Which of the chemical bond present in keratin of skin and nail which makes them different in consistency?

Keratin is the protein present in hair and nails. Keratin is rich in cysteine and the propeies of the keratin present in different tissues is due to the differences in the number and position of disulfide bonds. Cysteine has a sulfhydryl group that can bond with another cysteine sulfhydryl group to form a cystine "disulfide" bond. This bond can be either within one protein or between two different proteins and is impoant in making strong structures such as the protein keratin found in fingernails. Hair, which is composed of linear protein sequences, is curly or straight depending on the number of cystine disulfide bonds. Ref: Janson L.W., Tischler M.E. (2012). Chapter 1. Amino Acids and Proteins. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.