Spiders: Chemistry of Silk and Venom

Hello everyone. Welcome to our next blog post, where we will go in depth into the chemistry behind spiders, like their webs and their venom. In our previous blog post, you may have noticed we started with talking about spider webs; if you are still not so sure about the basics of spider silk, visit our first post so some of this makes sense! Anyway, let’s get right to it. We mentioned previously that spider silk contains chains of proteins linked together partially with intermolecular forces. Now, we will introduce the idea that spider silk is a “natural polypeptide”, polymeric protein. A polypeptide is a continuous peptide chain, or a long chain of proteins, like we stated earlier but are now just giving it an official name. A polymer is a chain of repeatable links, that are called monomers. These are usually terms that go along with biological compounds. So, since spider silk is a polymer, you should probably understand how polymers are created from monomers. This usually involves a continuous chain of covalent bonds. Covalent bonds are the sharing of a pair of electrons between two atoms in a molecule, and these bonds are what holds the proteins together to form polypeptides. These proteins are what provide the structure for the silk.

The aforementioned proteins are called fibroins. Fibroin is formed from a combination of two proteins, spidroin 1 and 2. Hm, wonder where they got that name from! Anyway, continuing on the chemical structure of spider silk, proteins are made up of long chains of amino acids that determine the properties of that protein. The major amino acids in the protein fibroin are glycine and alanine (as shown below); others include glutamine, serine, leucine, valine, proline, tyrosine, and arginine (you may remember this name from before).

Molecular structure of alanine

 

Molecular structure of glycine

There is a repetition of alanine and glycine in spider silk, called the beta sheet, which accounts for the crystalline fraction of spider webs. Here’s an interesting representation of what the chains and beta sheets look like on a microscopic level:

Basically, what you are seeing here is the proposed model for spider dragline silk, as you may recall as one of the specific types of silk. The Alanine are the red lines in this diagram, while the glycine are the blue lines. The important thing that we want you to notice from this diagram is mostly the fact that alanine and glycine are so closely intertwined in spider silk, as well as the geometry of the  molecular structures here, and how a lot of it eventually turns into a sort of helical structure, which is very stable, as demonstrated by spider silk’s strength.

But, who am I kidding, this isn’t nearly as much chemistry as you want to hear! Let’s get to the good stuff. According to an in-depth study of spider silk, there are hydrogen bonds between carbonyl and amide groups, and Van der Waal interactions in the short chain amino acids found between the molecules. Hm….I don’t think we’ve heard this term yet. Van der Waal interactions are forces that occur between molecules, and are basically a fancy way for saying intermolecular forces, or IMF. Though a large reason for spider silk’s strength is its beta sheet, or 180 degree turns to form helices, those intermolecular forces that we discussed previously seem to keep coming back.  If you’re further interested in the sizes and construction of the spider silk that give it more of its tensile properties, this link gives a good explanation as to why intermolecular/intramolecular forces as well as construction of the spider silk all contribute to its strength and tensile properties.

How about we mix it up a bit now, let’s go into more detail on spider venom. Just as a refresher, we have established so far that general venom corrodes tissue by the process of hydrolysis, or the breakdown of molecules or structures with the use of water. This also combines with enzymes, which make the reaction occur faster. But, it is appropriate to examine more information on this topic now. Though we have mentioned cytotoxic venom before, we will now discuss mostly neurotoxic venom, because it is proven that this is the most common venom that spiders possess. In general, this venom contains consists of three compounds: polyaminespolypeptides, and proteins. Wow, spiders must really be lazy, because their venom is made up of the same compounds that their silk is made out of! Since we don’t really want to quench your chemistry thirst completely yet, we’re just going to explain the structure of these three things in this blog (don’t worry, we’ll discuss functionalities again in our next blog post).

Polyamines consist of a hydrophobic, or resistant to water, carboxylic acid region and a hydrophilic, or attractive to water, amide chain. Polypeptides are similar in structure, but much heavier in molecular mass. What’s molecular mass, you say? Well, molecular mass is basically the mass of a molecule, which is determined by what elements and atoms are in it.A single polypeptide molecule folds up to form a beta sheet, the same one discussed previously. The last compounds are the neurotoxic proteins, highly toxic to invertebrates.  Here are pictures of all three compounds, in order of discussion:

General structure of a spider polyamine

Molecular structure of a spider polyamine

Molecular structure example of a polypeptide

General Structure of a Protein

NOTE: The R’s are side chains of carbons; peptide bonds are covalent bonds between carboxyl groups and amino groups of other molecules.

Now, let’s take some time to go over some of the key points we just mentioned. First, we went over the basics of spider silk, including its structure and composition. We learned that spider silk is a “natural polypeptide”, which ends up being many links and chains of polymers held together by covalent bonds. We also learned a little bit of the biology relating to spider silk (but who wants to be reminded of specific biological terms?), and within this biology, we found that the structure of spider silk is simply a repetition of simple chemical structures forming beta sheets. The last point we went over is the intermolecular forces (specifically hydrogen bonding) that give spider silk its adhesive properties, something that we have already mentioned in our first blog post.

Our second main topic of this post was the neurotoxic venomthat most spiders posses. We went over the three compounds that venom consists of, which are polyamines, polypeptides, and proteins; we also went over each compound’s purpose in the functioning of venom. If you really want to skip ahead and know more about venom, check out this link. Anyway, stay tuned for our next blog post for more in-depth breakdowns of the functionalities of all the parts of spider venom, as well as the tensile properties of spider silk and why it is resistant to solvents and enzymes. We’ll end this with a cinematic picture of a spider web (No worries, there are no scary spiders on it)! Until next time!

File:Spider web with dew drops03.jpg

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