In the amide group, rotation is restricted about the \(C-N\) bond, so that the \(\mathrm{C}, \mathrm{N}\), and \(\mathrm{O}\) atoms in that group are normally in the same plane. This rigidity helps to form the primary structure of proteins. The amino acid glycine can form a dipeptide, composed of two glycine monomers. (a) Draw the Lewis structure of the glycine dipeptide. (b) Explain how resonance structures can account for the restricted rotation and draw a second resonance structure for the dipeptide.

The formation of a peptide bond is a fundamental process in the synthesis of proteins. It involves linking together amino acids, which are the building blocks of proteins. This process is also known as a condensation reaction or dehydration synthesis because it results in the removal of a water molecule.

When two amino acids are to be joined, one amino acid’s carboxyl group \textnormal{(\text{COOH})} reacts with the other's amine group \textnormal{(\text{NH}\(_2\))}, releasing a water molecule \textnormal{(\text{H}\(_2\)O)} and forming a covalent bond known as a peptide bond. Specifically, the carbon atom from the carboxyl group of one amino acid bonds to the nitrogen atom of the amine group of the next amino acid. The dipeptide that results is the simplest form of a protein, comprising just two amino acid residues linked by a single peptide bond.

In the case of the glycine dipeptide, the same removal of water joins the two glycine monomers, showcasing the very essence of peptide bond formation in proteins.

Resonance structures are a concept in chemistry used to represent molecules where the actual distribution of electrons is an average of two or more hypothetical structures. These structures differ only in the positions of their electrons, not the positions of atoms.

Delocalized electrons are the electrons in a molecule that are not associated with a single atom or a single covalent bond. In the glycine dipeptide example, the resonance occurs within the peptide bond. Electrons from the nitrogen atom’s lone pair can delocalize to form a double bond with the carbon atom of the carbonyl group. This electron delocalization results in two or more resonance structures, each depicting different possible locations for the electrons.

This delocalization gives the peptide bond some characteristics of a double bond, which restricts rotation around the bond and contributes to the stability and rigidity of the protein structure.

Amino acids are organic compounds that serve as the monomers for proteins. Each amino acid has a central carbon atom, known as the \textalpha-carbon, to which four different groups are attached: an amino group (\textnormal{(\text{NH}\(_2\))}), a carboxyl group (\textnormal{(\text{COOH})}), a hydrogen atom, and a variable side chain or R group that determines the identity of the amino acid.

The sequence and chemical properties of the side chains in the amino acids ultimately determine the three-dimensional structure and function of the protein. Proteins can have several levels of structure: primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids, connected by peptide bonds. Secondary structure refers to local folded structures that form within the polypeptide chain due to interactions between amino acid side chains, such as \textalpha-helixes and \textbeta-sheets. Tertiary structure denotes the overall three-dimensional shape of a single protein molecule, and quaternary structure relates to the way in which multiple protein subunits interact to form a larger complex.