Figure 5.

Predominant molecular states of aggregated sphingosine at different pH values suggested by the data shown in this study. (A) At pH below pK_a (6.6), NH₂ moiety of sphingosine is protonated. At pH slightly above pK_a, sphingosine exists as an electrically neutral molecule with a deprotonated amine group. (B) As reported by Merrill et al. (31), intramolecular hydrogen bonds are formed between OH and NH₂ groups of sphingosine (broken lines). (C) Between pH 6.7 and 9.9, intermolecular hydrogen bonding becomes predominant (broken lines); such bonds are efficiently formed only when sphingosine molecules exist in close proximity, which increases the mean aggregate radius.