Important biomolecules (e.g. L-amino acids, D-sugars) are homochiral (have uniform chirality) and this property is essential for predictable self-assembly of chiral supramolecular structures. The importance of chirality to the structure of proteins, cellulose, DNA, and RNA is well-recognized, whereas, the role of the glycerol stereocenter in determining secondary structure has never been fully appreciated. This is ironic because the fundamental repeating unit of both DNA and RNA – the most-widely recognized chiral molecular architectures – can be viewed as phosphate-linked glycerol.
Homochirality is thought to be a key factor in the functionality of membrane lipids – i.e. membrane integrity and interactions with chiral molecules (e.g. proteins, sterols). In polar solvent, many enantiopure amphipathic lipids (e.g. phospholipids, sphingolipids) form visible helical or tubular structures with consistent handedness [Spector, Selinger, Schnur, 2003]. Likewise, the crystallization behavior of other glycerol-derived molecules (e.g. terminal aromatic glycerol ethers) is invariably influenced by their stereochemistry; i.e. “… the secondary hydroxyl… directly bonded to the chiral centre ensures the sensitivity of the crystal packing to chirality effects.”[Bredikhin, Bredikhina, Zakharychev, 2012] Clearly, stereochemistry is an important factor in structure-forming processes of glycerol-containing molecules.
For over 50 years, mainstream research in acylglycerol polymorphism has focused on the subcell (spacing between methylene units), chain-end match, and chain-to-basal plane angle – with little regard for the role the glycerol stereocenter might play. Nevertheless, the role of the glycerol stereocenter in the crystallization behavior of chiral acylglycerols has been investigated for at least one representative of each acylglycerol compound class: 1-monoacyglycerols [Iwahashi, Watanabe, Watanabe, Muramatsu, 1984], 1,2-diacylglycerols [Iwahashi, Ashizawa, Ashizawa, Kaneko, Muramatsu, 1984], 1,3-diacylglycerols [Craven, Lencki, 2011], and triacylglycerols [Schlenk Jr., 1965; Craven, Lencki, 2011]. Results from these enantiomeric phase behavior studies clearly demonstrate that glycerol stereochemistry is a major factor in the polymorphism of acylglycerols. These results are confirmed by comparison with space group assignments derived during crystal structure determination experiments on pure acylglycerols.
Prof. Robert Lencki rlencki@uoguelph.ca
