beta-keratin. Alpha-keratin is seen in humans and other mammals, beta-keratin is present in birds and reptiles. Beta-keratin is harder than alpha-keratin. Structurally alpha-keratin have alpha-helical coiled coil structure while beta-keratin have twisted beta sheet structure.bIn the case of β-sheets, this allows sterically-unhindered hydrogen bonding between the amino and carboxyl groups of peptide bonds on adjacent protein chains, facilitating their close alignment and strong binding. Fibrous keratin molecules can twist around each other to form helical intermediate filaments. The secondary structure of silk is an example of the beta pleated sheet. In this structure, individual protein chains are aligned side-by-side with every other protein chain aligned in an opposite direction. The chains are antiparallel, with an alternating C → N orientation. The protein chains are held together by intermolecular hydrogen bonding, that is hydrogen bonding between amide groups of two separate chains. This intermolecular hydrogen bonding in the beta-pleated sheet is in contrast to the intramolecular hydrogen bonding in the alpha-helix. The hydrogen on the amide of one protein chain is hydrogen bonded to the amide oxygen of the neighboring protein chain. The pleated sheet effect arises form the fact that the amide structure is planar while the "bends" occur at the carbon containing the side chain. The "side" chain R groups in silk are not very bulky. The basic primary structure of silk consists of a six amino acid unit that repeats itself. The sequence where every other unit is glycine in silk is: -gly-ala-gly-ala-gly-ala-. Although glycine and alanine make up 75-80% of the amino acids in silk, another 10-15% is serine and the final 10% contain bulky side chains such as in tyr, arg, val, asp, and glu. Keratins