Submitted to: Colloids and Surfaces A: Physicochemical and Engineering Aspects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 11, 2005
Publication Date: November 21, 2005
Citation: Evans, K.O. 2005. Room-temperature ionic liquid cations act as short-chain surfactants and disintegrate a phospholipid bilayer. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 274:11-17. Interpretive Summary: Phospholipids are molecules that have long chains of hydrocarbons attached to one of four oxygen atoms that are directly attached to a phosphorus atom. These hydrocarbon chains form a 'tail' on the lipid molecule. Attached to an oxygen atom on the opposite end of the phosphorus atom is another group of atoms that form a 'head-group'. At a high enough concentration in solution, these phospholipids can form an organized structure called a bilayer. This phospholipid bilayer is comprised of two single, separate layers of lipids compressed together such that the 'tails' of one layer points toward the 'tails' of the opposite layer. The 'head-group' of the lipids form the outer region of each layer. Research was conducted to investigate the stability of phospholipid bilayers in solvents called room-temperature ionic liquids. The objective was to determine if the selected phospholipid spherical bilayer could protect its entrapped contents by remaining relatively intact in the presence of a room-temperature ionic liquid. Room-temperature ionic liquids are new solvents that have generated considerable interest in recent years because they can be used to replace the traditional organic solvents used in the conversion of excess soybean oil produced in the U.S. into nutritional and cosmetic ingredients. The proteins that are used in this conversion can not be used in their natural environment because soybean oil is not soluble in water. Therefore, organic solvents have been considered as a replacement for water in the conversion process. However, the organic solvents used are hazardous, volatile, and come with considerable environmental and health concerns. Room-temperature ionic liquids, on the other hand, have low vapor pressure which makes them less volatile than organic solvents. It may be possible to prevent the protein from losing activity in a room-temperature ionic liquid by using a phospholipid bilayer as a 'protective covering'. The results of this investigation demonstrated that the chosen room-temperature ionic liquids did disrupt the stability of a phospholipid bilayer. In particular, these results highlight that one type of room-temperature ionic liquids left the bilayers relatively intact, whereas others did not. These fundamental results will be used by us and other scientists as a foundation to better select room-temperature ionic liquids that least interact with phospholipid bilayers that are used to protect protein activity.
Technical Abstract: The effect of room-temperature ionic liquids (RT-ILs) cations made of 1-alkyl-3-methylimidazolium (alkyl group of C4 ' C8) and anions (commonly found in RT-ILs) on the structural integrity of large unilamellar vesicles (LUVs) of 1,2-dielaidoylphosphocholine were studied. Vesicle integrity was determined by monitoring the release of calcein quenched by cobalt ions and vesicle size distribution after incubation in the presences of each cation or anion. Results demonstrated that the instability of a fluid bilayer increased as the length of alkyl group of the cation increased. LUVS were mostly stable in the presence of the anions as evidenced by continued retention of encapsulated materials. Size measurements revealed, in the case of 1-butyl-3-methylimidazolium cation (100 ' 1500 mM), encapsulated material was released through small holes in the bilayer. However, total vesicle disruption was the release mechanism in the case of 1-hexyl or octyl-3-methylimidazolium above 200 mM.