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42802-AC7
Poly(gamma-benzyl-L-glutamate) Ionomers

Masanori Hara, Rutgers, the State University of New Jersey

While PBLG is one of the most widely studied polypeptides, little work has been reported about PBLG ionomers. In the previous reporting period, we prepared PBLG ionomers that had the ion contents of 3.2, 6.0, 13.4, 45.0, and 78.0 mol%. The circular dichroism (CD) spectra of PBLG ionomers in a helicogenic solvent, dimethylformamide (DMF), have shown that PBLG ionomers form α-helix, with two minima at 208 and 222 nm and a crossover at about 200 nm. There is no evidence of β-structure and the amount of the α-helical structure decreased with increasing ionic content.  PBLG in a helicogenic solvent, DMF showed birefringence due to liquid crystalline structure (lyotropic). The birefringence was also observed for PBLG copolymer (acid), but was not observed for PBLG ionomer at similar concentrations. This is because the helical conformation of acid-form PBLG copolymer (neutral polymer) is similar to that of PBLG, but the helical structure of PBLG ionomer is partially disrupted by ionic groups (and ionic interactions), making polymer chains more disordered. In solid, both PBLG and PBLG copolymer (acid) were not completely processed due to lack of melting. In contrast, PBLG ionomers were easier to process due to possibly more flexible structure despite higher viscosity that is expected for ionomers; and, birefringence due to liquid crystallinity was observed for samples with low ionic contents (3.2 mol%). At very high ionic content (e.g., 45 mol%), the polymer was thermally processed but without showing birefringence, since chains became too flexible (more like random coils) to be liquid crystalline. These results suggest that PBLG ionomers having a proper amount of ionic groups can become liquid crystalline upon heating and under flow field (i.e., thermotropic). Our results suggest that the chain conformation of the PBLG ionomer  is more disrupted than PBLG or PBLG copolymer (acid); yet, much of the α-helical structure is retained and some ordered structure can be formed under flow field to be (thermotropic) liquid crystalline for ionomers with low ion contents. This is significant, since most polypeptides, and PBLG in particular, do not melt and thus not thermally processable. After our initial work as reported last year, we have further studied PBLG ionomers. First, light scattering behaviors of  PBLG ionomer solutions were studied. In a polar solvent, DMF, PBLG ionomer shows polyelectrolyte behavior, a big contrast to the behavior of  PBLG in DMF, which shows a typical neutral polymer behavior.  In another solvent, dichloroacetic acid  (DCA), PBLG shows molecular aggregation possibly due to attractions between ion pairs of polymer chains in this solvent. Second, the effect of ion content on the conformations of the PBLG ionomers  in the solid state was studied with CD. With an increase in the ion content from 0, to 6, 13, and 45 mol%, the peak height around 230 nm due to n-π* transition of the α-helix is decreased. This is due to disruption of α-helical structure upon introduction of ionic groups (and ionic interactions), which is consistent with our previous work on solution CD data and confirms our previous speculation about the correlation between the polymer conformations and liquid crystallinity of PBLG ionomer films. Third, thermal stability of the PBLG ionomer was compared with that of PBLG: the ionomer degrades at lower temperature, again due to partial disruption of &alpha-helical structure that provides thermal stability to PBLG.   This work is the beginning of my long-term research plan to combine ionomers (ionic polymers) and polypeptides, and provides evidence to substantiate my approach.  The work also helps PI move into a new area to him, i.e., bio-related polymers. Also, the work has provided co-workers (especially undergraduate students) opportunities for laboratory working on the interdisciplinary research area.

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