Effects of Nonionic Surfactants -Alkyl--Dimethylamine--Oxides on the Structure of a Phospholipid Bilayer. Small-Angle X-Ray Diffraction Study

Abstract

Effects of nonionic surfactants -alkyl--dimethylamine--oxides (CnNO, n is the number of alkyl carbons) on the structure of egg yolk phosphatidylcholine (EYPC) bilayers in the lamellar fluid phase was studied by small-angle X-ray diffraction as a function of HO:EYPC and CnNO:EYPC molar ratios. The bilayer thickness and the lipid surface area at the bilayer - aqueous interface were calculated from the repeat period of the lamellar phase, based on the model that water and EYPC+CnNO molecules form separated layers and that their molecular volumes are additive. In the studied range of =CnNO:EYPC molar ratios up to 1:1, and change linearly. The slopes and are equal to -0.876±0.027 nm and 0.347±0.006 nm for C6NO, -1.025±0.060 nm and 0.433±0.025 nm for C8NO, -0.836±0.046 nm and 0.405±0.018 nm for C10NO, -0.604±0.015 nm and 0.375±0.007 nm for C12NO, -0.279±0.031 nm and 0.318±0.005 nm for C14NO, -0.0865±0.070 nm and 0.2963±0.014 nm for C16NO, and -0.040±0.022 nm and 0.297±0.002 nm for C18NO, respectively, at full bilayer hydration. The peak-peak distance in the bilayer electron density profile, which relates to the P-P distance obtained from the first 4 diffraction peaks by the Fourier transform also depends linearly on , and the slope is -0.528±0.065 nm for C6NO, -0.680±0.018 nm for C8NO, -0.573±0.021 nm for C10NO, -0.369±0.075 nm for C12NO, -0.190±0.015 for C14NO, -0.088±0.016 nm for C16NO and -0.094±0.016 nm for C18NO. The effects of CnNO on and are the results of CnNO insertion into EYPC bilayers and depend on the hydrophobic mismatch between CnNO and EYPC hydrocarbon chains and on the lateral interactions of CnNO and EYPC in the bilayer.

Keywords: nonionic surfactants, -alkyl--dimethylamine--oxides, egg yolk phosphatidylcholine, bilayer, X-ray diffraction