Protein secondary structure is characterized by hydrogen bond patterns between:

• A. Backbone amine protons and carbonyl oxygens
• B. Backbone amine protons and side chain carbonyl oxygens
• C. Side chain hydroxyl groups and backbone carbonyl oxygens
• D. Side chain amine protons and backbone carbonyl oxygens

Answer: (A)

The characterization of protein secondary structure, such as alpha helices and beta sheets, is primarily defined by specific hydrogen bonding patterns. In these structures, hydrogen bonds form between the backbone amine (N-H) groups and the carbonyl (C=O) groups of the peptide bonds in the protein. This pattern establishes the stability and conformation of these secondary structures.

In alpha helices, for instance, each carbonyl oxygen forms a hydrogen bond with the amine hydrogen of an amino acid that is four residues earlier in the sequence. In beta sheets, hydrogen bonds can form between carbonyl oxygens of one strand and amine hydrogens of another, stabilizing the sheet-like arrangement.

Other options involve incorrect relationships. Side chain interactions or hydrogen bonds involving different parts of the molecule do not participate in forming the primary secondary structures; rather, these are typically involved in tertiary and quaternary structures. Therefore, the accurate description of hydrogen bonding in the context of protein secondary structure directly leads to the conclusion that the correct answer involves the backbone amine protons interacting with the carbonyl oxygens.