Chain length dependence of the helix orientation in Langmuir-Blodgett monolayers of alpha-helical diblock copolypeptides

The effect of chain length on the helix orientation of alpha-helical diblock copolypeptides in Langmuir and Langmuir-Blodgett monolayers is reported for the first time. Amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol% of stearyl substituents (PMLGSLG) of various block lengths were studied. The tilt angle between the helices and the substrate-normal decreases upon increasing the transfer pressure coincident with "double brush" formation. The hydrophobic block length strongly affects the maximum surface chain density and thereby the helix orientation of the diblock copolypeptides. Increasing the degree of polymerization of the hydrophobic block (DP(PMLGSLG)) results in an increase in the helix tilt angle tentatively attributed to the off-axis interactions of the unscreened peptide dipoles between the parallel aligned alpha-helices. In those cases where the alkyl side chains surround the PMLGSLG helices, the smallest helix tilt angle of 29 degrees for the PMLGSLG block and 67 degrees for the PLGA block were obtained for the diblock copolymer of DP(PLGA) 37 and DP(PMLGSLG) 24. For smaller DP values of the hydrophobic block, in particular the diblock copolymer having DP(PMLGSLG) 11, the long alkyl side chains are partially expelled from the brush layer and the alpha-helices of the PMLGSLG block are oriented nearly perpendicular to the interface.