Acyl Structure Regulates Galactosylceramide's Interfacial Interactions

Galactosylceramides (GalCer) with homogeneous acyl chains containing zero, one, or two cis double bonds have been synthesized and characterized at an argon-aqueous buffer interface using a Langmuir film balance. Both surface pressure and surface potential were measured as a function of molecular area at 24°C. N-Lignoceroylgalactosylsphingosine (N-24:0-GalSph), N-stearoylgalactosylsphingosine (N-18: 0-GalSph), and N-palmitoylgalactosylsphingosine (N-16:0-GalSph) form condensed films that are similar to that of bovine brain GalCer, which contains long saturated and mono-unsaturated acyl chains, almost half being hydroxylated. In contrast, a bovine brain GalCer subfraction (NFA-GalCer) that is devoid of the hydroxylated acyl chains displays an apparent two-dimensional phase transition near 9.0 mN/m at 54 Ų/molecule. To determine the role of acyl unsaturation in regulating NFA-GalCer's surface behavior, GalCer derivatives containing different mono-unsaturated acyl residues were investigated. N-Nervonoylgalactosylsphingosine (N-24:1^Δ15-GalSph) and N-docosenoylgalactosylsphingosine (N-22:1^Δ13-GalSph) show liquid-expanded to -condensed phase transitions in their force-area isotherms at 10 and 35 mN/m, respectively. Introduction of acyl chains that are short and saturated [e.g., N-decanoylgalactosylsphingosine (N-10:0-GalSph)] or that are long but contain two cis double bonds [e.g., N-linoleoylgalactosylsphingosine (N-18:2^Δ9,12-GalSph)] causes GalCer to display only liquid-expanded behavior at 24°C. The surface potentials [ΔV) of the condensed GalCer derivatives with long saturated acyl residues were quite similar and were over 100 mV higher than that of bovine brain GalCer. In contrast, ΔV values for GalCer derivatives containing mono-unsaturated acyl chains and displaying liquid-expanded behavior were 60 mV lower than that of bovine brain GalCer. Upon onset of the liquid-expanded to -condensed phase transition, the slope of the ΔV−A isotherms increased. Interestingly, in the condensed phase, N-24:1^Δ15-GalSph reached significantly higher levels of ΔV, resulting in a surface dipole moment approximately 4 times higher than that of bovine brain GalCer.