"Green" Synthesis of Unnatural Poly(Amino Acid)s with Zwitterionic Character and pH-Responsive Solution Behavior, Mediated by Linear Dendritic Laccase Complexes

This article describes the enzyme-catalyzed green synthesis of an unnatural poly(amino acid). dl-Tyrosine was polymerized under environmentally friendly conditions using lineardendritic laccase complexes as initiators and water as solvent. The influence of the dendron generation in the lineardendritic copolymers, the monomer concentration, and time and temperature on the polymer yields and molecular masses was investigated. Depending on the reaction conditions poly(tyrosine) with molecular mass (M-w) up to 82 kDa could be obtained in yields ranging between 45 and 69%. It was found that the lineardendritic laccase complexes can induce further chain growth upon addition of fresh monomer to the preformed poly(tyrosine) in a fashion resembling the classic living polymerization. The structure of the poly(tyrosine) was investigated by NMR, FT-IR, and MALDI-TOF and it was discovered that the polymer chains consist of phenol repeating units linked together by CC and CO bonds randomly distributed along the backbone of the polymers. The materials formed are completely water-soluble and behave as typical poly(zwitterions) changing charge and size with the medium pH. DLS measurements reveal that the zeta potential of the polymers can vary between +15 mV at pH 1.2 with hydrodynamic diameter (D-h) = 6.7 nm to -35 mV at pH 11.8 and D-h = 10 nm. The isoelectric point was found at pH = 2.32.6, where D-h of the polymer is at the minimum (2.4 nm).