Reinforcing effect of polyterpenoids on polyprenyl phosphate monolayers

Abstract

Monolayer properties of some of the conceivably most primitive lipids, sodium dipolyprenyl phosphates (sodium digeranyl phosphate (DGPNa), sodium difarnesyl phosphate (DFPNa), sodium digeranylgeranyl phosphate (DGGPNa)) and related phospholipids were investigated at the air/water interface. DFPNa and DGGPNa formed an expanded monolayer with a reasonable molecular area, while DGPNa, with shorter chains, could not form a monolayer on the water surface. π- A Isotherms of the mixed monolayers of these lipids and various polyterpenoids ( all-trans-retinol, cholesterol, bacteriohopane aminotriol, α-tocopherol) were measured at various mixing ratios. A condensation effect (the reduction of molecular area by mixing two components) was observed in DMPC/cholesterol, DMPC/bacteriohopane aminotriol, DGGPNa/cholesterol, and DGGPNa/bacteriohopane aminotriol systems. The length of the hydrophobic part (hydrophobic length) of these components was estimated by using Corey-Pauling-Kulton (CPK) models, and the relation between the observed condensation effect and the molecular structures was discussed. It was concluded that polyterpenoids have a condensation effect on the lipid monolayers when their hydrophobic moieties have similar lengths. For example, DGGPNa/bacteriohopane aminotriol and DMPC/cholesterol systems showed larger condensation effects, probably because the lipids and the polyterpenoids in these systems have the same hydrophobic length, hence the cooperative van der Waals force is more intense. It is very interesting to note that in the former case the reinforcing effect of bacteriohopane aminotriol is larger than that of cholesterol. The observed results confirm that DGGPNa has, at 20°C, monolayer properties similar to those of the typical phospholipid, DMPC, and they suggest that, at an early stage of life, membranes of sodium dipolyprenyl phosphates may have been reinforced with polyterpenoids with suitable structures.