Molecular parameters and conformation of globoside and asialo-G_M1

Comparative studies of the individual properties and intermolecular organization of asialo-G_M1 (Gg₄Cer) and globoside (Gb₄Cer) were made employing lipid monolayers and high-sensitivity differential scanning calorimetry. The surface pressure-area isotherm of Gb₄Cer is more expanded than that of Gg₄Cer. This results in greater molecular areas and compressibilities, and lower intermolecular interaction energies, for Gb₄Cer compared to Gg₄Cer at all surface pressures. This looser intermolecular packing may be responsible for a lower transition temperature (40.5 °C) and enthalpy of transition (ΔH_cal) (2.0 kcal mol^-1) found for the phase transition of Gb₄Cer compared to Gg₄Cer (54.0 °C and 4.2 kcal mol^-1). The surface potential per molecule and resultant molecular dipole moment vector values are greater for Gb₄Cer than for Gg₄Cer at comparable values of surface pressure and molecular area. All these results reflect the existence of a rigid L-shape in the oligosaccharide chain of Gb₄Cer that prevents a close intermolecular packing compared to the straight orientation of the polar head group of Gg₄Cer. Significant movements of the oligosaccharide chain may occur depending on the lateral surface pressure. At low surface pressures the orientation of the oligosaccharide chain of Gg₄Cer may be displaced an angle of up to about 40 ° compared to the perpendicular position adopted at high surface pressures. In agreement with an enhanced liquid character of the interface, the oligosaccharide chain of Gb₄Cer exhibits a greater freedom of movement and the displacement from the position perpendicular to the interface can reach to about 65 °.