Monday, April 22, 2013

Sulfur-containing Amino Acids

I was recently working on a submission for a pass-along story [1][2] I've been writing with some of my friends for a few years now, which we call Blood Faith.[3] It concerns a secret world of vampires (no relation to the creatures found in the Twilight books and movies [4]). It is written in epistolary fashion, much like the famous vampire novel Dracula or C. S. Lewis' novel The Screwtape Letters. The first several letters are pedagogic, explaining many aspects of the vampires' secret society, while the later letters transition into a narrative about several vampires who discover additional secrets that the rank-and-file vampires are unaware of. Even though we're far into the narrative at this point, I decided to go back and write another pedagogic letter about the effects of garlic on vampires. While researching various chemical compounds found in garlic, I discovered that the chemical structures for several of these compounds weren't available on Wikimedia Commons. So I made them myself.




alliin [5]

isoalliin [5]


You can see a sort of progression in the structures. The methiin has the shortest carbon chain coming off the left side of the sulfur (S).[6] The ethiin adds a second carbon and the propiin adds a third. The chemical prefix meth- means "one carbon", the prefix eth- means "two carbons", and the prefix prop- means "three carbons". The next up, alliin, is similar to propiin except that the last two carbons are double-bonded. Isoalliin also has a double bond, but not on the same two carbons as alliin. Cycloalliin is like propiin except that one of the carbons after the sulfur is bonded to the nitrogen (N). Alliin can be converted to a chemical called allicin [7], which is a major component of the smell of garlic.

All of these compounds, except cycloalliin, fit the definition of an amino acid: they have an amino group (NH2), a carboxyl group (O=C–OH), and a 'side chain' connected to the same carbon as the amino group. Cycloalliin is considered an imino acid because the side chain of cycloalliin curls around and bonds to the N. All of these compounds has a sulfur atom. And now you understand the title of this post.


[1] The gist of a pass-along story is that you write a portion of it and then pass it along to another author to continue. I've also heard these referred to as round-robin stories.

[2] I had poor experiences with pass-along stories while camping with my family. My sisters invariably made the story about Native Americans and a white bison. No matter which alternative direction I tried to take the story (kill the white bison, send the protagonist to space, etc.) my sisters always brought it back (the white bison had a baby white bison that didn't die, the protagonist came immediately back to Earth, etc.).

[3] You can read the individual installments of Blood Faith here (scroll down), if you like. The installment related to this post is currently titled Blood Faith 3.5.

[4] Read my reviews of the RiffTrax versions of the first three Twilight movies here, here, and here.

[5] There were already images for alliin and isoalliin on Wikimedia Commons, but I didn't discover that until after I'd uploaded my versions.

[6] In chemical drawings carbons are implied by angles and the ends of lines. Additionally, hydrogens bonded to carbons are implied: the carbon at the end of a line has three hydrogens bonded to it, a carbon at an angle (i.e. the intersection of two lines) has two hydrogens bonded to it, a carbon at the intersection of three lines has one hydrogen bonded to it, and a carbon at the intersection of four lines has no hydrogens bonded to it.

[7] See

Image attributions:

The Chemical Structure of Alliin is by Ninjatacoshell, available at 2.svg.

The Chemical Structure of Cycloalliin is by Ninjatacoshell, available at

The Chemical Structure of Ethiin is by Ninjatacoshell, available at

The Chemical Structure of Isoalliin is by Ninjatacoshell, available at 2.svg.

The Chemical Structure of Methiin is by Ninjatacoshell, available at

The Chemical Structure of Propiin is by Ninjatacoshell, available at

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