Answer:
A. Only L-amino acids (except for glycine) are used in proteins.
B. The partial double bond character of the α-C-N bond in the peptide group limits the conformations of the peptide group.
C. Hydrogen bonding within elements of secondary structure stabilize certain atomic geometries.
D. Although any one of 20 amino acids is possible at each position, only one is used.
Explanation:
Proteins are biomolecules composed of one or more chains of amino acid residues which are joined together by peptide bonds (this sequence represents the primary structure of the protein). Proteins are made up of 20 types of amino acids which have diverse properties. The secondary structure of a protein refers to the local structure of the protein backbone, which is stabilized by hydrogen bonding between amino groups (—NH2) and carboxyl groups (—COOH) in neighboring areas of the protein. The most common secondary structures in proteins include alpha helices, beta sheets, and random turns. Moreover, the tertiary structure in proteins describes the packing of these secondary structures. The peptide bond (CO–NH) is a stable covalent bond that has a rigid planar structure and acquires partial double-bond properties, thereby peptide bonds undergo very little rotation (i.e., rotation around peptide bonds is restricted). With the exception of glycine, all amino acids are stereoisomers, i.e., there exist mirror images of their structures which are labeled as L (left-handed) and D (right-handed) in order to differentiate between mirror images. All amino acids in proteins have the L-configuration.
