Comblike Alkyl Esters of Biosynthetic Poly(γ-glutamic acid). 1. Synthesis and Characterization

Abstract

Two series of comblike poly(α- n -alkyl γ-glutamate)s (PAAG- n, n being the number of carbons contained in the linear alkyl side chain), one with a D:L enantiomeric ratio approximately 9:1 and the other with a nearly racemic composition, were prepared and characterized for even values of n ranging from 12 to 22. Esterification of bacterial poly(γ-glutamic acid) to poly(α-ethyl γ-glutamate) followed by transesterification of the latter with long linear 1-alkanols afforded pure PAAG- n in almost full conversions and high yields. Thermal decomposition temperatures of PAAG- n were above 300 °C, and they increased slightly with the length of the alkyl side group. 1 H NMR studies in chloroform solution revealed the presence of regularly folded conformations that were disrupted upon addition of strong acids. In the solid state, PAAG- n with n ≥ 14 displayed melting of the alkyl side chains at temperatures increasing from 20 up to 80 °C for increasing values of n . Preliminary X-ray diffraction showed that PAAG- n adopt the typical layered structure of comblike polypeptides with alkyl groups crystallized in a paraffinic interlayer phase. The layer periodicity was found to steadily increase from 2.7 to 3.7 nm in agreement with a structure made of polypeptide chains in helical conformation with the alkyl side groups oriented nearly normal to the layers and extensively interdigitized. No significant differences either in structure or in thermal behavior were observed between the two investigated series.