"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 linear-dendritic laccase complexes as initiators and water as solvent. The influence of the dendron generation in the linear-dendritic 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 linear-dendritic 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 C-C and C-O 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 = 2.3-2.6, where D_h of the polymer is at the minimum (2.4 nm).