MadSci Network: Chemistry
Query:

Re: What is a chemical equation for hair relaxers

Date: Mon Dec 14 23:40:26 2009
Posted By: Steve Mack, Associate Staff Scientist
Area of science: Chemistry
ID: 1260850987.Ch
Message:

Hi Jocline,

The answer to your question will depend on the type of relaxer you are using. Historically, many hair relaxers have basically been solutions of lye (sodium hydroxide, NaOH, or potassium hydroxide, KOH), or other hydroxide-containing solutions1, so I will focus my answer on those. However, some modern relaxers use other chemistry, so check and see if you want information on these, and we will see what we can do.

We have many answers in our archives that discuss the chemical structure of hair, and the changes that have to be made to hair in order to curl (perm) straight hair. You can search for those answers, or read a few of these (892764277.Ch, 985546180.Eg, and 1170439971.Bc) for background.

So, from the answers in the archives, you will see that the structure of hair is in part due to the presence of cystine bonds between the filaments of protein (keratin) that make up the hair. A cystine bond is made between two cysteine amino acid residues. Cysteine molecules contain sulfur-containing thiol groups (-SH) groups, and it is the sulfur atoms in these thiol groups that bond to each other, forming the cystine bonds (also known as disulfide bonds), which we draw as (-S-S-). However, I've included a picture of an actual cystine molecule below.

These disulfide bonds are very strong, and hold the keratin fibers together. So, if you want to change the shape of your hair from curly to straight, you have to loosen the keratin fibers by breaking the disulfide bonds.

Lye-based relaxers break disulfide bonds by removing one of the sulfur atoms in a cystine (generating hydrogen sulfide [H2S] and hydrogen disulfide [H2S2]), and generating a new, more flexible bond, known as a lanthionine bond. I've included a picture of a lanthionine molecule here.

You can see here that the difference between a cystine and a lanthionine is the loss of one sulfur atom. This greatly weakens the connection between the hair fibers, allowing very curly hair to be straightened, or relaxed.

So, in terms of writing an equation, this reaction isn't as simple as something like:
2 H2 + O2 --> 2 H2O

If you look at the paper I cited below by Thakur and Balaram (and all you really have to look at is the abstract; you don't have to buy the paper), you will see that there are a lot of intermediate steps between the cystine and the lanthionine, with many other products of the reaction possible. The actual chemistry that they outline is quite complex, and when the reaction is occurring on someone's head the disulfide bonds are between proteins, which it will be difficult to account for in your report. So maybe we can summarize the overall reaction without going through the intermediate details.

The chemical formula of cystine is C6H12N2O4S2 and the formula for lanthionine is C6H12N2O4S. Two of the additional end products of this process will be C3H5NO2 and C3H5NO2S. So the equation could be summarized as:

2 C6H12N2O4S2 --> C6H12N2O4S + H2S2 + C3H5NO2 + C3H5NO2S

I hope that that helps you in your report Jocline. I've included some references below that I relied on in writing this answer.

Keep asking questions!

References:
Zviak, Charles, Editor. The Science of Hair Care, Marcel Dekker, New York (1986).

Thakur SS, Balaram P. (2009) Characterization of alkali induced formation of lanthionine, trisulfides, and tetrasulfides from peptide disulfides using negative ion mass spectrometry. Journal of the American Society for Mass Spectrometry 20 (5) 783-791.
doi:10.1016/j.jasms.2008.12.019


Historical Footnote: While he was serving in Europe during World-War II, my dad used to make conkoline hair relaxer out of lye and potatoes. If you left that in your hair too long, it would burn your scalp and leave you with bald patches, which is why there are many more gentle relaxers being made and used today.


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