We used vibrational sum-frequency generation (VSFG) spectroscopy to elucidate the possible effect of various levels of isotopic substitution (H/D) on the properties of the DPPC monolayer by probing DPPC/D2O interface. We found that deuteration of the choline group has a great impact on monolayer properties, while monolayers with deuterated alkyl chains do not exhibit any differences under our experimental conditions. In addition, deuteration of the choline group strongly affected the hydration of the phosphate group. We showed by probing symmetric stretching vibration of phosphate group that denser packing only slightly reduced the hydration of DPPC-d13 and DPPC-d75 monolayers. Moreover, addition of calcium ions, which generally cause a marked dehydration of the lipid monolayer, had no effect on lipid monolayers with deuterated choline group. We proposed that one way to explain this experimental finding could be deuteration induced changes in the structure of lipid's choline group, resulting in a well-hydrated but Ca2+ ion blocking structure. These results have important implications for various spectroscopic techniques, which commonly use deuteration of phospholipids to circumvent overlapping between vibrational bands.
Keywords: Isotopic labelling; Langmuir; Lipids; Monolayers; Phosphatidylcholine; SFG spectroscopy.
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