Galactosylceramides with homogeneous acyl chains: the effect of acyl structure on intermolecular interactions occuring at the argon/buffered saline interface

Galactosylceramides (GalCers) containing homogeneous acyl chains with no, one or two double bonds were synthesized and characterized at the argon-buffered saline interface using a Langmuir film balance. Surface pressure was monitored as a function of molecular area at various fixed temperatures between 10 and 30†C. In this temperature range, isotherms of GalCer containing palmitoyl acyl chains show condensed behavior. Calculated compressibility values verify the liquid-condensed nature of the films. Replacement of the long saturated acyl chains with long monounsaturated residues dramatically modifies surface behavior. N-nervonoyl galactosylsphingosine (N-24:1^Δ15 GalSph) and N-docosenoyl galactosylsphingosine (N-22:1^Δ13 GalSph) show discontinuities in thier force-area isotherms (24°C) at 10 and 35 mN m^-1 respectively. Compressibility data are consistent with the discontinuities being transitions from liquid-expanded to condensed monolayer states. As the overall chain length is decreased while keeping the cis double bond nine carbons from the terminal methyl group, progressively lower temperatures are needed to induced the surface phase transition. GalCer species with eicosenoyl and oleoyl acyl chains (e.g. N-20:1^Δ11 GalSph, N-18:1^Δ9 GalSph) show the tow-dimensional phase transition only if the temperature is lowered to the 10-15°C range. Changing the stereochemical configuration of the double bond without changing its position (e.g. N-18:1^Δ9(t) GalSph) rigidifies the film and is energetically equivalent to lowering temperature by about 30°C. Introducing acyl chains that are short and saturated (e.g. N-10:0 GalSph) or that are long but contain two cis double bonds (e.g. N-18:2^Δ9,12 GalSph) causes GalCer to display only liquid-expanded behavior over the entire temperature range (10-30°C) along with accompanying increases in compressibility, "lift-off" area and apparent collapse area. The results help to explain why changes in the acyl composition of membrane sphingolipids can be so disruptive in demyelinating diseases such as adrenoleukodystrophy.