Poly(hydroxyalkanoates): A Fifth Class of Physiologically Important Organic Biopolymers?

Abstract

Along with polyisoprenoids, polypeptides, polysaccharides, and polynucleotides, Nature contains a further group of biopolymers, the poly(hydroxyalkanoates). The commonest member of this group, poly[(R)‐3‐hydroxybutyrate] P(3‐HB), had been identified by Lemoigne as early as the 1920s, as a storage substance in the microorganism Bacillus megaterium made up of more than 12000 (3‐HB) units. However, the widespread distribution and significance of these biopolymers has only become clear recently. The work of Reusch, in particular, has shown that low molecular weight P(3‐HB) (100–200 3‐HB units) occurs in the cell membranes of prokaryotic and eukaryotic organisms. The function of P(3‐HB) in the latter sources is largely unknown; it has been proposed that a complex of P(3‐HB) and calcium polyphosphate acts as an ion channel through the membrane. Indeed, it has even been speculated that P(3‐HB) plays a role in transport of DNA through the cell wall. In the present article, the following subjects will be discussed: metabolism of P(3‐HB) and analogous polyesters in the synthesis and degradation of storage materials; P(3‐HB) as a starting material for chiral synthetic building blocks; synthesis of cyclic oligomers (oligolides) of up to ten 3‐HB units, and their crystal structure; high molecular weight bio‐copolymers of hydroxybutyrate and hydroxyvalerate (BIOPOL) as biologically degradable plastics; nonbiological production of polyhydroxyalkanoates from 3‐hydroxy carboxylic acids and the corresponding β‐lactones; specific synthesis of linear oligomers with a narrow molecular weight distribution, consisting of about 100 (R)‐3‐hydroxybutyrate units, by using an exponential coupling procedure; structure of the polyesters, and a comparison with other polymers; the experimental results which led to the postulation of a P(3‐HB) ion channel through the cell wall; modeling of P(3‐HB) helices of various diameters, by using the parameters obtained from the crystal structures of oligolides; formation of a crown ester complex and ion transport experiments with the triolide of 3‐HB. The article describes one example of the contributions that synthetic organic chemists can make to important biological problems in an interdisciplinary framework.