| Summary: | The aim of this study was to investigate molecular interactions between Quil A and phosphatidylcholine in the solid state using diffuse reflectance
infrared Fourier-transform spectroscopy (DRIFTS). Analysis of the interactions was characterized on the different regions of phosphatidylcholine:
hydrophobic chain, interfacial and headgroup regions. The spectra of the hydrocarbon region of phosphatidylcholine alone compared to that for
the binary mixture of Quil A and phosphatidylcholine were similar. These findings suggest that Quil A did not cause conformational disorder of
the fatty acyl chains of the phospholipid. In contrast, a shift in the wavenumber of the choline group and a broad band in this moiety indicate a
modification of the phospholipid in the headgroup region due to interaction between Quil A and phosphatidylcholine. These results suggest possibly
ionic interactions between the negatively charged glucuronic acid moiety of the Quil A molecule with the positively charged choline group. The
findings could also be the result of conformational changes in the choline group because of the intercalation of sugar moieties in Quil A between
the choline and phosphate groups due to hydrogen bonding. Shift of wavenumbers to lower values on the carbonyl group was observed suggesting
hydrogen bonding between Quil A and phosphatidylcholine. The difference in degrees of wavenumber shift (choline > phosphate > carbonyl group)
and observed broad bands indicated that Quil A preferentially interacted with phosphatidylcholine on the hydrophilic headgroup. Cholesterol
influenced such interactions at relatively high concentration (60%, w/w).
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