Effect of sugars on headgroup mobility in freeze-dried dipalmitoylphosphatidylcholine bilayers: Solid-state P-31 NMR and FTIR studies

The effect of the carbohydrates trehalose, glucose, and hydroxyethyl starch (HES) on the motional properties of the phosphate headgroup of freeze-drie d dipalmitoylphosphatidylcholine (DPPC) liposomes was studied by means of P -31 NMR, Fourier transform infrared spectroscopy (FTIR), and differential s canning calorimetry (DSC). The results show that trehalose, which is a stro ng glass former (T-g = 115 degrees C), elevates the onset of the lipid head group rotations and preserves some rotational mobility of the phosphate hea dgroups after cooling from the liquid-crystalline state. Glucose (T-g = 30 degrees C), a very effective depressant of the phase transition temperature of freeze-dried DPPC, markedly elevates the initiation of the temperature of headgroup rotations. On the other hand, the monosaccharide does not pres erve the headgroup disordering when cooled from the liquid-crystalline stat e. These effects are consistent with formation of hydrogen bonds between th e OH groups of the sugar and the polar headgroups of DPPC. They show, howev er, that hydrogen bonding is not sufficient for preservation of the dynamic properties of freeze-dried DPPC. HES, although a very good glass former (T -g > 110 degrees C), does not depress the phase transition temperature and affects only slightly the rotational properties of freeze-dried DPPC. This lack of effect of HES is associated with the absence of direct interactions with the lipid phosphates, as evidenced by the FTIR results. These data sh ow that vitrification of the additive is not sufficient to affect the dynam ic properties of dried DPPC.