3- Hydroxybutyrate common short-chain-length

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. 

PHB (or poly-3-hydroxybutyrate (P(3HB))) is the most common type of PHA produced and is an example of a short chain length homopolymer produced by A. eutropbus. PHB has poor physical properties for commercial use, as it is stiff, brittle and hard to process. This has led to an increased interest to produce heteropolymers with improved qualities. 

Fig. 2 Common PHA monomer structures. Short-chain-length monomers 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV). Medium-chain-length monomers 3-hydroxyhexanoate (3HHx), 3-hydroxyoctanoate (3HO), 3-hydroxydecanoate (3HD), 3-hydroxydodecanoate (3HDD)...

Figure 1. General molecular structure of polyhydroxyalkanoates. m = 1, 2, 3, yet m = 1 is most common, n can range from 100 to several thousands. R is variable. When m=, R-CH3, the monomer structure is 3-hydroxybutyrate, while m = 1 and R = C3H7, it is a 3-hydroxyhexanoate monomer. If R > C3H7, the PHA are called medium-chain-length PHA If R < C3H7, the PHA are referred to as short-chain-length PHA.

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