As with other resonance bond structures seen in
previous chapters, the actual structure is somewhere in between.
The measured bond lengths from protein chains are shown on the bottom
drawing of the three. The C-0 distance is close to that expected
for a double bond, but the C-N distance is between the single and
double bond values, skewed somewhat toward a double bond. The sharing
of the nitrogen lone pair with carbon evidently is incomplete, as
is the pushing away of the second electron pair from the C=O bond
onto the oxygen atom. Oxygen then acquires a slight excess of electrons
and a partial negative charge. The electron deficiency created at
nitrogen pulls the N-H bonding pair toward N, so the partial positive
charge ends up on the hydrogen atom, as shown. From a delocalization
viewpoint, the second electron pair of the C=O double bond and the
nitrogen lone pair both have been delocalized over the entire O-C-N
region (lower right). This delocalization gives the amide group
an extra 21 kcal mole-' of stability, which means that
one cannot twist the group about the C-N bond without supplying
the 21 kcal mole-' necessary to break the partial double
There are two important structural consequences
of this bonding. The planar amide group may be considered as a rigid
structural unit whose only degrees of freedom are swivels about
the connections to the alpha carbons, and the 0 and H of the amide
plane have slight negative and positive charges that aid in the
formation of the hydrogen bonds that hold different chains together.
Above: 1. A delocalized molecular-orbital
picture of bonding in the amide plane, with four electrons delocalized
over all three O, C, and N atoms.
2. Oblique view of the amide plane, showing the three 2pz
orbitals that participate in delocalized bonding.