鈥淭he itsy-bitsy spider climbed up the waterspout. Down came the rain and washed the spider out鈥︹��. But funnily her web remained unscathed. How?!
With Halloween having just passed, we saw spider web decorations completely overtake houses, adorn witch hats and even candy wrappers. Despite my fear of spiders, I have an immense amount of respect for them and not for their contributions to our environment. I admire them because spiders are quite the biochemists, and their webs are a product of a biochemical reaction that we can only attempt to reproduce in the lab.
Spiders are able to produce impressive protein fibers without fancy equipment, chemicals or training. The chemistry of spider webs has always intrigued scientists and now we finally understand it (sort of).
Spider silk is a protein fiber made of a combination of amino acids. Amino acids are the building blocks of proteins and will bond together in a specific order to make a functional biopolymer. There are 20 of these building blocks and they all share a common element to their structure that can be seen below.
The differential part of the amino acid is the 鈥淩鈥� group. This can be replaced with 20 different functional groups that give rise to a different amino acid. This differential group will also determine the properties that the amino acid possesses. Some of the amino acid classifications are hydrophobic (water fearing), polar (water loving), charged and aromatic.
Spider silk is made primarily with a combination of two amino acids, alanine and glycine, which both bring different properties to the silk. While you may think spider silk is weak and can be brushed away with your hand, it is actually extremely strong! When considering its thickness is 20 times thinner than a single strand of your hair (around 0.003 millimetres), the fact that it can withstand wind and rain, seems almost impossible. The American Chemical society even states that 鈥減ound for pound, spider silk is much stronger than many types of steel.鈥� Its strength is due to the presence of alanine in its composition. Alanine-rich regions in spider webs are so strong because this amino acid has the ability to make hydrogen bonds with other alanine units in the fiber. Hydrogen bonds are the bonding of an electronegative atom with hydrogen. These are the same bonds that keep water molecules together and are very strong. The hydrogen bonds of alanine occur in the backbone of two alanine units on adjacent strands. This holds the strands together, thus strengthening the structure of the protein.
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The strength of spider silk combined with its elasticity, provided by glycine, make it a unique material. Spiders store this web fluid in their internal silk glands until it is time to extrude them from their 鈥渟pinnerets鈥�, an organ that works similar to a piping bag. As a liquid, spider silk is quite weak but as it passed though the spinning duct, there are numerous intermolecular interactions that occur between the amino acids that compose the silk and make it that ductile fiber that can withstand more than it lets on.
If you were impressed with spiders because of their talents in chemistry, just wait until you find out they are also good at engineering! They will use different web compositions for different parts of their webs. For the frame they will use 鈥淢A silk鈥� which contains a high content of b- sheet structures which increase its strength. b- sheets occur though hydrogen bonding of the non-variable (common) part of the amino acid backbone thus increasing its strength. They will then use more flexible silk to construct the inner portion of the web. The type of web will be dictated by their amino acid content. I guess they take so much care in perfecting their webs as we do perfect our homes.
To enhance their predatory skills, there are specific strands in the center portion of the web that will be coated in a sicky aqueous layer that contains organic molecules, small glycoproteins, salts and fatty acids. This coating helps confine their prey to the web and seems to only trap them more as they continue attempting their escape. They navigate around their web using the tips of their feet which have a non-stick coating on them. They use a special claw at the edge of their foot to grip the webs and pull them against the springy hairs that cover their legs and push the web off the claw as it is released.
So maybe the itsy-bitsy spider should just stay in her web instead of trying to climb the waterspout. She will probably stop getting washed out.
Angelina Lapalme is a BSc student majoring in Bio-Organic Chemistry at 不良研究所.聽
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