Quantum chemical study of carbohydrate-phospholipid interactions

Carbohydrates on host membranes are fundamental to many important biological processes. Here, we seek a basic understanding of the nature of the interactions between carbohydrates and phospholipids to dissect their roles in molecular recognition. A hybrid quantum mechanics/quantum mechanics (QM/QM) scheme with two different levels of treatment was used to explore the conformations and energetics of carbohydrate-phospholipid complexes. We investigate the interactions of two phospholipids (POPC and DOPC) with mannose using density functional theory. Carbohydrate-phospholipid interactions are probed with respect to competing interactions with water. Our hybrid QM/QM approach demonstrates that mannose interactions with phospholipids can result in alterations in charge distributions and conformations of phospholipids. The results clearly reveal the interplay between conventional and nonconventional hydrogen bonding; moreover, nonpolar interactions are shown to be crucial in the recognition and further stabilization of carbohydrate-phospholipid complexes. The influence of the acyl chain on phospholipid headgroup orientation is clearly evident in our investigation. The significance of the conventional OH···O and nonconventional CH···O and CH···C interactions in the stabilization of the intermolecular complexes is deduced from the molecular electron density topology using Bader's atoms-in-molecules theory. Finally, we have compared the QM energies with molecular mechanics energies for the same interactions to aid in the refinement of the all-atom lipid-carbohydrate force fields.