Natural polymers include the RNA and DNA that are so important in genes and life processes. In fact, messenger RNA is what makes possible proteins, peptides, and enzymes. Enzymes help do the chemistry inside living organisms and peptides make up some of the more interesting structural components of skin, hair, and even the horns of rhinos. Other natural polymers include polysaccharides (sugar polymers) and polypeptides like silk, keratin, and hair. Natural rubber is, naturally a natural polymer also, made from just carbon and hydrogen. Let's look at each of the main families of natural polymers closely.
Chemically, chitin is poly(N-acetylglucosamine). Here's it's structure:
As you take a look more closely at each member of these families of natural polymers, remember this: nature was there first, by a long shot! One of our jobs as scientists is to figure out how nature does such a good job so that we can imitate it. For example, once we figured why silk had such neat properties we were able to make synthetic silk in the form of nylons. We still have a long way to go, though, before we can make synthetic RNA and DNA that will lead to synthetic life. While we may never get there, trying to figure out how is fun and leads to lots of important developments in synthetic polymers and other areas including medicine and biochemistry. This raises the important point that science is like life. It doesn't deal with just one thing, but everything mixed together. Polymer science isn't the only science, and it may not even be the most important science (although we in the business like to think it is!). It is one of the areas that can help us understand and use the knowledge we get from studying nature. In this way, we develop technology.
(Note: Just to clear up this whole science-and-technology question, science and technology are two different things despite being so closely related. Science is the act of gathering knowledge by observation and experimentation. Technology is putting this knowledge to use. Example: Using science we learn that hot gasses expand. Then using technology, we use the principle hot gasses expanding to make a gasoline engine that can power a car. See how it works?)
The amide molecular segment is unique in its structure and intermolecular interactions. Because of the hybridization of the nitrogen, carbon, and oxygen of the amide group, the segment is basically flat. More importantly, the hydrogen on the nitrogen and the carbonyl oxygen are capable of a strong interaction called a hydrogen bond. Because of this, the amide groups like each other so much that they form strong associations that give amide-containing polymers unusual properties. This type of interaction is also discussed in the section on nylons, and is the key similarity between natural and synthetic polyamides.
The differences between how nature does nylons and how we do it is striking. We mostly make nylons from molecules that have lots of CH2 groups in them. The section on nylons shows structures for nylon 6 and nylon 6,6, two of the most common synthetic polyamides. They possess four, five, or six CH2 groups between amide units. Nature, however, is much more economical, choosing to use only a single carbon between amide groups. What nature does differently is to substitute this carbon with lots of different functional segments and groups.
This results in two key properties. First, the individual segments and the entire molecule are optically active, or chiral. This means they are like gloves: there are both right and left and versions. For some reason, nature chose to use only the left hand version of the amino acids that are synthesized by plants and animals. The fact that only one of the two isomers is used leads to some neat stereochemical consequences. For example, natural polypeptides can form helical structures while nylons can't. The helical conformations increase the stability of the natural polypeptides. Did you know that some bacteria can survive in boiling water? This is because their natural polymers have been stabilized by such helical structures. The figure below shows one such helical structure, called an a-helix. Small segments of such helical structures are what nature uses to mold enzymes into certain shapes so that they can do their catalytic magic. For example, a flexible randomly coiled segment may be joined by two a-helix segments so that they can react together on some substrate.
Bundles of very thin individual polymers joined together to be stronger. This is the way we also make rope, using weak individual strands bound together in such a way that the total is both flexible and strong. The structure of silk molecules is unusual for a polypeptide. It possesses lots of the unsubstituted amino acid, glycine. Glycine segments are able to form flat extended chains that can pack together nice and tightly. This gives silk its unique strength and shiny flexibility.
Silk has such unique properties, especially in hot wet climates, that it dominated trade for centuries in east Asia. The silk trade between Japan and China controlled the economics of civilizations in that region for longer than either country cares to admit. Even in America, silk was important before World War II for use in silk stockings. When silk was used for parachute cord, women in America got very upset. This resulted in chemical companies synthesizing artificial silk, nylon, to make nylon stockings so that women's feet could stay warm and the men could go back to fighting their wars.
Another key difference between polypeptides and nylons is the way they are made. We humans make nylons in tons per day in huge chemical plants where simple molecules are joined together in large quantities to give products that we need or want. Nature is much more careful and concise in how she does things. For a living organism to make an enzyme, another enzyme or active species must be involved. The synthesis always involves a template, or recording, of how the individual amino acids are to be joined together to give the final polymer. This template, or map is a messenger RNA (mRNA). The message it carries, of course, is how the peptide-making enzyme involved should make the polypeptide.
Each amino acid is brought to the enzyme by a carrier molecule and is activated for incorporation by a whole cascade family of reaction steps. The enzyme adds a single amino acid, one at a time, as indicated by the mRNA. This is a slow and tedious process and takes a long time. Sometimes the enzyme gets frustrated, waiting for the right amino acid to come along, and slaps a wrong one on instead. To compensate this the enzyme is made to back up occasionally to check its work. If it has made a mistake, it has a process for clipping out the wrong amino acid and inserting the right one. We humans never do this. If we make a mistake, we simply grind it up and throw it away.
You're probably asking why we would want to do such a bad job of making synthetic analogs of what nature does so well. (If you aren't, just play along anyway.) There are lots of reasons, one of which is to figure out just how nature does it. Another is to figure out why peptides and enzymes work the way that they do. It's not always clear to us mere mortals why a given sequence of amino acids causes a polypeptide to assume a certain shape or structure. These structures are key to how the polypeptides do whatever job nature has devised for them. Sometimes, when we see the way nature puts these molecules together, we can make synthetic analogs that do the same thing but are easier to make. This has led to the development of new drugs and to treatments for some genetic diseases.
Nature also does things differently than us by synthesizing polypeptides in water. Most of our syntheses, in fact, don't use water. We synthesize our polyamides in toxic organic solvents. This leads us to a problem: what do we do with the organic solvents when we're through? Sometimes we burn the, but more and more we try to recycle these materials. Not only are they getting more expensive to buy in the first place (compared to cheap water which is everywhere, or almost everywhere) but we must be responsible for their recycle, purification, and final disposal. An example of how nature uses water in this way, and one which we still haven't figured out is the production of spider silk. Spiders spin their webs from solutions of polypeptides in water. These solutions are squeezed through the spider's tiny spinneret and elongated quickly to form the spider webs which we've all seen and sometimes gotten tangled in. What's really weird is that, once these spider webs form, they are no longer soluble in water. If we could just figure out how spiders first make spider silk in water and then spin their webs from it, we could make nylon the same way. This might save us a lot of waste disposal problems, and money. This is one area of basic research where we need lots and lots of help; maybe you can think of something we could try.
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