MCAT EK Question on Peptide Bond Formation

[aspiringsocialepi]

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Seeing as there's no one else who commented on this, I'm assuming I'm wrong about something. On page 12 in the Examkracker's BIO1 manual (topic is Amino Acids and Proteins), there is a statement that "nitrogen is most stable with four bonds". The statement is in reference to the partial double bond character of peptide bonds, due to delocalization of electrons between the nitrogen and carbonyl carbon. However from my knowledge, nitrogen is most stable with three bonds and a lone pair (otherwise it takes on a charge) . My deduction was that the delocalization is occurring because of nitrogen's lone pair. Can anyone explain to me where I'm wrong here?

 

[theonlytycrane]

The peptide bond is stabilized due to amide resonance using the lone pair on the Nitrogen.

This is different from a nitrogen atom with 4 bonds in the + form that is not part of an amide functional group.

 

[aldol16]

Looks like EK's wording isn't so good. The reason the peptide bond is stable is because of delocalization. The amide nitrogen has three sigma bonds and one lone pair. That lone pair can delocalized into the pi system to form a pi bond and that's stabilizing for the amide. This is not the same as, for example, tetramethylammonium, which has four sigma bonds, no lone pairs, and no pi bond.

 

[aspiringsocialepi]

Thank you everyone! Indeed, EK's wording was bad, but I'm glad I was on the right track.