Polyhydroxybutyrates

After many years of research initiated in the 1970s, ICI introduced polyhydroxybutyrate-valerate copolymers in 1990. Because of their particularly interesting manufaeturing technology, these materials are dealt with in depth separately in the next section. 

Table 31.3 Some selected properties of polyhydroxybutyrate-valerate copolymers (Biopol—Zeneca)...

In Europe, interest has centered particularly on polyhydroxybutyrate, which can be made into films for packaging as well as into molded items. The polymer degrades within 4 weeks in landfills, both by ester hydrolysis and by an ElcB elimination reaction of the oxygen atom p to the carbonyl group. The use of polyhydroxybutyrate is limited at present by its cost—about four times that of polypropylene. 

The lithium cnolate generated by deprotonation of 2-/m-butyl-6-methyl-l,3-dioxan-4-onc, readily available from polyhydroxybutyric acid (PHB), predominantly affords the diastereo-mers 7 when reacted with aldehydes. The diastereomeric ratios of aldol adducts 7/8, produced by reactions with aliphatic aldehydes, range from 87.5 12.5 to >99 1. Pure diastereoiners7are obtained by recrystallization in 25-74% yield116-118. Only marginal diastereoselectivities with respect to the carbinol center are obtained with aromatic aldehydes111-119. Benzoylation of the dioxanones 7, followed by reduction with lithium aluminum hydride, affords enan-tiomerically and diastereomerically pure triols 9 in >85% yield 11. ... 

H.,2010. Kinetic model for polyhydroxybutyrate (PHB) production by Hydrogenophaga pseudoflcwa and verification of growth conditions. Afr. J. Biot. 9, 3151-3157. 

Polylactic acid Polyglycolic acid Polycaprolactone Polyhydroxybutyrate Polyhydroxyvalerate... 

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

If the homopolymer decomposes at the fabrication temperature another approach is to make a copolymer that can be melt processed at a lower temperature. For example, polyhydroxybutyrate decomposes at the processing temperature (190°C), whereas the copolymer with valeric acid can be processed at 160°C without decomposition. These aliphatic polyesters are biodegradable and most importantly, the decomposition products are not toxic, hence their use in medical applications (e.g., sutures). 

Keywords. Polyhydroxyalkanoates, Polyhydroxybutyrate, Polyester, Transgenic plants, Metabolic engineering... 

This review will focus on the synthesis of polyhydroxybutyrate (poly(3HB)) and PHA co-polymers in plants. Most of the knowledge on PHAs, including the... 

Nawrath C, Poirier Y, Somerville CR (1994) Plastid targeting of the enzymes required for the production of polyhydroxybutyrate in higher plants. In Doi Y, Fukuda K (eds) Biodegradable plastics polymers. Elsevier, Amsterdam, p 136... 

The preparation of enantiomerically pure chemicals is also the theme of the next group of four procedures. The biopolymer polyhydroxybutyric acid, which is now produced on an industrial scale, serves as the starting material for the large scale synthesis of (R)-3-HYDROXYBUTANOIC ACID and (R)-METHYL 3-HYDROXYBUTANOATE. Esters of (-)-camphanic acid are useful derivatives for resolving and determining the enantiomeric purity of primary and secondary alcohols. An optimized preparation of (-)-(1S,4R)-CAMPHANOYL CHLORIDE is provided. The preparation of enantiomerically pure a-hydroxyketones from ethyl lactate is illustrated in the synthesis of (3HS)-[(tert)-BUTYL-DIPHENYLSILYL)OXY]-2-BUTANONE. One use of this chiral a-hydroxyketone is provided in the synthesis of (2S,3S)-3-ACETYL-8-... 

M. Khatipov, J. Miyake, M. Miyake, Y. Asada (1998) Polyhydroxybutyrate accumulation and hydrogen evolution by Rhodobacter sphaeroides as a function of nitrogen availability, BioHydrogen, edited by Zaborsky et al. 

The previous example related to manipulation of primary plant metabolism through creating a bypass within the existing system. There are also a range of studies that have looked at the production of novel substances within plants. One particularly relevant example is the production of a polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), in Arabidopsis thaliana. 

In order to produce PHAs in plants it is necessary to introduce the biosynthetic enzymes from bacteria. PHB represents the best characterized and simplest form of PHA, and the synthetic pathway (Figure 4.2) has been extensively studied in Ralstonia eutropha. 30,31 Starting from acetyl-CoA, a P-ketothiolase is required in order to form acetoacetyl-CoA. This is then reduced by a NADPH-dependent acetoacetyl-CoA reductase, which gives rise to 3-hydroxybutyryl-CoA. The latter intermediate is the substrate for the polymerization reaction catalyzed by polyhydroxybutyrate synthase.30 In Ralstonia eutropha, the thiolase, reductase, and synthase genes make up an operon.31... 

The three step pathway of polyhydroxybutyrate synthesis (PHB) as found in Ralstonia eutropha. 

POIRIER, Y., DENNIS, D.E., KLOMPARENS, K., SOMMERVILLE, C., Polyhydroxybutyrate, a biodegradable thermoplastic, produced in transgenic plants, Science, 1992, 256, 520-523. 

NAWRATH, C POIRIER, Y., SOMERVILLE C., Targeting of the polyhydroxybutyrate biosynthetic pathway to the plastids of Arabidopsis thaliana results in high levels of polymer accumulation, Proc. Natl. Acad. Sci. USA, 1994, 91, 12760-12764. 

OH 0, v 0 A r-(3-hydroxybutyryl)-L-homoserine lactone, HBHL or 3-OH, C4-HSL Vibrio harveyi Xenorhabdus nematophilus Bioluminescence, polyhydroxybutyrate metabolism. Virulence, extracellular lipase. 

PE general purpose, ammonia free PE heat resistant, glass fibre reinforced PE impact resistant, cotton filled PE mica filled PerEluoroAlkoxy PolyGlycolic Acid PolyHydroxyAlkanoate PolyHydroxyButyrate... 

Macromolecules may be classified according to different criteria. One criterion is whether the material is natural or synthetic in origin. Cellulose, lignin, starch, silk, wool, chitin, natural rubber, polypeptides (proteins), polyesters (polyhydroxybutyrate), and nucleic acids (DNA, RNA) are examples of naturally occurring polymers while polyethylene, polystyrene, polyurethanes, or polyamides are representatives of their synthetic counterparts. When natural polymers are modified by chemical conversions (cellulose —> cellulose acetate, for example), the products are called modified natural polymers. 

Polyhydroxybutyric acid (PHB) is a bacterial biopolymer which has gained much interest because of its potential use as a biodegradable plastic material. This compound is produced by various terrestrial bacteria and serves as an energy reservoir. PHB is usually highly polymeric (10,000 monomer units) and is stored in the bacteria as an insoluble material in inclusion bodies that are visible with an electron microscope 1111]. Although PHB has been inten-... 

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... 

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