V-shaped molecules. This is in strong contrast to the typical arrangement of the alkyl chains of phospholipids where the two alkyl chains are arranged parallel in the same direction (e.g., tuning fork configuration in bilayers). Additionally, it was found that the arrangement of the V-shaped molecules of the di-n-alkyl esters in neighboring stacks of the lamellar crystals is antiparallel for short chain lengths (n 10 and 12) and parallel for the longer (n 14 and 16). DSC reveals that the melting of the crystals increases systematically with increasing chain lengths from 48 to 82 °C. The contribution of each methylene group to the melting enthalpy (70−133 kJ/mol) is independent of the chain length (3.9 kJ per mol CH2).