Supramolecular receptors from α-amino acid-derived lipids

Abstract

Anionic lipids derived from L-, D-, DL-glutamic acids, L-aspartic acid, L-lysine, L-ornithine, and L-2,4-diaminobutyric acid were prepared and the way in which structurally related, solvatochromic cationic styryl dyes were incorporated into these lipid aggregates were investigated. The L- or D-glutamic acid-derived lipids with glutaric acid headgroups aggregated to form specific hydrophobic cavities which exhibited inclusion ability for the styryl dyes mainly based on the planarity recognition. The formation of such specific hydrophobic cavities can be achieved not only by introducing amide groups capable of complementary hydrogen bondings between neighbouring lipids in the aggregates but also by introducing appropriate spacer methylenes into, for example, glutarate headgroups. Side-chain methylenes of the amino acid residue were also found to play a significant role in the formation of specific hydrophobic cavities as well as the spacer methylenes. Such cavities were also formed for appropriately designed L-lysine- and L-ornithine-derived lipids. These results indicate that the specific incorporation is not peculiar to the glutamic acid-derived lipids but a general phenomenon for the aggregates from appropriately designed L-amino acid-derived lipids. A difference in the manner of assembly of L-glutamic acid residues between L-, D-isomers, and DL-mixtures in the lipid aggregates is suggested not only by the λ_max shift of incorporated styryl dyes but also by aggregate morphologies as evidenced by the TEM observations. These results suggest that the two-dimensional assembly of pure enantiomeric α-amino acid residues in appropriately designed lipids can produce specific hydrophobic cavities unless the DL-mixture phase-separates into two enantiomeric components.