Abstract
In the current study, differential scanning calorimetry and <sup>31</sup>P NMR were used to characterize the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers in the presence of butanol isomers. Small amphiphilic molecules at critical concentrations, including n-, iso-, sec- and tert-butanol, induce the interdigitated phase in which the acyl chains of the phospholipids interpenetrate. The effects of the inducer molecule structure on lipid phase behavior were investigated by comparing these isomers in the DPPC system. The threshold concentration for the onset of interdigitation for each isomer was determined by the disappearance of the pretransition, the biphasic effect, and the onset of a large hysteresis between the heating and cooling scans of the gel-to-liquid main transition. The threshold concentration was the lowest for n-butanol and increased with branching of the isomers with tert-butanol having the highest concentration necessary for induction of interdigitation. At low concentrations of the alcohol isomers, the temperature of the main transition (TM) decreased with increasing alcohol concentration. Following the threshold concentration the behavior of DPPC in the presence of each isomer varied. In contrast to short chain alcohols, n-butanol did not exhibit clear biphasic behavior. However, with increased branching of the isomers, the reversal of the TM at the threshold was greater; DPPC in the presence of tert-butanol had the most biphasic effect. As solution concentration of tert-butanol increased, there was an abrupt shrinking of the hysteresis, initially with well-resolved shoulder peaks indicating mixed phases. The disappearance of the shoulder peaks was correlated with a breakdown of bilayer structure identified using <sup> 31</sup>P NMR.
