What describes the primary (1°) structure of proteins?

The primary structure of proteins refers specifically to the linear sequence of amino acids that are linked together by peptide bonds. This sequence is determined by the genetic code and dictates how the protein will fold and function. Each amino acid in this sequence contributes to the overall characteristics of the protein, such as its chemical properties and potential interactions with other molecules.

While the other answers describe important aspects of protein structure, they do not pertain to the primary structure. For example, the presence of 3D polypeptide structures, such as alpha-helices and beta-sheets, relates to the secondary structure of proteins, which arises from hydrogen bonding within the amino acid chain. Likewise, the final 3D shape of a protein belongs to the tertiary or quaternary structures, which involve more complex folding and interaction between multiple polypeptides. Therefore, the primary structure is fundamentally defined by the sequence of amino acids, making this answer the correct choice.