Comblike Ionic Complexes of Poly(γ-glutamic acid) and Alkanoylcholines Derived from Fatty Acids

A series of ionic complexes with almost stoichiometric composition have been “synthesized” from bacterial poly­(γ-glutamic acid) and alkanoylcholines derived from fatty acids with 12, 14, 16, and 18 carbon atoms. The complexes were stable up to temperatures close to 200 °C and were non-water-soluble but readily soluble in organic solvents. In the solid state they adopted the biphasic structure typical of comblike systems with the polypeptide chains arranged in sheets and periodically separated by a paraffinic phase which was partially crystallized for C14, C16, and C18. The window width of the layered structure was estimated by SAXS to be within the 3.6–4.5 nm range, and WAXS showed that the alkyl tails were crystallized in a hexagonally packed lattice with a d₁₀₀ spacing of 0.41 nm. These complexes displayed reversible melting of the paraffinic phase in the 40–65 °C range at temperatures with values increasing with the length of the alkanoyl group. A slight expansion of the intersheet distance occurred at melting followed by contraction upon heating at higher temperatures but without significant alteration of the layered structure. The ¹³C CP-MAS NMR analysis revealed the underlying polymethylene anti-to-gauche conformational conversion that takes place at the thermal transition. An energy-based simulation study of the crystallization process afforded a molecular picture of the complex and evidenced the favoring effect of the choline structure on the packing of the alkyl side chains.