What’s the biz with all this frizz? 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 – making me wonder why moisture makes my hair seem to want to fly. And the answer leads to a hopefully helpful discussion of covalent & noncovalent “bonds.”
Inside each strand of your hair are actually more strands – long strands of a protein called keratin, and these strands demonstrate several different kinds of bonds & attractions. The strongest of these bonds can be found within the strands themselves. Like all proteins, keratin is made up of building blocks called amino acids – there are 20 of these letters (at least common ones) and, kinda like cursive letters, they all have a generic part that allows them to link together and a unique “Side chain” that sticks off and gives them different properties and binding opportunities.
A type of strong, covalent bond called a peptide bond links together their backbones to give a chain we call a polypeptide or protein. These intrAmolecular bonds are strong, involving full-on electron sharing between next-door-neighboring atoms in the chain, so your hair doesn’t disintegrate when you take a shower (good except for when it clogs up your drain!)
But some of the bonds *between* strands of keratin (intErmolecular bonds) *are* disrupted by water. These disruptable ones are a form of “noncovalent bond” called hydrogen bonds, so let’s quickly discuss this terminology…
Covalent bonds like the ones we talked about *within* strands of keratin are the type of bond that hold atoms together to form molecules and they involve atoms sharing pairs of electrons. Electrons are one of 3 types of subatomic particles (atom parts) – the others are neutrons (which are neutral) and protons (which are positively charged). Protons and neutrons hang out together in an atom’s dense central core called the atomic nucleus and the electrons whizz around them in an “electron cloud.” I like to think of electrons kinda like dogs that the protons in the nucleus have to reign in using their positive charge as a “leash.”
The outermost electrons are called valence electrons and they’re the most energetic and reactive (these are the super hyper dogs). Atoms can give, take, and share electrons to get their “optimal number” of electrons, and when they share electrons they form covalent bonds (share 1 pair for a single bond & 2 pairs for a double). These covalent bonds are strong and not easily breakable. So they’re good for holding atoms together to form molecules – everything from water to proteins. But they wouldn’t be good for things you want to be able to stick and unstick and stick and unstick. And such reversible sticking is crucial for the interactions between molecules (intermolecular interactions). I mean, imagine if each time 2 molecules stuck together they were permanently stuck – your body would be one giant clump that couldn’t do anything!
Thankfully, covalent bonds are not the only way atoms can interact. Enter noncovalent bonds which are relatively weak bonds formed between molecules without sharing electrons. It can be more helpful to think of them more as attractions than “bonds” because they’re “easily” reversible. Just how easy depends on the strength of the attraction.
The attraction comes from oppositely-charged charges and/or partial charges liking each other. If the charges are full and “permanent” (eg. Na⁺ & Cl⁻) you can get an “ionic bond” (aka salt bridge). These are relatively strong, but still not as strong as covalent bonds. At the weak end are “London dispersion forces” which involve temporary charges that randomly form because electrons in atoms move around a lot and sometimes they end up clustered together leading to transiently partly charged regions of the (overall neutral) molecule. This partial charge separation situation is referred to as a dipole, and London dispersion forces involve temporary dipoles. Sounds like these wouldn’t have much oomph, but they let geckos walk up walls! Noncovalent bonds may be individually weak, but when you have a lot of them each contributing it can really add up for some serious stickiness.
In the middle strength-wise you have hydrogen bonds (aka H-bonds) which I want to tell you more about because, although they’re responsible for a lot of water’s awesomeness, they’re also the culprits in my hair frizz fiasco. H-bonds are similar to those London forces, but they involve permanent dipoles. Dipole-dipole interactions & London forces are collectively referred to as van der Waals interactions, and H-bonds are a special form of dipole-dipole interactions. They’re not “really” special, they’re just defined by where their dipoles come from, and they just get their own name because they come up a lot in biochemistry.
As we talked about above, each molecule of water (H₂O) is held together by covalent bonds between 1 O & 2 H’s. But these covalent bonds are “polar covalent bonds” because they involve unfair sharing. O is highly electronegative (electron-hogging), like a dog walker with really tight leashes. When O & H form a covalent bond, the O pulls the shared electrons closer to it. And electrons are negative so the O part of water becomes partly negative (δ⁻) & the H’s partly positive (δ⁺).and opposites attract. So you now have something that can bind to both + & – things. And those things don’t have to be fully + or – (and, as we saw with London dispersion forces) partial charges, even temporary ones work too, but with the case of water, these partial charges in a neutral molecule are “permanent” and thus H-bonds are stronger than London forces).
So water molecules stick to other water molecules, giving you things like surface tension (think insect walking on water). And water can stick to other things, coating those things like when you dissolve sugar in it – each sugar molecule gets a nice water coat. And it can stick to a water and another thing, giving you things like capillary action (think water “sneaking up” the inside of a straw). And, it can stick to 2 non-water things, “gluing” them together.
But remember, this glue is non-covalent. Unlike the peptide bonds creating the chains, hydrogen bonds don’t involve the involved partners actually merging electron clouds or anything, they just like to hang out because of charge oppositeness.
And it’s easier to break things off if you haven’t already tied the knot. So hydrogen bonds are weaker and easier to disrupt than covalent bonds. But you can then subsequently reform them (although probably with a different actual water molecule because there are usually a ton of them swimming around and once you break off the relationship that first one isn’t gonna just stick around and wait for you to get your hair into shape).
And it’s not just when it’s part of a water molecule that hydrogen can form H-bonds. Whenever they’re sharing electrons with a really greedy sharer (an electronegative atom) like oxygen or nitrogen they get partially positive. And attracted to something electronegative that has a “lone pair” of electrons, which O & N often do.
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.
A common place you see H-bonds come into play in biochemistry is “base pairing” – H-bonds are what holds complementary DNA (or RNA) strands together.
But in the case of hair, we’re talking about proteins. 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 one polypeptide chain folds up (quick 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 which can make the terminology a bit complicated).
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
Because it’s a gas, water vapor’s free to explore, and it “wants” to go places that are less crowded (in reality its movement’s random but if you take a bunch of randomly moving molecules more move away from the crowded areas because there were more there to move to begin with – and if a molecule wanders into a crowded place it’s more likely to collide with another molecule and get “bounced back” to somewhere less crowded). But net result’s the same no matter how you think of it -> molecules move from places there are a lot of copies of them (high concentration) to places there are fewer of them (areas of low concentration).
There’s not a lot of water in your hair but, especially on humid days, there is a lot of water in the air. 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…
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! (that always confused me!)
Hair can form more H-bonds when there’s more water BUT a 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.
This kind of bonding is different from the perms we talked about before when discussing redox (reduction and oxidation) reactions. http://bit.ly/dttreducingagents
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.
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.
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 bonds are holding it there. You can’t just “wash out” these bonds because they’re stronger, hence the term “perm” for permanent waves.
You might have noticed that today included a lot of terminology. In fact, it was inspired in part by me spending the weekend working on building my bumbling biochemist glossary of de-jargonized jargon… It includes links to relevant posts and some graphics & I hope it can help people. http://bit.ly/bumblingbiochemistglossary