Polyhydroxyalkanoates

Polyhydroxyalkanoates occur in cell membranes and constitute a minor class of biopolymers whose importance has only been recognised in recent years. The most common and widespread variety appears to be poly(3-hydroxybutyrate) (poly(3-HB)). 

A poly(3-HB)/calcium polyphosphate complex has been isolated from E. coli and other species. This complex, whose structure is thought to consist of two high molecular weight concentric helices, is believed to form ion channels (Chapter 11.1) in natural membranes. 

Poly (3-HB) or other hydroxyalkanoates may be capable of forming other helical cavities with polyphosphate chains. These may have other diameters which might allow the transport of different ions or molecules such as DNA (see Chapter 11.1). 

7 General molecular structure of polyhydroxyalkanoates (PHA). /n = 1,2, 3, yet /77 = 1 is most common, n can range from 1 00 to several thousands. R is variable. When /n = 1,7 = CH3, the monomer structure is 3-hydroxybutyrate, while/77 = 1 and R = C3H7, it is a 3-hydroxyhexanoate monomer. 

Many structure variations of PHA have also been synthesized. Due to the small number of these uneonventional PHA, Uttle physieal eharacterization and application research has been carried out so far. By reviewing the PHA research carried out to date, it is clear that many works have been directed towards the design, biosynthesis, and properties of biodegradable and biocompatible materials, these materials ean be explored for bioengineering new optical and other smart chiral materials. Additionally, the funetional groups of these unconventional PHA provide a lot of opportunities for further chemical modifications. 

2 PHA monomer structures derived from vernolic acid identified in euphorbia oils (on the left) and from ricinolic acid identified in castor oil (on the right). Both were used for growing Pseudomonas aeruginosa 44T1 

4 A monomer structure of PHA synthesized by Pseudomonas stutzeri 1317 grown in soybean oil.  

5 Monomer structures of PHA synthesized by Pseudomonasputida grown on phenoxyalkanoates (a), a mixture of nonanoic acid and 10-undecynoic acid (b) and a mixture of nonanoic acid and fluorinated acid cosubstrates (c), respectively.  

3 Polymers Obtained by Microbial Production 5.5.3.1 Polyhydroxyalkanoates (bacterial polyesters) 

Polyhydroxybutyrate [PHB] is the most common type of polymer that falls in the category of polyhydroxyalkanoates. PHAs are generally classified into short-chain-length PHA (sCL-PHA) and medium-chain-length PHA (mCL-PHA) by the different number of carbons in their repeating units. 

Several types of bacterial polyesters that are produced by biosynthesis are poly-3-hydroxybutyrate, poly-4-hydroxybutyrate, poly-3-hydroxyvalerate, poly-3-hydroxyhexanoate, poly-3-hydroxy-heptanoate, etc., and their respective copolymer combinations. Due to their ability to degrade naturally in variety of environments, they will find a lot of applications in disposal items, short-term packaging, and also considered biocompatible in contact with living tissues and can be used for biomedical applications (e.g., drug encapsulation, tissue engineering) (Chauhan, 2012). 

Another important family of biobased polyesters is the polyhydroxyalkanoates (PHAs) the most common members of this family are polyhydroxybutyrate (PHB) and its copolymer, polyhydroxybutyrate-valerate (PHBV). 

The leading producer of PHAs (mostly PHBV) is Metabolix, which originated from research and development on PHAs at Massachusetts Institute of Technology. Metabolix sells PLA In sheet and Injection molding grades, as well as additives for other plastics, under the Mirel name [6]. 

Depending on the particular structure, PHAs can be nearly amorphous or can be up to about 70% crystalline, and can range from soft and elastic to stiff and brittle. Metabolix PHAs are reported to be better water vapor barriers than most biodegradable plastics. WVTRs for 50 micron PHA film at 23°C and 90% RH range from 20-150 g/m2 d. 

The PHA family generally has better properties than PLA, in terms of both strength and barrier. These polymers biodegrade rapidly In a variety of environments. However, cost and availability remain significant issues. As of the time of this writing, there does not appear to be any significant use of PHAs in packaging, but they are a family of polymers to watch for the future. 

leading to the epoxidized unit, 1-alkene is oxidized at the unsaturated end to form the corresponding 1,2-epoxyalkane 126. The saturated end of 126 can be oxidized, and the resulting co-epoxyalkanoic acid 127 is further converted to the precursor of the epoxidized unit 128 through the p-oxidation pathway. Pathway 

leading to the saturated unit, involves the conversion of the epoxy group of the 1,2-epoxyalkane 126 to the diol, followed by carboxylation and p-oxidation to give 129, the immediate precursor of the saturated unit 130. However, the formation of a certain amount of the saturated unit 130 from acetyl-CoA, which was sequentially removed through the p-oxidation pathway in any pathway mentioned above, cannot be excluded [145]. 

Doi and co-workers [5] exposed PF3BV of different copolymer compositions to seawater (1.5 m depth) at temperatures between 14-25 C (depending on the season). There was no clear detectable influence of the degradation rate on the hydroxyvalerate (HV) content of the copolymer. 

Erosion rates (removal of polymer material from each surface of the film sample) 

PH As are a family of linear polyesters of 3, 4, 5, and 6-hydroxyacids, synthesized by a wide variety of bacteria through the fermentation of sugars, hpids, alkanes, alkenes, and aUsanoic acids. They are recyclable, natural materials, and can be easily degraded to carbon dioxide and water. This makes them as excellent replacements for petroleum-derived plastics in terms of processability, physical characteristics, and biodegradability. In addition, these polymers are biocompatible and hence have several medical applications [238], leading to vast interest in PHAs in bionanocomposites as well. The main polymers studied are poly(3-hydroxybutyrate), PHB and poly(hydroxybutyrate-cohydroxyvalerate). 

PHBV although, as they become more commercially available, PHO and poly(hydroxybutyrate-co-hydroxyhexanoate), PHB-co-PHH, have also been employed in bionanocomposites. 

Zhang et al. [243] prepared bionanocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx), PHB-co-PHH, with OMMTs (day 20A and day 25A), mica, and talc by solution mixing. Wide-angle X-ray scattering results and TEM images confirmed that these two clays were intercalated and finely distributed in the PHB-co-PHH matrix. It was also found that the layered days led to remarkable improvements in mechanical properties even at very low loadings however, in some cases, the layered fillers slightly decreased the thermal stability of the bionanocomposites. 

The isothermal degradation and kinetics of PHB-based bionanocomposites were studied by Erceg et al. [245]. Their bionanocomposites consisted of OMMT Cloisite 30B (30B) and PHB prepared by solution casting and isothermally degraded at 230, 235, 240, and 245 °C. They found that the addition of 30B increased the thermal stability of PHB with the most pronounced effect being the addition of 1 wt% 30B. 

Carbon nanotubes have also been used to prepare bionanocomposites with PHBV. The addition of carbon nanotubes may improve the thermal stability and act as nucleating agents to PHBV [252, 253]. 

PHAs can be nsed for the manufacture of films, coated paper, compost bags and can also be molded into bottles and razors (Mooney, 2009). Additionally, since they are biocompatible, they can also be nsed as implants without causing inflammation (Ramakrishna et al., 2001). 

PHAs require expensive bacterial fermentation and isolation processes resulting in more expensive production costs if compared to other petroleum-derived polymers. Thus, in the beginning of the 2000s, an alternative strategy for lowering the production costs proposed to 

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Other blends such as polyhydroxyalkanoates (PHA) with cellulose acetate (208), PHA with polycaprolactone (209), poly(lactic acid) with poly(ethylene glycol) (210), chitosan and cellulose (211), poly(lactic acid) with inorganic fillers (212), and PHA and aUphatic polyesters with inorganics (213) are receiving attention. The different blending compositions seem to be limited only by the number of polymers available and the compatibiUty of the components. The latter blends, with all natural or biodegradable components, appear to afford the best approach for future research as property balance and biodegradabihty is attempted. Starch and additives have been evaluated ia detail from the perspective of stmcture and compatibiUty with starch (214). 

Steinbuchel, A. and Lutke-Eversloh, T. 2003. Metabolic engineering and pathway construction for biotechnological production of relevant polyhydroxyalkanoates in microorganisms. Biochemical Engineering Journal 16 81-96. 

Fermentation of Sweet Sorghum into Added Value Biopolymer of Polyhydroxyalkanoates (PHAs)... 

Keshavarz, T., Roy, L, 2010. Polyhydroxyalkanoates bioplastics with a green agenda. 

Braunegg, G., 2008. Polyhydroxyalkanoate production from whey by Pseudomonas hydrogenovora. Bioresour. Technol. 99, 4854-4863. 

Steinbuchel, A., 1991.Polyhydroxyalkanoic acids. In Biomaterials. Edited by D. Byrom. 

Yoon, S. C., Choi, M. H., 1999. Local sequence dependence of polyhydroxyalkanoic acid... 

SCHEME 8.11 Ring-opening polymerization of (J-lactones to produce polyhydroxyalkanoates (PHAs). 

Steinbuchel A and Valentin HE., Diversity of bacterial polyhydroxyalkanoic acids. Ferns Microbiol lett, 1995, 128(3), 219-228. 

Willibald B, Holler E (1996) Paper 3/01. In Witholt B (Ed) Preprints Internationd Symposium of Bacterial Polyhydroxyalkanoate 96, Davos, Switzerland... 

Biochemical and Molecular Basis of Microbial Synthesis of Polyhydroxyalkanoates in Microorganisms... 

Keywords. Polyhydroxyalkanoic acids, Microbial polyesters, PHA, PHA synthase, Metabolic engineering, PHA granules, Ralstonia eutropha, Pseudomonas aeruginosa... 

Many bacteria are able to synthesize polyesters of hydroxyalkanoic acids and to accumulate these water insoluble polyhydroxyalkanoic acids (PHA) in the cytoplasm as inclusions and as storage compounds for energy and carbon these in-... 

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