Until now we have drawn the peptide bond as an amide with double-bond character between the peptide carbon and the oxygen, and single-bond character between the peptide carbon and nitrogen atoms. Table 3.2 provides typical bond lengths for carbon—oxygen and carbon—nitrogen double and single bonds. Based on these data, one would expect the peptide carbon— oxygen bond length to be 1.22 Å and the carbon—nitogen bond length to be 1.45 Å. In fact, however, when x-ray crystallography was first applied to small peptides and other amide-containing molecules, it was found that the carbon— oxygen bond length was longer than expected, 1.24 Å, and the carbon— nitrogen bond length was shorter than expected, 1.32 Å (Figure 3.6). These values are intermediate between those expected for double and single bonds. These results were rationalized by the chemist Linus Pauling by invoking two resonance structures for the peptide bond; one as we have drawn it at the beginning of this section, with all the double-bond character on the carbon— oxygen bond, and another in which the double bond is between the carbon and the nitrogen, and the oxygen is negatively charged. Thus the system is actually delocalized over all three atoms, OCN. Based on the observed bond lengths, it was concluded that a peptide bond has about 60% CO character and about 40% CN character.