Nature's Polymers

Nature does a great job itself making polymers. Let's look at a few important ones that are all around us.


Proteins

Proteins are in all kinds of places in our bodies - muscles, hair, fingernails, enzymes. As pointed out previously, proteins are characterized by their amide links - except when talking proteins and biochemistry they have another name - peptide links. Our body uses amino acids as the monomer units to build proteins. We have a total of 20 different naturally occurring amino acids in our bodies. This is a case where the chemical, physical, and functional properties of the proteins are based on the sequencing of the amino acid monomer units. All 20 of those amino acids in our body share the same exact structure except for an identifying side-chain or R group.

Wool is actually a quite complex biological fiber. We can say though that the primary component fiber giving it strength is a protein, keratin. Keratin is also the key component of other tough biological things like hair, fingernails, feathers, claws, horns, and hooves. Silk is also in the same family of proteins/keratins.

Below is a structural view of a protein chain where I'm showing two monomer units (amino acids) and the amide links (peptide links for proteins) are highlighted.


Cellulose

Cellulose is the main natural polymer in wood and plant material. It is an a single monomer of D-glucose and propagates via a condensation type reaction splitting out water and making an ether link. The link is called a β(1→4) link. Very strong polymer and it cannot be digested by most mammals.

Cotton and flax are two more natural fibers that are also mostly cellulose.


Starch

Starch is a carbohydrate and like cellulose is a polymer made of D-glucose monomer units. The difference in starch and cellulose is very subtle. Starch is connected via an α(1→4) link. The difference in the α vs β linkages makes all the difference in the world to us mammals that consume starch as a food/fuel for our bodies.


DNA and RNA

DNA is a template for all living things. DNA is an abbreviation for deoxyribonucleic acid. It is quite complex but can be broken down into its individual part. First, it is a polymer (right? or I wouldn't be listing it here). The main chain of the polymer is a copolymer consisting of phosphate (yes, that PO43- you memorized) and deoxyribose which is a sugar. The deoxyribose also has a side group that is a nitrogen-containing base. Here is a helpful schematic showing the 3 parts of the repeat unit.

There is a special name for this monomer unit in biochemistry speak - it is a nucleotide. Nucleotides are the monomers that make up DNA. Once a series of nucleotides are linked, you have a DNA strand. Almost all DNA strands find their compliment and create a double-helix DNA strand that then is the basis of chromosomes and genes. Now, let's have a closer look at the specifics here for each piece of a nucleotide.

Phosphate, PO43-, we are already familiar with this one so I'll move on. The sugar unit is deoxyribose (for DNA) or ribose (for RNA). These are 5-carbon sugars unit with the structures shown below.

deoxyribose

ribose

The carbons are numbered to aid in identifying. Note that substituent on carbon-2 is the only difference in deoxyribose (a H) and ribose ( an OH) - shown in blue. Off the carbons 3 and 5 are hydroxyl groups (shown in green) that hook into the phosphate group. And last, the 1-carbon has the hydroxide (in purple) that links to the nitrogen-containing bases.

Now the bases. There are only four of these for DNA and they are adenine, guanine, cytosine, and thymine which are abbreviated as A, G, C, and T.

adenine

guanine

cytosine

thymine

Note that the position for attachment to the sugar unit is the red H. RNA is very analogous to DNA with the exception that uracil (U) is used in place of thymine. RNA tends to only form single strands and does not pair up with its compliment for a double-helix. It does fold and link up with itself in some of its 3D structure though.

Below is a full structure of a single nucleotide using adenine as the base.



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