Ghalib's observations of Hydrogen Bonding
Ghalib (1797-1869) in one of his most famous ghazals says:
بسکہ ہوں غالب اسیری میں بھی آتش زیر پا
موے آتش دیدہ ہے حلقہ میری زنجیر کا
This verse can be roughly translated as follows: I am restless in captivity as if there were a fire under my feet. It is this fire that has caused steel to curl to form the links of the chain around my ankles much like a hair that coils when exposed to heat!
I know hairs are proteins and like a good proteins person, I am, at this time, more interested in knowing why does a hair curl when exposed to heat? Just to verify Ghalib's observation, I took out a hair and exposed it to heat to see it curl.
I have been looking on the Internet and have found the following explanation of this phenomenon.
A hair is a composed of keratin protein fibers. Keratin has a coiled coil structure (http://proteopedia.org/wiki/index.php/Keratins). The keratin in the hair is held together by hydrogen bonds. These hydrogen bonds act as glue and can change due to heat or water. On the application of heat, the hydrogen bond structure is disrupted which causes the hair to change its shape. This is the principle on which curling irons are based! However, the hair can recover its shape after some time especially with the addition of water if the amount of the initially applied heat is small!
However, the coiled coil protein structure of keratin is stabilized by hydrophobic interactions. I wonder what role these might play in the change in the shape of the hair when heat is applied.
