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In organic chemistry, a peptide bond is an type of linking two consecutive from C1 ( number one) of one alpha-amino acid and N2 ( number two) of another, along a or chain.

It can also be called a eupeptide bond to distinguish it from an , which is another type of amide bond between two amino acids.


Synthesis
When two amino acids form a through a peptide bond, it is a type of condensation reaction. In this kind of condensation, two amino acids approach each other, with the non- (C1) of one coming near the non-side chain (N2) moiety of the other. One loses a and oxygen from its carboxyl group (COOH) and the other loses a hydrogen from its amino group (NH2). This reaction produces a molecule of water (H2O) and two amino acids joined by a peptide bond (−CO−NH−). The two joined amino acids are called a dipeptide.

The amide bond is synthesized when the of one amino acid molecule reacts with the of the other amino acid molecule, causing the release of a molecule of (H2O), hence the process is a dehydration synthesis reaction. The formation of the peptide bond consumes energy, which, in organisms, is derived from ATP.

(1987). 9780805396140, The Benjamin/Cummings Publishing Company, Inc.. .
Peptides and are chains of held together by peptide bonds (and sometimes by a few ). Organisms use to produce nonribosomal peptides,
(2024). 9781493933730
and to produce proteins via reactions that differ in details from dehydration synthesis.
(2024). 9780716735205, W. H. Freeman. .

Some peptides, like , are called ribosomal peptides as they are made by ribosomes, but many are nonribosomal peptides as they are synthesized by specialized enzymes rather than ribosomes. For example, the tripeptide is synthesized in two steps from free , by two : glutamate–cysteine ligase (forms an , which is not a peptide bond) and glutathione synthetase (forms a peptide bond).


Degradation
A peptide bond can be broken by (the addition of water). The hydrolysis of peptide bonds in water releases 8–16 /mol (2–4 /mol) of . This process is extremely slow, with the at 25 °C of between 350 and 600 years per bond.

In living organisms, the process is normally by known as peptidases or , although there are reports of peptide bond hydrolysis caused by conformational strain as the peptide/protein folds into the native structure. This non-enzymatic process is thus not accelerated by transition state stabilization, but rather by ground-state destabilization.


Spectra
The of absorption for a peptide bond is 190–230 nm, which makes it particularly susceptible to UV radiation.


Cis/trans isomers of the peptide group
Significant delocalisation of the of electrons on the nitrogen atom gives the group a partial double-bond character. The partial double bond renders the amide group planar, occurring in either the cis or . In the unfolded state of proteins, the peptide groups are free to isomerize and adopt both isomers; however, in the folded state, only a single isomer is adopted at each position (with rare exceptions). The trans form is preferred overwhelmingly in most peptide bonds (roughly 1000:1 ratio in trans:cis populations). However, X-Pro peptide groups tend to have a roughly 30:1 ratio, presumably because the symmetry between the Cα and Cδ atoms of makes the cis and trans isomers nearly equal in energy, as shown in the figure below. The associated with the peptide group (defined by the four atoms Cα–C'–N–Cα) is denoted \omega; \omega = 0^\circ for the cis isomer ( conformation), and \omega = 180^\circ for the trans isomer ( conformation). Amide groups can isomerize about the C'–N bond between the cis and trans forms, albeit slowly (\tau \sim 20 seconds at room temperature). The \omega = \pm 90^\circ requires that the partial double bond be broken, so that the activation energy is roughly 80 kJ/mol (20 kcal/mol). However, the activation energy can be lowered (and the isomerization ) by changes that favor the single-bonded form, such as placing the peptide group in a hydrophobic environment or donating a hydrogen bond to the nitrogen atom of an X-Pro peptide group. Both of these mechanisms for lowering the activation energy have been observed in peptidyl prolyl isomerases (PPIases), which are naturally occurring enzymes that catalyze the cis-trans isomerization of X-Pro peptide bonds.

Conformational is usually much faster (typically 10–100 ms) than cis-trans isomerization (10–100 s). A nonnative isomer of some peptide groups can disrupt the conformational folding significantly, either slowing it or preventing it from even occurring until the native isomer is reached. However, not all peptide groups have the same effect on folding; nonnative isomers of other peptide groups may not affect folding at all.


Chemical reactions
Due to its resonance stabilization, the peptide bond is relatively unreactive under physiological conditions, even less than similar compounds such as . Nevertheless, peptide bonds can undergo chemical reactions, usually through an attack of an electronegative atom on the , breaking the carbonyl double bond and forming a tetrahedral intermediate. This is the pathway followed in and, more generally, in N–O acyl exchange reactions such as those of . When the functional group attacking the peptide bond is a , or , the resulting molecule may be called a or, more specifically, a thiacyclol, an oxacyclol or an azacyclol, respectively.


See also
  • The Proteolysis Map

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