Proline free AA vs. Proline residue

[nfg05]

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Could somebody explain to me why proline cannot participate in H-bonding when incorporated into a polypeptide? I see the amine functionality in the free amino acid, but according to EK there is amide functionality in a proline residue. I googled for the structure of a proline residue but couldn't find it anywhere.

 

[ThePandaFactor]

the amino group is a secondary amine. so when it forms the peptide bond it loses its only donor hydrogen.