Last week, we talked about the three major types of bonds that keratin molecules can make: disulfide bonds (strongest), salt bonds, and hydrogen bonds (weakest). This week, we’ll see how keratin molecules can interact to make hair curly or straight.
In natural hair types, these disulfide bonds are the major players in terms of protein-interactions for creating straight or curly hair. In curly hair, the disulfide bonds are generally formed perpendicularly between sulfur atoms on the neighbouring keratin chains. This makes the strand more tightly coiled.
Also, the distribution of these bonds is important; if they are evenly distributed, then the hair is straight. If the bonds are more heavily distributed on one side that the other, this will lead to a kink or a curl to form. Finally, curly hair is found to have more sulfur atoms participating in disulfide bonds with neighbouring strands than straight hair.
Hook of the Follicle
Remember when we were talking about how the follicle plays a part in the type of hair you have? I mentioned that the follicle can sometimes have a hook, which leads to curly hair. A reason why this is true is that the hook forces the keratin molecules on the hair shaft to be brought closer to other keratin chains, inducing more disulfide bonds which leads to curlier hair.
If you imagine a line that traces the shape of the hairs in the image above and then draw multiple lines that are perpendicular, or at a right angle, to the lines representing the hair, you can see that the sulfurs (represented by the multiple lines) seem closer together. This allows the hair to form more disulfide bonds at a shorter length than the straight hair with its neighbours, thus creating curly or wavy hair.
Salt (Ionic) Bonds
This bond offers weak interactions between neighbouring keratin chains. These bonds are actually more important in terms of elasticity as well as cosmetic purposes.
These bonds are even weaker than the salt/ionic bonds, but they do form between neighbouring keratin chains too! These bonds, like the ionic bonds, contribute to our hairs’ elasticity. They can form temporary bonds, but these can be broken very easily or will break on their own after some time.
So, you can see that the disulfide bonds contribute a lot to our hair types. Hydrogen bonds have the ability to form temporary bonds, which are helpful for short-term curls or waves, but nothing permanent!
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