Formation
The bond forms by condensation: a carboxyl group and an amino group react, and a molecule of water is released. In a cell this is done by the ribosome; in synthesis it is driven by coupling reagents that activate the carboxyl group so the reaction proceeds efficiently.
Why it is rigid
The bond has partial double-bond character because electrons delocalise between the carbonyl and the nitrogen. That prevents free rotation and holds the six atoms around the bond in a plane. The backbone therefore bends at the alpha carbons rather than at the peptide bonds themselves, which constrains the shapes a chain can adopt.
Why it breaks
Condensation released water, and the reverse reaction — hydrolysis — consumes it. Given water, time, heat, or an unfavourable pH, peptide bonds break. Certain sequences are more susceptible than others, and bonds involving aspartic acid are a well-known example.
This is the underlying reason peptides are supplied freeze-dried. Removing water does not make hydrolysis impossible, but it removes the reagent it requires.
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