Planarity of peptide bond

A2.5-A model refined to an R-factor of 0.18 should be a detailed model. "Good canonical properties7 means good agreement with accepted values of bond lengths, bond angles, and planarity of peptide bonds. 

Here, it ould be noted that a Qrclization of a linear peptide affects the bond length, the bond angle, and the configuration and planarity of peptide bond, and as a consequence these variables might deviate from their standard values. Go and... 

The a-helix is probably the most significant result of the work of Pauling and Corey, although authors before them had already discussed spiral structures. Innovations introduced by Pauling s model are, first, the abandonment of whole numbers of amino acids per turn second, the use of exact distances and bond angles and, third, the strict adherence to the planarity of peptide bonds. Subsequently, it became evident that the a-helix is also involved in the structure of soluble proteins. 

A polypeptide chain assumes the a-helical conformation because this is where H bonds can be formed most effectively. The planar conformations of peptide bonds with the =0 and -H groups trans to each other contributes to the stability of the a helix. 

Examination of the geometry of the protein backbone reveals several important features. First, the peptide bond is essentially planar (Figure 2.2d). Thus, for a pair of amino acids linked by a peptide bond, six atoms lie in the same plane the a-carbon atom and CO group ol the first amino acid and the NH group and a-carbon atom of the second amino acid. The nature of the chemical bonding within a peptide accounts for the bond s planarity. The peptide bond has considerable double-bond character, which prevents rotation about this bond and thus constrains the conformation of the peptide backbone. 

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. 

Fig. 17 Planar Structure of peptide bond and its dipolar representation...

For instance, the N-C bond in peptide bond is 0.132 nm long whereas in aliphatic amines -C-NH2- similar bond has a length 0.147 nm. This shortened bond and planar nature of peptide bond allows conjugation of C=0 and N-C bonds. Hence, there is considerable overlap of their electron shells and there is a shift of electron density from N to C to O. This makes the N-C bond partially double C=0 bond partially single. In addition, the peptide bond attains a dipolar character as shown in Fig. 17. 

The geometry of the peptide bond is planar and the mam chain is arranged m an anti conformation (Section 27 7)... 

Figure 3-4. Dimensions of a fully extended polypeptide chain. The four atoms of the peptide bond (colored blue) are coplanar. The unshaded atoms are the a-carbon atom, the a-hydrogen atom, and the a-R group of the particular amino acid. Free rotation can occur about the bonds that connect the a-carbon with the a-nitrogen and with the a-carbonyl carbon (blue arrows). The extended polypeptide chain is thus a semirigid structure with two-thirds of the atoms of the backbone held in a fixed planar relationship one to another. The distance between adjacent a-carbon atoms is 0.36 nm (3.6 A). The interatomic distances and bond angles, which are not equivalent, are also shown. (Redrawn and reproduced, with permission, from Pauling L, Corey LP, Branson PIR The structure of proteins Two hydrogen-bonded helical configurations of the polypeptide chain. Proc Natl Acad Sci U S A 1951 37 205.)...

The peptide bond is characterized by a fixed planar structure, as was discovered by X-ray crystallography of peptides more than 60 years ago. The arrangement of the atoms in the peptide bond is due to resonance stabilisation the lowest-energy state of the system is that in which the four atoms forming the peptide linkage lie in a plane, while the C-N bond has partial double bond character. 

Peptides and proteins are composed of amino acids polymerized together through the formation of peptide (amide) bonds. The peptide bonded polymer that forms the backbone of polypeptide structure is called the a-chain. The peptide bonds of the a-chain are rigid planar units formed by the reaction of the oc-amino group of one amino acid with the a-carboxyl group of another (Figure 1.1). The peptide bond possesses no rotational freedom due to the partial double bond character of the carbonyl-amino amide bond. The bonds around the oc-carbon atom, however, are true single bonds with considerable freedom of movement. 

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