Synthesis, Self-Assembling Properties, and Atom Transfer Radical Polymerization of an Alkylated L-Phenylalanine-Derived Monomeric Organogel from Silica: A New Approach To Prepare Packing Materials for High-Performance Liquid Chromatography

• Published in: Chemistry : a European journal 2008-01, Vol.14 (4), p.1312-1321
• Main Authors: Rahman, M. Mizanur, Czaun, Miklós, Takafuji, Makoto, Ihara, Hirotaka
• Format: Article
• Language: English
• Subjects: Alkylation; Chromatography, High Pressure Liquid - instrumentation; Chromatography, High Pressure Liquid - methods; Gels; liquid chromatography; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Conformation; Phenylalanine - analogs & derivatives; Phenylalanine - chemical synthesis; Phenylalanine - chemistry; polyaromatic hydrocarbons; polymerization; Polymers - chemistry; self-assembly; Silicon Dioxide - chemistry; Spectroscopy, Fourier Transform Infrared; Temperature; Thermogravimetry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200701302

The monomer N′‐octadecyl‐Nα‐(4‐vinyl)‐benzoyl‐L‐phenylalanineamide (4) based on L‐phenylalanine has been simply but effectively synthesized, and its self‐assembling properties have been investigated. FTIR and a variable‐temperature 1H NMR spectroscopic investigation demonstrated that the aggregation of compound 4 in various organic solvents is due to the formation of intermolecular hydrogen bonds among the amide moieties. UV/Vis measurements indicated that the multiple π–π interactions of the phenyl groups also contribute to the self‐assembly. As was observed by 13C cross‐polarization magic‐angle spinning (CP/MAS) NMR and variable‐temperature 1H NMR measurements, the ordered alkyl chains also played an important role in the molecular aggregation by van der Waals interactions. Compound 4 was polymerized by surface‐initiated atom transfer radical polymerization from porous silica gel to prepare a packing material for HPLC. The results of thermogravimetric analysis showed that a relatively large amount of polymer was grafted onto the silica surface. The organic phase on silica was in a noncrystalline solid form in which the long alkyl chain exists in a less‐ordered gauche conformation. Analysis of chromatographic performance for polyaromatic hydrocarbon samples showed higher selectivity than conventional reversed‐phase HPLC packing materials. Pack it in: An L‐phenylalanine‐based vinylic monomer (1) self‐assembles by hydrogen bonding. Its surface‐initiated radical polymerization can be carried out from silica particles to produce a stationary phase (Sil‐poly1) for HPLC applications. The chromatographic performance of Sil‐poly1 for polyaromatic hydrocarbons (PAHs; see figure) shows higher selectivity than conventional reversed‐phase HPLC packing materials.