Poly-3-hydroxybutyrate structure

The important bacterial storage material poly-hydroxybutyric acid is related metabolically and structurally to the lipids. This highly reduced polymer is made up of D-(3-hydroxybutyric acid units in ester linkage, about 1500 residues being present per chain. The structure is that of a compact right-handed coil with a twofold screw axis and a pitch of 0.60 nm.a Within bacteria it often occurs in thin lamellae 5.0 nm thick. Since a chain of 1500 residues stretches to 440 nm, there must be 88 folds in a single chain. Present in both cytoplasmic granules and in membranes,b polyhydroxybutyrate can account for as much as 50% of the total carbon of some bacterial In E. coli and many other bacteria polyhydroxybutyrate is present in a lower molecular mass form bound to calcium polyphosphates, proteins, or other macromolecules.d e It has also been extracted from bovine serum albumin and may be ubiquitous in both eukaryotes and prokaryotes.d/e The polymer may function in formation of Ca2+ channels in membranes.b/d... 

Fig. 4. The molecular structures of some aliphatic polyesters a) poly(glycolide), PGA b) poly(lactide), PLA c) poly(lactide-co-glycolide), PLGA d) poly(e-caprolactone), PCL e) poly(hydroxybutyrate), PHB and f) poly(hydroxybutyrate-co-hydroxyvalerate), P(HB-co-HV)...

The important bacterial storage material poly-hydroxybutyric add is related metabolically and structurally to the lipids. This highly reduced polymer is made up of o-P-hydroxybutyric acid units in ester linkage, about 1500 residues being present per chain. The structure is that of a compact right-handed coil with a twofold screw axis and a pitch of... 

Rebrov, A.V Dubinskii, V. A. Nekrasov, Y. P Bonartseva, G. A. Shtamm, M. and Antipov, E. M. [Structure phenomena at elastic deformation of highly oriented poly-hydroxybutyrate. Polym. Scl (Russian). 2002, 44A, 347-351. 

The nomenclature of PHAs and their classification are still in the process of evolving, because new structures are continuing to be discovered. The main biopolymer in the PHA family is a homopolymer polyhydroxybutyrate (PHB). The most conunon copolymer is poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV), whose chemical structure is presented in Figure 9.4. However, there are others, such as poly(hydroxybulyrate-co-hydroxyhexanoate) (PHBHx), poly(hydroxybutyrate-co-hydroxyoctanoate) (PHBO) and poly(ltydroxybulyrate-co-hydroxyoctadecanoate) (PHBOd), for example. 

Luo S, Grubb DT, Netravah AN (2002) The effect of molecular weight on the lamellar structure, thermal and mechanical properties of poly(hydroxybutyrate-co-hydroxyvalerates). Polymer 43 4159 166... 

Abate, R., Ballistreri, A., Giuffrida, M., Impallomeni, G., and Montaudo, G., Separation and Structural Characterization of Cyclic and Open Chain Oligomers Produced in the Partial Pyrolysis of Microbial Poly(hydroxybutyrates), Macrotnolecules, 28, 7911, 1995. 

This paper revises the wide variety of properties, structures and biodegradation behavior of thermoplastic starch in combination with polymers of vinyl alcohol and with poly-s-caprolactone and of some aliphatic polyesters like poly-hydroxybutyrate-valerate, poly-lactic acid, poly-s-caprolactone and polybutylene succinate. 

Poly(hydroxybutyrate-CD-hydroxyvalerate) (PHBV), a random copolymer comprised of 3-hydroxybutyrate (HB) and hydroxyvalerate (HV) units, can be obtained by a fermentation medium [85,90,110] using bacteria such as Alcaligenes eutrophus. According to their feedstock during synthesis, different structures are obtained, many of which are isotactic and others with random stereo sequences [111-114]. 

The mechanical performance required of biocomposites is dependent on specific structural applications. Crude inferences can be made by comparing properties of materials that these biocomposites are intended to substitute. Mechanical data and/or allowable design values of wood and engineered wood products were used to evaluate potential applications of hemp fabric/cellulose acetate composites and hemp fabric/poly (hydroxybutyrate) composites (Table 13.2). From the comparisons, it can be inferred that these biocomposites, despite not passing the design values of wood structural material, can potentially substitute engineered wood products (of the same size) such as plywood and oriented strand boards to partially capture existing markets as crates, pallets, and formwork [38]. 

Furukawa, T, Sato, H., Murakami, R, Zhang, J., Duan, Y. X., Noda, I., Ochiai, S., Ozaki, Y. (2005). Structure, dispersibility, and crystallinity of poly(hydroxybutyrate)/poly(L-lactic acid) blends studied by FT-IR microspectroscopy and differential scanning calorimetry, Macromol., 38,6445-6554. 

Starting liom the respective blends, the PVA/poly(lactic acid) (PLA), PVA/poly (capro lacton) (PCL), and PVA/poly(hydroxybutyrate) (PHB), and applying the standard treatment including extraction of PVA with water, the 3-D structures have been observed (Fig. 6.8). 

F Koosha, RH Muller, SS Davis, MC Davies. The surface chemical structure of poly (/J-hydroxybutyrate) microparticles produced by solvent evaporation process. J Control Rel 9 149, 1989. 

The polyesters synthesized by microorganisms were first investigated by Lemoigne, who isolated the reserve polymers in Bacillus megaterium in 1925 and identified the polymer to be poly(3-hydroxybutyrate), poly(3HB) [1]. The chemical structure of poly(3HB) is shown in Fig. 1. 

First, the short chain length PHAs, poly(HASCL), are composed of monomeric units containing up to 5 carbon atoms. The most well-known representatives are poly(3-hydroxybutyrate) (PHB), and its copolymers with hydroxyvalerate. Of all the PHAs, PHB is by far the most commonly encountered in nature [18]. It is the simplest PHA with respect to chemical structure, having a methylene (-CH3) group as the pendent R-unit in Fig. 1. Owing to its enzymatic synthesis, PHB has an exceptional stereochemical regularity. The chains are linear and the chiral centers all are in the R-stereochemical conformation, which implies that this polymer is completely isotactic. 

The i-poly(3HB) depolymerase of R. rubrum is the only i-poly(3HB) depolymerase that has been purified [174]. The enzyme consists of one polypeptide of 30-32 kDa and has a pH and temperature optimum of pH 9 and 55 °C, respectively. A specific activity of 4 mmol released 3-hydroxybutyrate/min x mg protein was determined (at 45 °C). The purified enzyme was inactive with denatured poly(3HB) and had no lipase-, protease-, or esterase activity with p-nitro-phenyl fatty acid esters (2-8 carbon atoms). Native poly(3HO) granules were not hydrolyzed by i-poly(3HB) depolymerase, indicating a high substrate specificity similar to extracellular poly(3HB) depolymerases. Recently, the DNA sequence of the i-poly(3HB) depolymerase of R. eutropha was published (AB07612). Surprisingly, the DNA-deduced amino acid sequence (47.3 kDa) did not contain a lipase box fingerprint. A more detailed investigation of the structure and function of bacterial i-poly(HA) depolymerases will be necessary in future. 

Poly(3-hydroxybutyrate) is a biopolymer produced by numerous bacteria in nature as an intercellular carbon and energy reserve and belongs to the class of poly (hydroxyalkanoate)s (PHAs). In 1925, the French microbiologist Maurice Lemoigne discovered and characterized PHB extracted from Bacillus megaterium. However, it is produced by a various number of microorganisms such as Cupriavidus necator or Ralstonia eutroph. PHAs are biodegradable polyesters with a structure as shown in Fig. 1. 

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