MONOLAYER STUDY OF CLEAVABLE PHOSPHOLIPIDS

Monolayer characteristics were determined for three isomeric cleavable phospholipids containing a ketal group on one of the alkyl chains: n- 4-yl)octanoyl]]-sn-glycero-3-phosphatidylcholine (1a); exadecanoyl]-sn -glycero-3-phosphatidylcholine(1b); and -4-yl)methanoyl]]-sn-glycero-3 -phosphatidylcholine (1c). The monolayer characteristics were also det ermined for two of the daughter surfactants in which a vic-diol group replaces the ketal group: oxyloctadecanoyl)-sn-glycero-3-phosphatidylc holine (2a) and roxyoctadecanoyl)-sn-glycero-3-phosphatidylcholine (2b ). Surfactants 1a-c displayed surprisingly high solubility in the wate r 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 appare nt areas/molecule were larger than expected based on estimates from CP K models, even assuming that all alkyl chains are flat on the interfac e and are never elevated above it to any appreciable degree, even at h igh compressions. Surface potential measurements generally remained co nstant throughout the range of compression. The disparities between me asured and estimated molecular areas are probably due to two effects: chirality in the systems and occlusion of void areas present in many o f the planar conformational states.