On the crystal structure of poly(α-benzyl γ,DL-glutamate) of microbial origin

The conformation and crystal structure of the benzyl ester of microbial poly(gamma,DL-glutamic acid) with a nearly racemic composition was investigated by X-ray diffraction, polarized infrared spectroscopy and molecular modeling simulations. Three different molecular arrangements were characterized. Form I was found to be a structure made of hydrogen-bonded sheets of chains in a nearly extended conformation with features similar to the well-known gamma-form of nylons. Form III consisted of a hexagonal packing of 37/10 helices stabilized by intramolecular hydrogen bonds set between i and i + 3 amide groups. Form II was observed as a poorly oriented phase and identified with the 5/2 helical form previously described for optically pure poly(alpha-benzyl gamma,L-glutamate). Energy calculations indicated that the helical conformations adopted by PAB(DL)G are incompatible with a polymer having a statistical distribution of D and L units and that a polymer made of D- and L-stereoblocks is the most appropriate to describe the crystal structure present in form I.