title: Deposition and aggregation of aspirin molecules on a phospholipid bilayer pattern.
abstract: Aspirin and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) are deposited from their alcoholic mixed solution onto highly oriented pyrolytic graphite (HOPG) by spin coating. The film structure and morphology are characterized by atomic force microscopy (AFM). The barely soluble DMPE forms a highly oriented stripe phase as a result of its one-dimensional epitaxy with the HOPG lattice. The bilayer stripe pattern exposes the cross section of the lipid bilayer lamellae and enables the direct visualization of the molecular interactions of drug or biological molecules with either the hydrophobic or the hydrophilic part of the phospholipid bilayer. The bilayer pattern affects the aspirin molecular deposition and aggregation. AFM shows that the aspirin molecules prefer to deposit and aggregate along the aliphatic interior part of the bilayer pattern, giving rise to parallel dimer rods in registry with the underlying pattern. The nonpolar interactions between aspirin and the phospholipid bilayer are consistent with the lipophilic nature of aspirin. The bilayer pattern not only stabilizes the rodlike aggregate structure of aspirin at low aspirin concentration but also inhibits crystallization of aspirin at high aspirin concentration. Molecular models show that the width of the DMPE aliphatic chain interior can accommodate no more than two aspirin dimers. The bilayer confinement may prevent aspirin from reaching its critical nucleus size. This study illustrates a general method to induce a metastable or amorphous form of an active pharmaceutical ingredient (API) by chemical confinement under high undercooling conditions. Metastable and amorphous solids often display better solubility and bioavailability than the stable crystalline form of the API.
authors.affiliation: Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202, USA. gzmao@eng.wayne.edu
authors.forename: Guangzhao
authors.initials: G
authors.lastname: Mao
authors.forename: Dongzhong
authors.initials: D
authors.lastname: Chen
authors.forename: Hitesh
authors.initials: H
authors.lastname: Handa
authors.forename: Wenfei
authors.initials: W
authors.lastname: Dong
authors.forename: Dirk G
authors.initials: DG
authors.lastname: Kurth
authors.forename: Helmuth
authors.initials: H
authors.lastname: Möhwald
mesh_terms: D001241:Aspirin / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D000432:Methanol / Q000737:chemistry; D018625:Microscopy, Atomic Force; D010714:Phosphatidylethanolamines / Q000737:chemistry; D010743:Phospholipids / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D008051:Lipid Bilayers; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D001241:Aspirin; D000432:Methanol; C018525:1,2-dimyristoylphosphatidylethanolamine