The formation of disulfide bonds has a direct application
in producing curls in hair by the permanent wave process. Hair
keratin consists of many protein alpha-helices. Three alpha-helices
are interwoven into a left-handed coil called a protofibril.
Eleven protofibrils are bonded and coiled together to make a
microfibril. Hundreds of these microfibrils are cemented into
an irregular bundle called a macrofibril. These in turn are mixed
with dead and living cells to make a complete strand of hair.
Although it may seem incredible, in order for hair to grow
6 inches in one year, 9-1/2 turns of a -helix must be produced
The alpha-helices are extensively cross-linked with disulfide
bonds from cysteine. These bonds enable keratin to have a somewhat
elastic nature. If the alpha -helices stretch unevenly past each
other, the disulfide cross-links return them to the original
position when the tension is released.