Final answer:
B) purines The correct answer is purines. Purines are not found in proteins, which are composed of amino acids linked by peptide bonds. The a-helices, disulfide bridges, and hydrogen bonds are all found in proteins.
Step-by-step explanation:
The correct answer is B) purines. Purines are nitrogenous bases that are found in DNA and RNA, but they are not found in proteins. Proteins are composed of amino acids linked together by peptide bonds. The a-helices and disulfide bridges are examples of the tertiary structure of proteins, which refers to the three-dimensional folding of the protein molecule. Hydrogen bonds are also important in the structure and stability of proteins.
The element not found in proteins is B) purines. Proteins are made up of amino acids linked by peptide bonds. They have structures such as α-helices and can form disulfide bridges. Proteins also contain hydrogen bonds, which help to maintain their tertiary structure. Purines, on the other hand, are nitrogenous bases found in nucleic acids like DNA and RNA, not in proteins.