Photoresponsive polymers: Azobenzene‐containing poly(L‐lysine)

Abstract

Poly(L‐lysine) was reacted with various azo‐reagents, including p‐phenylazobenzoic acid, p‐phenylazobenzoyl chloride, and p‐phenylazobenzoic N‐hydroxy‐succinimide ester, to give polypeptides containing 5–44 mol % azobenzene units in the side chains. The conformation of the azo‐modified polypeptides was investigated in connection with their photochromic behavior caused by the trans ⇌ cis photoisomerization of the azo groups present in the side chains. In methanol/water solvent mixture, the 20% azo‐poly(L‐lysine) adopts the α‐helix conformation. The helix stability was found to be higher when the azo side chains are in cis than when they are in trans configuration. So irradiation at 340 nm (trans‐to‐cis isomerization), and alternately at 450 nm (cis‐to‐trans isomerization), produced reversible variations of the α‐helix content. In hexafluoro‐2‐propanol/water/sodium dodecyl sulfate mixture, the 43% azo‐poly(L‐lysine) adopts a β‐structure, as indicated by CD spectra. Irradiation at 340 nm caused the disruption of the β‐structure and promoted the α‐helix conformation. The effect was reversed upon irradiation at 450 nm. The photoinduced β ⇌ helix change was explained on the basis of the different geometry and hydrophobic character of the trans and the cis azobenzene units.