Answer: The statemens correct are: A) In a β-pleated sheet, the side chains extend above and below the sheet,
C) The α-helix is held together by hydrogen bonds between the amide N-H and C=O groups
D) The secondary level of protein structure refers to the spatial arrangements of short segments of the protein
E) Peptide bonds stabilize secondary structure
Explanation: The statement B is incorrect, because the amino acid side chains extend outside the alpha-helix, not inside
