Polyhydroxyalkanoate polymer accumulation

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Polyhydroxyalkanoates (PHAs) are polyesters synthesized by many bacteria. These polymers are accumulated intracellularly imder nutrient stress and act as a carbon and energy reserve. PHAs are non-toxic, biodegratkble and biocompatible. They are produced from renewable sources. They have high degree of polymerization, are highly crystalline, isotactic and insoluble in water. Because of these properties, PHAs have a real potential in medical and pharmaceutical applications, such as drug delivery systems and tissue engineering. 

Another, more promising, method of reducing the accumulation of polymers in the environment is to develop polymers that are biod adable and can be recycled by natural processes. Biodegradable polymers are polymers that can be broken down by enemies produced by soil microorganisms. Much research is directed at the development of such polymers, and many have aheady been developed. Most biod adable polymers exhibit ester or amide moieties, which can be hydrolyzed by enzymes. Examples include a class of compounds called polyhydroxyalkano-ates (PHAs), which are polymers of 3-hydroxy carboxylic acids. 

The PHAs, PHB is the most extensively characterized polymer, mainly because it was the first to be discovered. The diversity of bacterial PHAs has changed dramatically. Until 1970 s, 3HB was considered as the only constituent of PHAs. hi 1980 s, PHAs having other monomers besides 3HB were shown to be accumulated by many bacteria with addition of certain precursors in the production medium. Figure 2 shows widely studied polyhydroxyalkanoates, which are PHB, PHV, and copolymer of PHB-co-PHV Today more than 150 different monomers of PHAs are synthesized by different microorganisms which include the following ... 

A homopolymer of (R)-3-hydroxybutyrate (PHB) is the most common type of PHA that bacteria accumulate in nature and has been studied and characterized extensively by many researchers. Poly (P-hydroxybutyrate) (PHB) has a number of interesting characteristics and can be used in various ways similar to maity conventional synthetic plastics now in use. The properties of polyhydroxyalkanoates viz. PHB, PHV and comparison with synthetic polymer polypropylene (PP) have been described by Sasikala and Ramana (1996). 

A wide range of microorganisms have been shown to be able to accumulate polyhydroxyalkanoates. The structure of the sidechain can be modified by careful choice of the microorganism and carbon substrate. Many excellent reviews have already been published on the biosynthesis of these materials [6, 7,8] and Table 5.1 represents a summary of the range of copolymers that have been reported. It is therefore the intention of the current authors not to concentrate on the biochemical aspects of these polymers but instead to deal with the physical and material properties. 

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