Cross-Linking-Induced Permanently Perpendicular Helix Orientation in Surface-Grafted Polyglutamate Films

Using a chemical cross-linking procedure, surface-grafted polyglutamate films with a permanently perpendicular helix orientation were prepared. A surface-grafted α-helical polyglutamate film containing polymerizable side groups was synthesized by ring-opening terpolymerization of 50 molar% γ-methyl-l-glutamate N-carboxyanhydride (NCA), 30% γ-stearyl-l-glutamate NCA and 20% γ-4-vinylbenzyl-l-glutamate NCA initiated from a silicon substrate functionalized with primary amino groups. The average tilt angle of the end-grafted helices in this film is approximately 66°, indicating a nearly parallel helix orientation with respect to the substrate surface. After swelling of the grafted terpolyglutamate film in stearyl methacrylate and subsequent radical cross-linking, the average helix tilt angle decreases to about 11°, indicating an almost perpendicular helix orientation. The film thickness increases accordingly from 151 Å before to approximately 390 Å after cross-linking. Extensive solvent treatment of the cross-linked film shows that the perpendicular helix orientation is permanent.