The peptide bond isamino acidIntermolecularaminoandcarboxylFormed by dehydration and condensationChemical bond, because the condensation product is calledpeptide, hence the name peptide bond.The peptide bond refers to theπ electronAnd the adjacent C-N bondThe isolated pair electrons together form the delocalized π bond (πthreefour）。[6]
The chemical bond formed by the dehydration and condensation of the carboxyl group of one amino acid and the amino group of another amino acid is called peptide bond, which is written as - CO - NH -.[2]
The peptide bond is characterized by:[2]
1. The lone pair electrons on the nitrogen atom have obvious conjugation with the carbonyl group.[2]
2. C-N bond in peptide bond is longer than C=N bond and shorter than adjacent C-N single bond;C-N bond in peptide bond has partial double bond property and cannot rotate freely.[2]
3. The four atoms constituting the peptide bond are in the same plane.[2]
4. In most cases, peptide bonds exist in trans structure.[2]
Peptide bond
Peptide bond plane
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The basic chemical bond forming the polypeptide chain in the polypeptide molecule is the peptide bond. The group (- C-CO-NH-C-) formed by the peptide bond and two adjacent carbon atoms is calledPeptide unit。Peptide chain is composed of many peptide units, which form the backbone of polypeptide chain.By comparing some simple peptide and protein peptide bondsX-ray crystal diffraction methodThrough analysis, it is proved that the spatial structure of peptide unit has the following three significant characteristics:[3]
1. The peptide unit is a planar structure.The six atoms constituting the peptide unit are located in the same plane, forming a planar structure of peptide bond, calledPeptide bond plane。[3]
2. The peptide bond has the property of local double bond and cannot rotate freely.The length of C-N bond in peptide bond is 0.132nm, which is shorter than the adjacent C-N single bond (0.147nm), and longer than the general C=N (0.127nm), between the two.This indicates that the x electron of carbonyl group is delocalized, which makes the peptide bond have double bond property locally, so the rotation between C and N is blocked to some extent.[3]
3. Peptide bondTrans configuration。Since the peptide bond cannot rotate freely, the atoms on the peptide bond plane can appear cis trans isomerism, and the O and H or two Cs connected with the C-N bond.The atoms are generally in a relatively stable trans configuration.[3]
C-N and C-C on both sides of the peptide bond plane can not rotate except for C-N bond.All bonds are bonds, so adjacent peptide bond planes can surround C.Rotation, the skeleton of the main chain of the peptide chain can also be regarded as a series of passing C.It is composed of peptide bond planes connected by atoms.The three-dimensional structure produced by the rotation of the peptide bond plane can take on various states, resulting in different conformations of protein molecules.[3]
The naturally occurring peptides have different molecular sizes, some of which areProtein degradationSome of them are active substances with special physiological and pharmacological effects.As far as known polypeptides are concerned, most are open chain peptides, a few are branched open chain peptides, and cyclic polypeptides are very rare.The chemical properties of peptides are similar to those of amino acids in some aspectsAmino acid residueR group has a great influence on the properties of peptides.Like amino acids, peptides also contain acidic and basic groups such as COO and NH, so they also exist in the form of dipole ions, with their own isoelectric points.The acid-base properties in aqueous solution mainly depend on the number and properties of side chain ionizable R groups.[3]
Formation principle
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The amide bond formed by the removal of a molecule of water from the carboxyl group of one amino acid and the amino group of another amino acid is also called peptide bond.[4]
Peptide bond has special properties.From the bond length, the peptide bond length (0.132nm) is between the C-N single bond (0.146nm) and the double bond (0.124mm), which has the property of partial double bond and cannot rotate freely;From the bond angle, the included angle between bonds in peptide bonds is 120 °.Therefore, the six atoms (CnC. O, N, H, Ca) are always on the same plane, forming a rigid "peptide bond plane", also known as“Amide plane ”Or peptide unit.In the peptide unit, the hydrogen and oxygen atoms connected with C-N are inversely distributed with the two carbon atoms.[1]
The peptide bonds in protein molecules mainly form many amino acid units at the C-position of amino acids, which are connected by peptide bonds to form polypeptide chains.The peptide bond isprotein structureThe main chemical bond in;In fact, polypeptide chain is the basic skeleton of protein.[4]
peptide bond formation
Since the atoms in the peptide bond are in resonance state, the peptide bond is highly stable in the peptide bond,C-N single bond has about 40% double bond property,C=O double bonds also have about 40% single bond properties.There are two important results: ① the amino group (NH) of peptide bond has no obvious tendency of dissociation or protonation in the range of pH0~14; ②The C-N bond in the peptide bond has double bond property and cannot rotate freely, so that the atoms constituting the peptide bond are roughly fixed on a plane, often called "peptide bond plane", which plays an important role in limiting the number of conformations of polypeptide chains.[4]
The simplest peptide is a dipeptide composed of two amino acids, which contains a peptide bond;Peptides containing three, four and five amino acid residues are called tripeptide, tetrapeptide and pentapeptide respectively.The amino acids in the peptide chain are called amino acid residues because they are no longer complete molecules due to the formation of peptide bonds.Generally speaking, the end of a polypeptide chain with free amino group is the amino end or N end of the polypeptide chain;The other end with free carboxyl group is the carboxyl end or C end of the peptide chain. Peptides are named according to their amino acid residues, usually starting from the N end of the peptide chain. For example, the following pentapeptides can be named as serine glycyl tyrosine alanyl leucine.[4]
The free amino and carboxyl groups in the peptide chain and other groups in the side chain of the residue can undergo chemical reactions similar to the corresponding groups in the amino acid.N-terminal amino acid residues can also interact withNinhydrinColor reaction occurs.Biuret reactionIt is usually used to measure the color reactions of peptides and proteins: generally, compounds containing two or more peptide bonds can be combined with basic CuSOfourThe solution generates purplish red or bluish purple compounds, which is called biuret reaction.This reaction can be used to determine the content of protein.The existence of peptide bonds in protein molecules can be proved by biuret reaction, infrared spectrum analysis and X-ray diffraction. The polypeptide chains in protein molecules can be hydrolyzed into amino acids or peptide segments with smaller molecules by acids, bases or proteases. This method is often used to study the primary structure of proteins.[4]
In addition to the partial hydrolysis of protein, which can produce various simple peptides, there are various peptides of different lengths in nature, which have various special physiological functions.glutathione(γ - Glutamylcysteinylglycine) is a kind of tripeptide that widely exists. Because it contains SH group, it is often expressed as GSH.This tripeptide may play an important role in the redox process in vivo.[4]
In addition, many hormones, such asOxytocin、Vasopressin、Bradykinin、EnkephalinAnd so on are polypeptides.There is anotherCyclic polypeptideIt is produced by the condensation of the terminal amino group and the terminal carboxyl group of the open bond polyphthalide to form a peptide bond, such asantibiotics、BacitracinIt is cyclodecapeptide.[4]
hydrolysis
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The formation of peptide bonds requires energy consumption, which is provided by ATP in vivo.The peptide bond can be broken by hydrolysis, and 8 – 16 kJ/mol (2 – 4 cal/mol) can be decomposed and released[4]But the process is quite slow.The lifetime of peptide bond in aqueous solution is about 1000 years.It is very difficult to hydrolyze peptide bonds without enzymes, so this process is usually mediated by an enzyme called protease, such as subtilisin, which is often added to washing powder to cut many protein pollutants.[7]
Difference between amide bond and peptide bond
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Amide bondBoth C and N atoms in (- CO-NH -) are sptwoHybrid, with a planar structure, a p - π conjugate system is formed between the lone pair electrons on the nitrogen atom and the carbonyl group.Due to the delocalization of electron pairs on the N atom,The bond length of the CN bond is shorter than that of the C-N bond in the amine, and it has the property of partial double bond.In addition, the electron absorption of oxygen also reduces the density of the electron cloud on nitrogen, thus weakening the alkalinity of nitrogen.[5]
All peptide bonds are amide bonds, and amide bonds include peptide bonds but are not equivalent to peptide bonds.The range of amide bond is larger than that of peptide bond.[5]
