Peptide bond

A peptide bond (amide bond) is a covalent chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, thereby releasing a molecule of water (H2O). This is a dehydration synthesis reaction (also known as a condensation reaction), and usually occurs between amino acids. The resulting C(O)NH bond is called a peptide bond, and the resulting molecule is an amide. The four-atom functional group -C(=O)NH- is called a peptide link. Polypeptides and proteins are chains of amino acids held together by peptide bonds, as is the backbone of PNA.
Figure 1: Dehydration synthesis (condensation) reaction forming an amide
Figure 2: Resonance forms of a typical peptide group. The uncharged, single-bonded form (typically ~60%) is shown on the left, whereas the charged, double-bonded form (typically ~40%) is on the right.
Figure 3: A hydrogen bond (dotted line) to an X-Pro peptide group favors the single-bonded resonance form (left) over the double-bonded form (right).
Figure 4: An electronegative substituent near the amide nitrogen favors the single-bonded resonance form (left) over the double-bonded form (right).

A peptide bond can be broken by amide hydrolysis (the adding of water). The peptide bonds in proteins are metastable, meaning that in the presence of water they will break spontaneously, releasing 2-4 kcal/mol [1] of free energy, but this process is extremely slow. In living organisms, the process is facilitated by enzymes. Living organisms also employ enzymes to form peptide bonds; this process requires free energy. The wavelength of absorbance for a peptide bond is 190-230 nm.


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