Conformation of polyamino acids containing fluorine

Abstract

The conformation of poly(γ-4-fluoro-benzyl- l-glutamate) (F-PBLG) and poly(γ-4-trifluoromethyl-benzyl- l-glutamate) (CF 3-PBLG) was investigated. Circular dichroism (c.d.), infra-red (i.r.) spectra and X-ray analysis indicated that the secondary structure of F-PBLG and CF 3-PBLG was α-helical both in the solid state and in chloroform solution, similar to PBLG, while it was random coil in trifluoroacetic acid (TFA) and dichloroacetic acid (DCA). In the chloroform-TFA solvent system, these polypeptides changed their conformation from helix to coil; the minimum TFA concentrations required for helix-coil transition were 10–12% in CF 3-PBLG, 14% in F-PBLG and 12–13% in PBLG. The results suggest that CF 3 groups have a pronounced effect on the stability of α-helical structure of these polypeptides. The helix-coil transition of the polypeptides was accompanied by a perturbation of 19F nuclear magnetic resonance chemical shift.