Hydrogen bonding of helical vinyl polymers containing alanine moieties: a stabilized interaction of helical conformation sensitive to solvents and pH

Abstract

Three alanine-bearing vinyl terphenyl helical polymers, poly{(+)-2,5-bis[4′-((N-(L)-α-methoxycarbonylethylamino)carbonyl)phenyl]styrene}, (PSt-(Ll)-M), poly{(-)-2,5-bis[4′-((N-(D)-α-methoxycarbonylethylamino)carbonyl)phenyl]styrene} (PSt-(Dd)-M) and poly{(+)-2,5-bis[4′-((N-(L)-α-tert-butoxycarbonylethylamino)carbonyl)phenyl]styrene} (PSt-(Ll)-B), were synthesized. Formation of hydrogen-bonding of those polymers in various solvents as well as the corresponding monomers was testified by using ¹H NMR and FT-IR. The role of hydrogen-bonding between amino acid residues of these polymers in the stability of their helical conformation as well as steric repulsion of bulky side groups were investigated by varying the solvent nature. PSt-(Ll)-M displays a larger optical rotation and stronger Cotton effect in CHCl₃ than in the polar solvent methanol, indicating that hydrogen-bonding stabilize the helical conformation. The helicities of PSt-(Ll)-M change with the solvent polarity and also are reversible during the recycled titration process in the binary solvents methanol/CHCl₃. However, PSt-(Ll)-B has similar optical properties in various solvents, implying that the steric repulsion of bulky side groups other than hydrogen bonding also play an important role in stabilization of the helical conformation. The helicity of poly{(+)-2,5-di[4′-((N-(L)-α-carboxyethylamine)carbonyl)phenyl]styrene} (PSt-(Ll)-A) obtained from PSt-(Ll)-B is also sensitive to pH when KOH solution was added into its methanol solution.