The non-R group of amino acid polymers, consisting literally of -N-C-C-N-C-C-N-.
The peptide bond is one of the two C-N bonds, i.e., as indicated with the tilde and highlighted in yellow, ~N-C-C~N-C-C~N-. Note that in addition to the nitrogen atoms with their associated hydrogen atoms, which provide partial positive charges for hydrogen bonding, or even full charges to the extent that the nitrogen acquires an additional proton, there also is a carbonyl group projecting from the same carbons involved in the peptide bonds. That is, this too is a representation of a short section of polypeptide backbone ~N-C-(C=O)~N-C-(C=O)~N-C-(C=O)~N-C-(C=O)~N-.
The polypeptide backbone is the key contributor to protein secondary structure, which involves backbone-to-backbone hydrogen bonding.
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