Effects of non-ionic surfactants N-alkyl- N, N-dimethylamine- N-oxides on the structure of a phospholipid bilayer: small-angle X-ray diffraction study

Abstract

Effects of non-ionic surfactants N-alkyl- N, N-dimethylamine- N-oxides (C n NO, 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 H 2O:EYPC and C n NO:EYPC molar ratios. The bilayer thickness d L and the lipid surface area at the bilayer–aqueous interface S L were calculated from the repeat period, d of the lamellar phase, based on the model that water and EYPC+C n NO molecules form separated layers and that their molecular volumes are additive. In the studied range of m=C n NO:EYPC molar ratios up to 1:1, d L and S L change linearly. The slopes Δ L= ∂d L/ ∂m and Δ S = ∂S L/ ∂m are equal to −0.876±0.027 nm and 0.347±0.006 nm 2 for C 6NO, −1.025±0.060 nm and 0.433±0.025 nm 2 for C 8NO, −0.836±0.046 nm and 0.405±0.018 nm 2 for C 10NO, −0.604±0.015 nm and 0.375±0.007 nm 2 for C 12NO, −0.279±0.031 nm and 0.318±0.005 nm 2 for C 14NO, −0.0865±0.070 nm and 0.2963±0.014 nm 2 for C 16NO, and −0.040±0.022 nm and 0.297±0.002 nm 2 for C 18NO, respectively, at full bilayer hydration. The peak–peak distance in the bilayer electron density profile, which relates to the P–P distance d PP, obtained from the first four diffraction peaks by the Fourier transform also depends linearly on m, and the slope Δ PP= ∂d PP/ ∂m is −0.528±0.065 nm for C 6NO, −0.680±0.018 nm for C 8NO, −0.573±0.021 nm for C 10NO, −0.369±0.075 nm for C 12NO, −0.190±0.015 for C 14NO, −0.088±0.016 nm for C 16NO and −0.094±0.016 nm for C 18NO. The effects of C n NO on Δ L, Δ S and Δ PP are the results of C n NO insertion into EYPC bilayers and depend on the hydrophobic mismatch between C n NO and EYPC hydrocarbon chains and on the lateral interactions of C n NO and EYPC in the bilayer.