Peptide bond planarity

First attempts to construct amide chemical shift surfaces additionally face the problem that at least live angles have to be studied the peptide bond planarity a>, and one pair each of (tp, yjT) angles on each side of the nitrogen. The only encouraging result is that the influence of the two pairs is practically additive. 2 A study of the main influence of V i) and revealed... 

Figure 3. Phi (p) and Psi ( /) torsional angles. Free rotation can occur around the bonds between the a-carbon of an amino acid and the carbonyl carbon (ly), and between the a-carbon and the nitrogen ((()). Resonance with the carbonyl group makes the peptide bond planar.

EXAMPLE 4.3 Why is the atom-group of a peptide bond planar ... 

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. 

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. 

As described in the beginning of this chapter, the peptide bond is rigid, polar, and prefers a planar structure with hydrogen of the amino group and oxygen of the carbonyl almost trans. It is easily understood that this conformational preference and rigidity has profound implications to the tertiary and quaternary structure of proteins and similarly on the binding of smaller peptides to receptors. 

In D O solution, the cyclic trimeric derivative of azetidine-2-carboxylic acid [cyclo(Aze)3] displayed (78MI2) more than one interconverting conformation, with peptide bonds slightly deviated from planarity. Circular di-chroism in methanol showed (78MI3) absorption very similar to that of... 

In cyclic peptides the cis- and frans-peptide bonds occur, with the cis form preferred in small cycles, and the trans form preferred in large systems (75AX(B)2035). The peptide bond is expected (68MI6) to deviate slightly from planarity in order to alleviate short-range interactions and bond-angle strain. 

In cyclic triprolyl derivatives, the peptide bonds were predicted (68MI6) to be slightly deviated from planarity. They have been found (78JA2548 79AG(E)538 82JA6297) to be cis both in solid and in solution. 

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