Surface grafting of poly(L-glutamates). 2. Helix orientation

In this paper the average helix orientation of surface-grafted poly(gamma - benzyl L-glutamate) (PBLG), poly(gamma -methyl L-glutamate) (PMLG), and pol y(gamma -methyl L-glutamate)-co-(gamma -n-stearyl L-glutamate) (PMLGSLG 70/ 30) was investigated by means of FT-IR transmission spectroscopy. The theor etical relation between the average tilt angle (theta) and the absorption p eak areas of three different backbone amide bands could be calculated becau se their transition dipole moment directions with respect to the helix axis were known. From the normalized absorptions, the average tilt angles of gr afted helices of PBLG, PMLG, and PMLGSLG 70/30 were determined. The somewha t larger average angle of PMLG helices of 35 +/- 5 degrees with respect to the substrate compared to the value of 32 +/- 5 degrees of PBLG was due to the higher grafting density of PMLG. Because of the smaller helix diameter as a result of the smaller size of the methyl side group, more PMLG helices grew on the same surface area. Sterical hindrance and unfavorable polar in teractions between unidirectional aligned helices forced the PMLG helices i n a more upright arrangement. The even more perpendicular orientation of PM LGSLG 70/30 (48 +/- 6 degrees) could be the result of incorporation of main ly gamma -methyl L-glutamate N-carboxyanhydride (MLG-NCA) monomers during t he initiation step. Incorporation of the much larger gamma -n-stearyl L-glu tamate N-carboxyanhydride (SLG-NCA) monomers afterward lead to enlarged ang les with respect to the substrate. Due to swelling, a pronounced change in helix orientation of grafted PMLGSLG 70/30 in n-hexadecane was observed, re sulting in an almost perpendicular helix orientation.