Page "Keratin" Paragraph 18

In addition to intra-and intermolecular hydrogen bonds, keratins have large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally-stable crosslinking — a role sulfur bridges also play in vulcanized rubber.

Human hair is approximately 14 % cysteine.

The pungent smells of burning hair and rubber are due to the sulfur compounds formed.

Extensive disulfide bonding contributes to the insolubility of keratins, except in dissociating or reducing agents.