Poly(α-butyl β-L-aspartate): A second alkoxycarbonyl nylon-3 derivative in helical conformation

A structural study was carried out on poly(α‐butyl β‐L‐aspartate), which is a stereoregular nylon‐3 derivative with an alkoxycarbonyl group attached to the β‐carbon of the main‐chain repeating unit. Two crystal forms, namely hexagonal and tetragonal, made of 13/4 and 4/1 helices, respectively, and bearing a structural resemblance to the polypeptide α‐helix, were characterized by X‐ray diffraction. Though these forms are basically identical to those previously reported for poly(α‐isobutyl β‐L‐aspartate), the crystal transition from hexagonal to tetragonal known to take place in this polymer by effect of heating, was not observed in the present case. Modeling calculations revealed that although similar conformations are favoured for the two polymers in both crystal forms, the relative stability of the 13/4 helix to the 4/1 helix was found to be significantly reduced in the case of the butyl derivative. The presence of helical conformation in solution has been evidenced by both 1H NMR and circular dichroism and the helix‐coil transition promoted by acids was investigated by means of these techniques.