Peptide bonds partial double-bond character

B is correct. The amino add bias and shapes of the secondary structures (a-hetix and -pleated sheets) is partially explained by the rigid structure of the peptide bond, whose double bond character prevents rotation. This rigidity provides steric constraints on hydrogen bond formation for some amino acids that precludes them from partidpating, or even breaking secondary structure. 

Although peptides are written as if a single bond finked die a-carboxyl and a-nitrogen atoms, this bond in fact exhibits partial double-bond character ... 

The partial double-bond character of the bond that links the carbonyl carbon and the nitrogen of a peptide renders four atoms of the peptide bond coplanar and restricts the number of possible peptide conformations. 

The torsion angle co, which is common to peptides and nonpeptidic amides, always prefers a planar over a nonplanar conformation due to the partial double-bond character of the amide bond (Fig. 6.3, right). Thus, a peptide bond resembles an amide bond in conformational and electronic terms [2] [3], However, peptides differ from amides in that both the carbonyl C-atom and the amido N-atom are nearly always bound to an sp3-hybridized C-atom. As a result, the trans-conformer (a>=180°) is consistently preferred over the cw-conformer, the energy difference usually being ca. 90 kJ mol 1 (Fig. 6.3). The relationship between the partial double-bond character of the amide bond (Fig. 6.3, right) and hydrolysis will be considered in Sect. 6.3. 

Such a difference in partial double-bond character has implications for the mechanism, and, hence, the reaction rate, of acid-catalyzed hydrolysis (Fig. 6.15). In delocalized peptide bonds (Fig. 6.15,a), protonation involves the carbonyl O-atom with its partial negative charge. In non-delocalized peptide bonds (Fig. 6.15,b), protonation involves the N-atom, which is rendered more basic by the lack of delocalization [73],... 

The observed lengths in nanometers determined by X-ray diffraction measurements are given in Fig. 2-5 (top). The partial double-bond character of the C-N bond has important consequences. The peptide unit is nearly planar as is indicated by the dashed parallelogram in Fig. 2-5. 

The partial double bond character of a peptide bond has its greatest effect in which structure of an enzyme ... 

The partial double bond character of the peptide bond ... 

Because of resonance, which gives the N-C bond a partially double bond character, the peptide bond is planar. Furthermore, the a-carbons are almost always trans. These two features of the peptide bond play a dominant role in determining protein structure. The other covalent linkage of importance is the disulfide bond that joins different parts of the protein chain in enzymes secreted outside the cells. Special note should also be made of an imino acid proline, which creates a very rigid peptide bond. 

Resonance stabilization of an amide accounts for its enhanced stability, the diminished basicity of the nitrogen atom, and the restricted rotation (partial double-bond character) of the C—N bond. In a peptide, the amide bond is called a peptide bond. It holds six atoms in a plane the C and O of the carbonyl, the N and its H, and the two associated a carbon atoms. 

The partial double bond character of each third bond in the peptide backbone would cause some limitation in the possible geometries of the chain but this in itself is certainly not sufficient to determine a preferred conformation. A well defined architecture requires further limiting factors and such can be found in the restricted rotation around the N —Q and the Q—C bonds. Negative (repulsive) interaction between side chains of neighboring amino acid residues hinders free rotation around these bonds and the dihedral angles 0 and W, corresponding to rotation out of the plane around the N —Q and Q—C bonds respectively... 

The partial double-bond character of the peptide bond forces the bond to adopt a planar arrangement. Consequently, conformational changes in the peptide chain can only involve other main-chain bonds they cannot involve peptide bonds. In addition, the peptide group can form hydrogen bonds to hydroxyl or amino groups or to the oxygen atoms of other peptide groups, etc. 

Thus, his theory suggested that the C-N bond in the peptide linkage had a partial double bond character, and he knew that the bonds around a double bond were all in the same plane. This allowed him to greatly reduce the number of possible models that had to be considered for alpha keratin. Still, Pauling was unable at this time to find a model that would fit the X-ray diffraction pattern. 

Although the peptide unit is mostly considered to be plane (co = 1 °) because of the partial double bond character of the amide bond, careful analyse by means of X-ray revealed deviaticms of torsion angles from planarity of up to 10°. 

The partial double-bond character prevents free rotation about the peptide bond, so the carbon and nitrogen atoms of the peptide bond and the two atoms to which each is attached are held rigidly in a plane (Figure 22.6). This regional planarity affects the way a chain of amino acids can fold this has important imphcations for the three-dimensional... 

Because of the partial double-bond character of the carbon-nitrogen bond, all atoms involved in a peptide bond lie in the same plane. Carbon atoms of each peptide bond are in a trans configuration, to reduce the steric repulsions. 

Due to the partial double bond character of the amide bonds, the distance connecting consecutive a-carbon atoms remains constant, being unaffected by the internal bond rotations. Virtual bonds C -C defined in this manner are often adopted in treating the configurational characteristics of peptide chains. 

A peptide chain consists of a succession of -C(=0)-N-C - atoms that are coplanar, with the C-N bond being shorter than that of a normal amide. There is a partial sharing of the 7t-electrons between the C=0 and the C-N bond, giving the latter double-bond character (-40%), so that it is unable to rotate freely. The NH-proton and the oxygen atom are in the same plane but in a trans relationship (Figure 6.24). 

A protein is a linear sequence of amino acids linked together by peptide bonds. The peptide bond is a covalent bond between the oi-amino group of one amino acid and the a-carboxyl group of another. The peptide bond has partial double bond character and is nearly always in the trans configuration. The backbone conformation of a polypeptide is specified by the rotation angles about the Ca-N bond phi, (j>) and Ca-C bond psi, amino acid residues. The sterically allowed values of 0 and yr are visualized in a Ramachandran plot. When two amino acids are joined by a peptide bond they form a dipeptide. Addition of further amino acids results in long chains called oligopeptides and polypeptides. 

The concept of cis-trans (Z-E) isomerism, originally used for the description of the relative geometry of olefins, has been extended to many other functions which feature a double bond character (pseudo double bonds), such as amides, as well as single bonds with a partial or complete limited rotation due to steric or stereoelec-tronic effects. Cis-trans isomerization (CTI) therefore exists in non-re-bonded or overcrowded molecules that switch from a given stable conformational state to another. This is the case of biaryl compounds which have been utilized in organic chemistry as the basis of molecular switches and rotors [1,2]. Nature has also exploited CTI of single bonds to increasing molecular diversity, in particular with the bulky thyroxin, a thyroid hormone, and the well-known disulfide bond which plays a critical role in the structure of peptides and in the conformation of proteins. 

The Peptide Bond Has Partial Double Bond Character... 

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