Synthesis of silver and gold nanoparticles–enzyme–polymer conjugate hybrids as dual-activity catalysts for chemoenzymatic cascade reactions

Abstract

Novel hybrids containing silver or gold nanoparticles have been synthesized in aqueous media and at room temperature using enzymes or tailor–made enzyme–polymer conjugates, which directly induced the formation of inorganic silver or gold species. The choice of pH, protein, or bioconjugate strongly affected the final metallic nanoparticles hybrid formation. Using Candida antarctica lipase (CALB) in a solution, nanobiohybrids containing Ag₂O nanoparticles of 9 nm average diameter were obtained. The use of tailor-made bioconjugates, for example, the CALB modified with dextran-aspartic acid polymer (Dext6kDa), resulted in a nanobiohybrid containing smaller Ag(0)/Ag₂O nanoparticles. In the case of nanobiohybrids based on gold, Au(0) species were found in all cases. The Au–CALB hybrid contained spherical nanoparticles with 18 nm average diameter size, with a minor range of larger ones (>100 nm) while the AuNPs–CALB–Dext6kDa hybrid was formed by much smaller nanoparticles (9 nm, minor range of 22 nm), and also nanorods of 20–30/40–50 nm length. Using Thermomyces lanuginosus lipase (TLL), apart from the nanoparticle formation, nanoflowers with a diameter range of 100–200 nm were obtained. All nanobiohybrids maintained (dual) enzymatic and metallic activities. For instance, these nanobiohybrids exhibited exquisite dual-activity for hydrolysis/cycloisomerization cascades starting from allenic acetates. By merging the transition metal reactivity with the inherent lipase catalysis, allenic acetates directly converted to the corresponding O-heterocycles in enantiopure form catalysed by AgNPs–CALB–Dext6kDa, taking advantage of a kinetic resolution/cyclization pathway. These results showed the high applicability of these novel hybrids, offering new opportunities for the design of novel reaction cascades.