Chemistry Reference
In-Depth Information
Unfortunately, despite their high potential for commercial applications,
most of the PHAs produced, especially those belonging to crystalline short
chain length PHAs or medium chain length PHAs with higher monomeric
compositions of 3-hydroxybutyric acid, were said to be highly hydrophobic,
and exhibited brittleness, a low heat distortion temperature and poor gas-
barrier properties. This resulted in slow degradability and resorbability
within the extracellular matrixes as well as limited malleability and ductility
during industrial processing.
10,11
As such, these kinds of neat biopolymers
fail to meet the industrial demands especially in aggressive environments.
Therefore, several approaches were devised to enhance the physico-
chemical properties of the PHA in order to overcome these shortcomings by
modifying the biopolymers through different processes.
9
For example, metabolic engineering and culture condition manipulation
were employed to produce modified PHAs with salient features. PHAs with
methyl side-chains such as PH6N (poly(3-hydroxy-6-methyl-nonanoate))
was obtained from Pseudomonas oleovorans fed with methylated alkanoic
acids or in a mixture with nonanoic acid as a carbon source.
12
PH6N
crystallizes much faster than neat PHN (poly-3-hydroxynonanoate),
13
and the melting temperature was higher (T
m
¼
65 1C) than that of PHN
(T
m
¼
58 1C).
14
Studies have shown that PHAs containing an epoxidized
group were accumulated inside P. oleovorans when fed with octanoate and
10-undecenoic acid
15
and inside P. cichorii YN2 when fed with 1-heptene to
1-dodecene as a sole carbon source.
16
Alternatively, P. oleovoransaccu-
mulated sulfanyl PHA upon feeding with m-(n-thienylsulfanyl) alkanoic
acid.
17
Furthermore, the bacteria was found to accumulate brominated
PHA when fed with a mixture of o-bromoalkanoic acids and nonanoic or
octanoic acid.
18
Changing the carbon source to octane and 1-chlorooctane
or nonanoic acid and fluorinated acid co-substrates resulted in the accu-
mulation of chlorinated or fluorinated PHA in P. oleovorans, respect-
ively.
19,20
Feeding propylthiooctanoic acid (PTO) or propylthiohexanoic
acid to metabolically engineered Ralstonia eutropha led to the production of
thiol functionalized PHA with enhanced chemical properties.
21
On the
other hand, addition of low molecular weight diols such as ethylene glycol
to the fermentation medium resulted in the bacterium producing a bi-
functional telechelic hydroxyl-terminated PHA.
22,23
The presence of these types of functional groups in specialized PHAs
allows for further functional group modification. However, such bio-
synthetic approaches could only produce the specified polymer in minute
quantities and most of the time, the whole-cell metabolic framework of the
fermentation process itself limits the degree of freedom in designing the
PHA with other functional groups of interest. As such, alternative routes for
modification need to be applied. In order to extend their applications in
aggressive environments where the neat polymers have failed, modification
(functionalization) of the polymer via chemical,
24
physical
25
or enzymatic
26
processes have been employed. For example, sugars with pyranose struc-
tures, such as galactose and mannose, were reported to be ligands specific
d
n
2
r
4
n
g
|
1
.
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