Orientation of the GM1 ganglioside in Langmuir–Blodgett monolayers: a PM IRRAS and computational study

The GM1 ganglioside in the monolayer at the air/water interface shows a liquid expanded-liquid condensed phase transition. Due to a combination of the PM IRRAS results with quantum chemical calculations the structure and orientation of the GM1 molecule in the monolayer on the sub-molecular level is provided. The PM IRRAS studies of GM1 monolayers demonstrate that the phase transition is accompanied by a reorientation of sugar residues with simultaneous changes in the network of hydrogen bonds. The calculation of the IR spectrum of the GM1 molecule allowed us to describe individual ν_as(COC) modes in sugar residues of the GM1 pentasaccharide. The visualization of the dipole moment vector of each analyzed ν_as(COC) band allowed us to discuss the orientation of the polar head group of the GM1 in the monolayer. In the liquid expanded state the planes of two sugar residues: β-Gal-(1–3)-βGlc-(1–1)-Cer of the GM1 molecule are tilted by ca. 55° with respect to the surface normal. The plane of the β1,4-GalNAc sugar ring is inclined by 70° towards the gold surface. The phase transition to the liquid condensed state causes simultaneous reduction (by 10°) of the tilt of planes of sugar residues. The plane of the α2,3-Nue5Ac residue (sialic acid) has a nearly perpendicular orientation to the gold surface. Upon the transition to the liquid condensed state the strength of hydrogen bonds formed by the carboxylate group in the α2,3-Nue5Ac residue decreases. In parallel, the strength of hydrogen bonds formed by N atoms of amide groups of the GM1 molecule increases. These events may be explained by loosening and/or breaking of hydrogen bonds between the carboxylate and hydroxyl groups in the α2,3-Nue5Ac residue in a densely packed monolayer, due to steric hindrances. The ceramide group coupling the polar head group with the hydrophobic hydrocarbon chains, forms strong hydrogen bonds. The CO bond of the amide group is almost perpendicular to the gold surface orientation. The hydrocarbon chains of the GM1 exist in a liquid disordered state and their physical state and orientation are not affected by the phase transition.