Fig. 1.

A: Cartoon representation of the human epidermis, B) chemical structures of the investigated lipid species with the deuterium-labeled segments in blue, lignoceric acid-d₄₇ (LA-d₄₇), cholesterol-d₆ (Chol-d₆), N-lignoceroyl-D-erythro-sphingosine-d₄₇ (Cer[NS]-d₄₇) and N-(32-linoleoyloxy)dotriacontanoyl-D-erythro-sphingosine (Cer[EOS]) labeled either at the sphingosine-d₉ moiety (-sphing-d₉), the upper acyl chain close to the headgroup (-upper-d₃₁) or the middle part (-middle-d₃₂) of the ultralong chain (black line indicating the border between upper and middle part) or at the linoleoyl-d₃₁ moiety(-lin-d₃₁). Cer[EOS] with ¹³C-labeled linoleate was also studied. Panel C) shows our synthesis strategy exemplified for deuterated Cer[EOS]-middle-d₃₂.