Polymerization of hyperbranched polysaccharides by combined biocatalysis

PhD ceremony: Mr. J. van der Vlist, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen

Title: Polymerization of hyperbranched polysaccharides by combined biocatalysis

Promotor(s): prof. K. Loos

Faculty: Mathematics and Natural Sciences

In this thesis it is shown that enzymes are essential laboratory tools for the in vitro synthesis of polysaccharides with control over macromolecular properties. Here we present a method to enzymatically polymerize hyperbranched polysaccharides with control over stereoregularity, degree of branching and molecular weight. Moreover, the possibility to construct hybrid materials consisting of a hyperbranched polyglucan part connected to a synthetic substrate (e.g. polymer, surface, etc) is shown.

Using an enzymatic catalyzed tandem polymerization in which the unique properties of the enzymes potato phosphorylase and glycogen branching enzyme (GBE_DG; from Deinococcus geothermalis) are combined, a hyperbranched polyglucan was polymerized consisting of (1→4) linked alfa-D-glucose residues with branches at the glucose C6 hydroxy group. In this tandem polymerization, phosphorylase catalyzes the addition of (1→4) linked alfa-D-glucose residues from a short oligosaccharide, using glucose-1-phosphate (G-1-P) as donor substrate (monomer). GBEDG introduces in situ branch points at the growing polymer chain. More specifically, GBEDG catalyzes the formation of alfa(1→6) branch points by the hydrolysis of an alfa(1→4) linked glycosidic linkage and subsequent inter- or intra-chain transfer of the non reducing terminal fragment to the C6 hydroxyl position of an alfa-glucan.

A property of phosphorylase, essential for the research as outlined in this thesis, is the donor substrate (primer) dependency. Polymerization is impossible without an oligomeric alfa(1→4) linked D-glucose primer of at least 3 glucose residues. By taking advantage of this property hybrid materials were constructed by anchoring the oligomeric primer to a substrate prior to the enzymatic tandem polymerization with. This resulted in hyperbranched multi arm structures, diblock copolymers consisting of a hyperbranched polyglucan part and hyperbranched brush polymers anchored to Si wafers.