Science in little things #3 – Hairstyle

This time I think women will understand me better than men, anyway… Do you know that dissatisfaction you experience while looking at your hair? They never look like you want them to. When they are long, you would like to cut them. After you have them cut, you wish them to grow again as fast as they can. If you have straight hair, you believe they look like spaghetti and you envy curly girls, while curly girls beg to know how you straight yours so perfectly.

I found that I am really happy about my hair only when I let them do exactly what they want, without trying to force them into something. However, over the years, I did a lot of experiments and tried some drastic procedures: such as perm.

Have you ever tried it? I had and the result was… well, better to forgive and forget. Do you know how it works and especially how it is that once done you can’t undone it?  Chemistry is behind it, fancy that!

Hair are filaments whose key structural component is keratin. Keratin is a fibrous protein and as such it is composed by a chain of amino acids, that arranges in an helical structure. There are twenty different amino acids in nature, which have a common structure but also differ in some chemical groups. Cysteine is one of these amino acids and has an important role in hair shape.

 File:Amminoacido cisteina formula.svgThis image is from Wikipedia(

Do you see that group made of sulfur and hydrogen (-SH)? It’s called thiol group. When two of these groups are near and in the right conditions, a oxidation can occur and the result is a disulfide bond (-S-S-). Now, imagine a straight chain of amino acids, with cysteine hither and thither. What happens when these disulfide bonds form? The chain folds up, right? Right. That’s why hair have waves and curls and the more disulfide bonds there are the more curly your hair are.

File:Disulfide Bridges (SCHEMATIC) V.1.svg

This image is from Wikipedia (

Are you still with me? Ok, let’s go then.

Perm requires a chemical treatment in order to break and reform disulfide bonds, so that hair shape can change. Hair are treated with a solution of sodium thioglycolate and wrapped around rods: this causes the reduction of disulfide bonds back to -SH groups. After a while, a neutralizer (a mild solution of hydrogen peroxide) is applied so that disulfide bonds can form again in the new position, which is maintained thanks to the rods.

Once the hair gain the new shape, you can’t go back to straight hair unless you repeate the chemical treatment. So please, think very carefully before doing perm! I don’t have a picture of my perm experiment at hand, but rest assured that it’s not something you would easily forget.

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