Structural Study on Poly(β-l-aspartate)s with Short Alkyl Side Chains:  Helical and Extended Crystal Forms

The conformation of three poly(α-alkyl β-l-aspartate)s, abbreviated PAALA-n, with the alkyl group being methyl, ethyl, and propyl (n = 1, 2, and 3), was examined by X-ray diffraction, polarized infrared spectroscopy, and molecular mechanics calculations. The crystal structure was modeled and refined against diffraction data with the linked-atom least-squares methodology (LALS). Two crystal forms reminiscent of the well-known α-helix and pleated β-sheet structures typical of polypeptides were characterized for the methyl derivative. The helical form is a pseudohexagonal lattice of right-handed 17/4 helices stabilized by intramolecular hydrogen bonds. The β form is made of chains in nearly extended conformation and hydrogen bonded to each other to form sheets which stack together in a metrically orthorhombic crystal lattice. In addition to these two forms, a second folded form made of right-handed 4/1 helices arranged in a tetragonal structure was characterized for the ethyl derivative. Poly(α-propyl β-l-aspartate) was found to display a crystal dimorphism entailing the hexagonal and tetragonal helical forms whereas no signs of extended conformation were detected for this polymer. In this case, the hexagonal form was found to be composed of right-handed 13/4 helices, which is the conformation usually observed in poly(β-l-aspartate)s with linear alkyl side chains of medium and larger sizes. The structural behavior exhibited by the three compounds under study is consistent with the general pattern described for PAALA-n with n ranging from 4 to 22 which have been previously studied by us in full detail.