Which of the following statements about hydrogen bonding with respect to the protein alpha-helix structure is accurate?

• A. Hydrogen bonds are formed only at the ends of the alpha-helix.
• B. Hydrogen bonds are established between the R-groups of amino acids.
• C. Hydrogen bonds destabilize the alpha-helix structure.
• D. Hydrogen bonds are formed between the carbonyl oxygen and amide hydrogen in the backbone.

Answer: (D)