The Peptide Bond
Individual amino acids can be linked by forming covalent bonds. The bond is formed between the α-carboxyl group of one amino acid and the α-amino group of the next one. Water is eliminated in the process, and the linked amino acid residues remain after water is eliminated (Figure 3.8). A bond formed in this way is called a peptide bond. Peptides are compounds formed by linking small numbers of amino acids, ranging from two to several dozen. In a protein, many amino acids (usually more than a hundred) are linked by peptide bonds to form a polypeptide chain (Figure 3.9). Another name for a compound formed by the reaction between an amino group and a carboxyl group is an amide.
The carbon–nitrogen bond formed when two amino acids are linked in a peptide bond is usually written as a single bond, with one pair of electrons shared between the two atoms. With a simple shift in the position of a pair of electrons, it is quite possible to write this bond as a double bond. This shift-ing of electrons is well known in organic chemistry and results in resonance structures, structures that differ from one another only in the positioning of electrons. The positions of double and single bonds in one resonance structure are different from their positions in another resonance structure of the same compound. No single resonance structure actually represents the bonding in the compound; instead all resonance structures contribute to the bonding situation.
The peptide bond can be written as a resonance hybrid of two structures (Figure 3.10), one with a single bond between the carbon and nitrogen and the other with a double bond between the carbon and nitrogen. The peptide bond has partial double bond character. As a result, the peptide group that forms the link between the two amino acids is planar. The peptide bond is also stronger than an ordinary single bond because of this resonance stabilization.
This structural feature has important implications for the three-dimensional conformations of peptides and proteins. There is free rotation around the bonds between the α-carbon of a given amino acid residue and the amino nitro-gen and carbonyl carbon of that residue, but there is no significant rotation around the peptide bond. This stereochemical constraint plays an important role in determining how the protein backbone can fold.
When the carboxyl group of one amino acid reacts with the amino group of another to give an amide linkage and eliminate water, a peptide bond is formed. In a protein, upward of a hundred amino acids are so joined to form a polypeptide chain.
The peptide group is planar as a result of resonance stabilization. This stereochemical feature determines a number of features of the three-dimensional structure of proteins.