Biopolymers are polymers produced by living
organisms; in other words, they are polymeric biomolecules. Since they are polymers, biopolymers contain
monomeric units that are
covalently bonded to form larger structures. There are three main classes of
biopolymers, classified according to the monomeric units used and the structure
of the biopolymer formed: polynucleotides (RNA and DNA), which are long
polymers composed of 13 or more nucleotide monomers; polypeptides, which are short
polymers of amino acids; and polysaccharides, which are often
linear bonded polymeric carbohydrate structures.
Biopolymers vs Synthetic polymers
A major defining difference between biopolymers
and other polymers can be found in their structures. All polymers are made of
repetitive units called monomers. Biopolymers often have a well-defined structure, though
this is not a defining characteristic (example:ligno-cellulose): The exact
chemical composition and the sequence in which these units are arranged is
called the primary structure, in the case of proteins. Many biopolymers spontaneously
fold into characteristic compact shapes (see also "protein folding"
as well as secondary structure and tertiary structure), which determine their biological functions and depend in
a complicated way on their primary structures. Structural biology is the study of the structural properties of the
biopolymers. In contrast most synthetic polymers have much simpler and
more random (or stochastic) structures. This fact leads to a molecular mass
distribution that is missing in biopolymers. In fact, as their synthesis is
controlled by a template directed process in most in vivo systems all biopolymers
of a type (say one specific protein) are all alike: they all contain the
similar sequences and numbers of monomers and thus all have the same mass. This
phenomenon is called monodispersity
in contrast to the polydispersity encountered in synthetic polymers. As a result biopolymers
have a polydispersity index of 1.
Conventions and nomenclature
Polypeptides
The convention for a polypeptide
is to list its constituent amino acid residues as they occur from the amino
terminus to the carboxylic acid terminus. The amino acid residues are always
joined by peptide bonds. Protein, though used colloquially to refer to any polypeptide,
refers to larger or fully functional forms and can consist of several
polypeptide chains as well as single chains. Proteins can also be modified to
include non-peptide components, such as saccharide
chains and lipids.
Nucleic
acids
The convention for a nucleic acid
sequence is to list the nucleotides as they occur from the 5' end to the 3' end
of the polymer chain, where 5' and 3' refer to the numbering of carbons around
the ribose ring which participate in forming the phosphate diester linkages of
the chain. Such a sequence is called the primary structure of the biopolymer.
Sugars
Sugar-based biopolymers are often
difficult with regards to convention. Sugar polymers can be linear or branched
and are typically joined with glycosidic bonds. The exact placement of the linkage can vary, and the
orientation of the linking functional groups is also important, resulting in α-
and β-glycosidic bonds with numbering definitive of the linking carbons'
location in the ring. In addition, many saccharide units can undergo various
chemical modification, such as amination,
and can even form parts of other molecules, such as glycoproteins.
Structural characterization
There are a number of biophysical
techniques for determining sequence information. Protein sequence can be determined by Edman degradation, in which the N-terminal residues are hydrolyzed from the
chain one at a time, derivatized, and then identified. Mass spectrometer
techniques can also be used. Nucleic acid sequence can be determined using gel electrophoresis
and capillary electrophoresis. Lastly, mechanical properties of these
biopolymers can often be measured using optical tweezers or atomic force microscopy. Dual polarisation interferometry
can be used to measure the conformational changes or self-assembly of these
materials when stimulated by pH, temperature, ionic strength or other binding
partners.
Biopolymers as materials
Some biopolymers- such as polylactic acid
(PLA), naturally occurring zein, and poly-3-hydroxybutyrate can be used as plastics, replacing the need for polystyrene
or polyethylene based plastics.
Some plastics are now referred to as
being 'degradable', 'oxy-degradable' or 'UV-degradable'. This means that they
break down when exposed to light or air, but these plastics are still primarily
(as much as 98 per cent) oil-based and are not currently certified as 'biodegradable'
under the European
Union directive on Packaging and Packaging Waste
(94/62/EC). Biopolymers will break down, and some are suitable for domestic composting.
Biopolymers (also called renewable
polymers) are produced from biomass
for use in the packaging industry. Biomass comes from crops such as sugar beet,
potatoes or wheat: when used to produce biopolymers, these are classified as non food crops.
These can be converted in the following pathways:
Sugar beet > Glyconic acid >
Polyglyconic acid
Many types of packaging can be made
from biopolymers: food trays, blown starch pellets for shipping fragile goods,
thin films for wrapping.
Environmental
Impacts of Biopolymers
Biopolymers can be sustainable,
carbon neutral and are always renewable,
because they are made from plant materials which can be grown indefinitely.
These plant materials come from agricultural non food crops.
Therefore, the use of biopolymers would create a sustainable
industry. In contrast, the feedstocks for polymers derived from petrochemicals
will eventually deplete. In addition, biopolymers have the potential to cut carbon emissions and reduce CO2 quantities in the atmosphere:
this is because the CO2 released when they degrade can be reabsorbed
by crops grown to replace them: this makes them close to carbon neutral.
Biopolymers are biodegradable, and
some are also compostable. Some biopolymers are biodegradable:
they are broken down into CO2 and water by microorganisms.
Some of these biodegradable biopolymers are compostable:
they can be put into an industrial composting process and will break down by
90% within six months. Biopolymers that do this can be marked with a
'compostable' symbol, under European Standard EN 13432 (2000). Packaging marked
with this symbol can be put into industrial composting processes and will break
down within six months or less. An example of a compostable polymer is PLA film
under 20μm thick: films which are thicker than that do not qualify as
compostable, even though they are biodegradable.
In Europe there is a home composting standard and associated logo that enables
consumers to identify and dispose of packaging in their compost heap.
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