Final answer:
The zinc finger does not belong to the three principal forms of protein secondary structures, which are α-helix, β-sheet, and beta turns; unlike the zinc finger, these are defined by the pattern of hydrogen bonds in the protein's backbone. OPTION C IS ANSWER
Step-by-step explanation:
The option that does NOT belong to the three principal forms of protein secondary structures is c. zinc finger. The three principal forms of secondary structures in proteins include the alpha (α-helix), the beta (β-sheet), and beta turns.
These structures are characterized by patterns of hydrogen bonds between the amino and carbonyl groups along the protein's backbone.
The α-helix structure forms a helical shape stabilized by hydrogen bonds, while the β-pleated sheet is composed of strands connected by hydrogen bonds, forming sheets.
Beta turns are not mentioned in the information provided, but they are another type of secondary structure that allow the polypeptide chain to reverse direction. In contrast, a zinc finger is a small protein structural motif characterized by the coordination of one or more zinc ions to help stabilize the fold. OPTION C IS ANSWER