Poly(phenylacetylene)s bearing a peptide pendant: Helical conformational changes of the polymer backbone stimulated by the pendant conformational change

Optically active, cis-transoid poly(phenylacetylene) derivatives bearing a poly(gamma-benzyl-L-glutamate) [poly(PBGA(m))] or poly(L-glutamic acid) [poly(PGA(m))] chain as the pendant were prepared by polymerisation of the corresponding macromonomer with a rhodium catalyst followed by hydrolysis of the pendant ester groups. Their conformational changes in solution, induced by a helix-coil transition of the pendant polypeptides, were investigated using circular dichroism (CD) and absorption spectroscopies. A series of macromonomers with a different peptide chain lengths was synthesised by the polymerisation of the N-carboxyanhydride of gamma-benzyl-L-glutamate with a phenylacetylene bearing an alanine residue as the initiator. The obtained macromonomers (PBGA(m)) were further polymerised with a rhodium catalyst in N,N-dimethylformamide (DMF) to yield novel poly (phenylacetylene)s [poly(PBGA(m))] with a poly(gamma-benzyl-L-glutamate) pendant. The poly(PBGAm) exhibited an induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone in dimethyl sulfoxide (DMSO), probably due to the prevailing one-handed helix formation. The Cotton effect signs of a DMSO solution of the poly(PBGA(m)) were inverted and accompanied by a visible colour change in the presence of an increasing amount of chloroform or DMF containing lithium chloride. The results suggest that poly(PBGA(m)) may undergo a conformational change such as a helix-helix transition with a different helical pitch responding to a change in the alpha-helix content of the poly(gamma-benzyl-L-glutamate) pendant. Moreover, a water-soluble poly(PGA(m)) also showed a similar, but dramatic change in its helical conformation with a visible colour change stimulated by a helix-coil transition of the pendant poly(L-glutamic acid) chains by changing the pH in water.