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The fundamental building blocks of proteins, amino acids, are linked together through a specific type of covalent bond known as a peptide bond. This crucial linkage, central to the structure and function of all proteins, is characterized by a particular functional group. Understanding what functional group is found in peptide bond is key to comprehending the chemistry of life.

At its core, a peptide bond is a special case of an amide group. This amide functional group, represented by the four-atom structure -C(=O)NH-, is formed when the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another amino acid. This reaction, often referred to as a dehydration reaction or peptide bond formation, results in the loss of a water molecule. The resultant molecule is an amide, and the bond itself is termed a peptide bond.

The process of peptide bond formation involves the interaction of specific functional groups. The amine and carboxylic acid functional groups within individual amino acids are the reactive sites. More precisely, it is the α-carboxyl group of one amino acid and the α-amino group of another that participate in forming this bond. The amino group and carboxyl group are the site of peptide bond formation. When these groups join together, they form amide bonds, creating a chain of amino acid units known as a polypeptide chain. This chain, essentially a series of linked amino acids, forms the backbone of proteins.

While the primary linkage is an amide group, it's worth noting that other functional groups can interact with a peptide bond. For instance, if the functional group attacking the peptide bond is a thiol, hydroxyl, or amine, the resulting molecule may be a cyclol or a more specific derivative. However, the defining characteristic functional group of the peptide bond itself remains the amide group.

The significance of the peptide bond extends beyond its chemical definition. These bonds are crucial for the stability and three-dimensional structure of proteins, which in turn dictates their biological roles. For example, Glutathione, an important tripeptide, is present in significant concentrations in all tissues and plays a vital role in cellular defense. It is composed of glutamic acid, cysteine, and glycine, linked by peptide bonds. The very existence of complex biological molecules and processes relies on the robust nature of these peptide bonds.

In summary, the functional group that defines a peptide bond is the amide group. This linkage is formed between the carboxyl group of one amino acid and the amino group of another, and it is the fundamental connection that builds the polypeptide chains essential for life. The amide group is called a peptide bond, highlighting its specific role in linking amino acids.