Thermal lipid order-disorder transitions in complexes of various disulfidetethered macrocyclic diacylglycerol analogues and dipalmitoyl phosphatidylcholine. role of diacylglycerol chain motions

The role of diacylglycerol (DAG) chain motions in binary mixtures of DAG wi th dipalmitoylphosphatidyl choline (DPPC) has been studied using differenti al scanning calorimetry (DSC). The DAGs used in this study were synthesized to have different extents of chain restriction. This was accomplished via tethering of two long hydrocarbon chains by a reducible disulfide bond at d ifferent depths in the chain. The disulfide tether provides the advantage o f comparing the effect of incorporation of each of these chain restricted D AGs in DPPC vesicles with their reduced analogue, in which the S-S linkage could be opened up to -SH by in situ treatment with a reducing agent, dithi othreitol (DTT). DSC analysis with such mixtures reveals that restriction o f chains at different depths in the DAGs lead to very different properties of the overall DPPC/DAG coaggregate. DAGs in which the disulfide tether is located at the middle of the chain have only partial chain restriction. Inc orporation of this type of DAG in DPPC destabilizes the resulting vesicles while the end-tethered DAGs upon incorporation in DPPC vesicles lead to sta bilization of the coaggregates. Similar studies with the reduced, open-chai n analogues reveal that release in chain restriction leads to extra stabili zation in the coaggregates.