MONOLAYER STUDY OF CLEAVABLE PHOSPHOLIPIDS

Monolayer characteristics were determined for three isomeric cleavable phospholipids containing a ketal group on one of the alkyl chains: 1-octadecanoyl-2-[trans-[8-(2,2-dimethyl-5-octyl-1,3-dioxolan-4-yl)octanoyl]]-sn-glycero-3-phosphatidylcholine (1a); I-octadecanoyl-2-[16-(2,2-dimethyl-1,3-dioxolan-4-yl)hexadecanoyl]-sn-glycero-3-phosphatidylcholine(1b); and 1-octadecanoyl-2-[trans-[1-(2,2-dimethyl-5-pentadecyl-1,3-dixolan-4-yl)methanoyl]]-sn-glycero-3-phosphatidylcholine (1c). The monolayer characteristics were also determined for two of the daughter surfactants in which a vic-diol group replaces the ketal group: 1-octadecanoyl-2-(threo-9,10-dihydroxyloctadecanoyl)-sn-glycero-3-phosphatidylcholine (2a) and 1-octadecanoyl-2-(17,18-dihydroxyoctadecanoyl)-sn-glycero-3-phosphatidylcholine (2b). Surfactants 1a-c displayed surprisingly high solubility in the water subphase; stable monolayers could not be formed even at 7-8-degrees-C. Reproducible surface pressure isotherms were obtained for 1a-c, 2a and 2b using a subphase of aqueous 30% polyethylene glycol. The apparent areas/molecule were larger than expected based on estimates from CPK models, even assuming that all alkyl chains are flat on the interface and are never elevated above it to any appreciable degree, even at high compressions. Surface potential measurements generally remained constant throughout the range of compression. The disparities between measured and estimated molecular areas are probably due to two effects: chirality in the systems and occlusion of void areas present in many of the planar conformational states.