The biochemistry behind hair frizz & curls: hydrogen bonds, disulfide bonds, & intermolecular forces
As a kid, my soccer teammates would call me “Einstein” - not because of smarts but because my hair would get super frizzy & curly! Especially on humid days - why? leads to a discussion of covalent & noncovalent “bonds.”⠀http://bit.ly/frizzandmolecularattrac...
Definition-wise, “hydrogen bond" is just a special name for when you have a molecule with partially-positive H (the H donor) (which is partially positive because it's hooked to an electronegative (electron-greedy) atom such as O or N) attracted to "lone pair" of electrons on an electronegative "acceptor" molecule. In the case of hair, we’re talking about the keratin protein. And proteins have lots of Os & Ns bound to H’s. In fact, H-bonds between carbonyl (C=O) & nitrogens in the peptide backbone give proteins their "secondary structure" (things like alpha helices & beta strands) that form when 1 polypeptide chain folds up (terminology note: proteins are made as long chains of amino acids called polypeptides which fold up to form functional proteins. So different proteins have different chains, but some proteins have multiple chains). ⠀
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H-bonds can also form between different peptide chains and between the peptide chains and water. So water can link keratin strands or unlink them - you can get keratin strands with a water molecule bridging them or keratin strands where they’re stuck to water but that water’s stuck to other things. But in order to have water stuck to keratin you’ve got to get it in! When it’s humid, there’s a lot of water in the air in the form of water vapor (water in its gas form). More on humidity here: https://bit.ly/humidityterms
When you shower or step out on a humid day, your hair absorbs a lot of water. So the water wanders into your hair - and finds keratin. The keratin has partly + & partly - parts, which the water’s partly + & partly - parts love, so they can bind. And this binding may require displacing previously-formed bridging H-bonds. Since water can form hydrogen bonds to multiple things, it can “bridge” strands of keratin together or just bind more water and leave the keratin strands “slippery” - for now…⠀
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Water molecules can bridge different keratin strands together or parts of the same keratin strand that have “folded up” on themselves - so your hair gets curlier. You hair’s more likely to absorb more moisture if it’s dry to begin with & if your hair absorbs more water than it can handle, your hair can break - the outer layer of hair strands, called the cuticle, is prone to such breakage which makes your hair look frizzy. This leads to the counterintuitive situation where you’re advised to keep your hair moist to avoid frizz caused by moisture in the air!
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Hair can form more H-bonds when there’s more water BUT at the same time, the water “lubricates” the strands of keratin so there’s “more competition” for them. So water molecules push off other molecules and take their place, etc. making it harder to form stable interstrand bonds. So when your hair’s wet it’s easier to “shape” and when it dries and there’s less water competition, the bonds are less likely to be “competed off” so your hair stays in that shape - hence “bedhead” after going to sleep after a shower.⠀
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This kind of bonding is different from perms - In a perm, you’re actually breaking and reforming stronger, covalent bonds - but not the peptide bonds keeping the chains chain-y. Instead, you’re breaking and reforming weaker (though still stronger than the H-bonds) disulfide bonds involving the thiol (-SH) side chains of 2 nearby cysteine (Cys) amino acids. Disulfide bonds are important for giving your hair strength and it’s “usual shape.” Depending on what -SH binds to what -SH the hair will take different shapes, but these shapes are strong and not wash-out-able with water.⠀
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Instead, if you want to “permanently” change the shape of your hair, you need to use a stronger de-bonder. You don’t want to break all the covalent bonds or your hair would disintegrate, instead you just want to break the disulfide bonds. Thankfully these are weaker and have an “Achille’s heel” we can target. The S in -SH is sulfur and it can take electrons (oxidize) & give electrons (reduce) more easily than the atoms in the peptide bonds. And since sharing electrons is how covalent bonds work, if you mess with how many sulfur has, you can mess with the bond. So we can use redox chemistry to break & reform them.⠀
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You break them using a reducing agent which turns the S-S into -SH + HS-. Then you physically mold the hair into the shape you want (e.g. curl it around curlers), hold it there, and add an oxidizing agent to reform the disulfide bonds. But now that the hair’s in a different shape, different parts of different keratin strands are nearby so the Cys’s get new binding partners. Then you remove the curlers and your hair stays curled because the newly-formed stron bonds are holding it there. http://bit.ly/dttreducingagents
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