Geometry of the Peptide Bond

We pointed out earlier that simple amides have a planar geometry, that the amide C—N bond is shorter than usual, and that rotation around that bond is restricted (Sec. 10.20). Bond planarity and restricted rotation, which are consequences of resonance, are also important in peptide bonds. 

The rather rigid geometry and restricted rotation of the peptide bond help to impart a definite shape to proteins. 

The characteristic bond angies and bond iengths in peptide bonds. 

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

The structures of the basic building blocks of the architecture of proteins were determined by Linus Pauling and R. B. Corey many years before the solution of the structures of globular proteins.13 They solved the structures of crystalline small peptides to find the dimensions and geometry of the peptide bond. Then, by constructing very precise models, they found structures that could fit the x-ray diffraction patterns of fibrous proteins. The diffraction patterns of fibers do not consist of the lattice of points found from crystals, but a series of lines corresponding to the repeat distances between constantly recurring elements of structure. 

Review Secs. 17.13 and 17.14. Secondary structures of importance are hydrogen bonds and the geometry of the peptide bond. The tertiary structures of fibrous sproteins are dominated by p-sheets (the presence of small amino acids with nonpolar side chains) and, in many cases, significant amounts of cysteine (the amino acid where R=CH2SH) cross-linked by disulfide bonds. 

The C-N peptide bond has an interesting property It is planar and very rigid. This special geometry of the peptide bond makes it very stable and ideal to maintain the structure of proteins. 

Resonance structures like these are commonly cited as leading to the planar geometry of the peptide bond and nucleic acid bases. 

The planar geometry of the peptide bond is analogous to the planar geometry of ethylene (or any other alkene), where the double bond between sp hybridized carbon atoms makes all of the bond angles -120° and puts all six atoms in the same plane. 

Figure 25.4 shows the structure of Ala-Gly as determined by X-ray crystallography. An important feature is the planar geometry of the peptide bond, and the most stable conformation with respect to this bond has the two a-carbon atoms anti to each other. Rotation about the amide bond is slow because delocalization of the unshared electron pair of nitrogen into the carbonyl group gives partial double-bond character to the carbon-nitrogen bond. 

The geometry of the peptide bond (Figure 4.4a) is important. The C-N bond length in the peptide bond is shorter than the 0.145 nm recorded for a single covalent C-N bond. This is a result of the propensity of the oxygen atom to withdraw electrons. The electrons associated with the C-N bond and the C=0 group resonate between two struc-... 

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