Primary structure:

The primary structure of a protein is its amino acid sequence. Biochemists are interested in sequence because it leads to 3-D structure, biochemical function (catalysis, etc.), and to control of that function.

Peptide bond formation results from elimination of water between the carboxyl group of one amino acid and the amino group of another.

An oligopeptide is a chain of amino acids containing a few amino acids.

A polypeptide is a chain of amino acids with many amino acids.

Polypeptides greater in molecular weight than 10,000 daltons are called proteins.

All peptides (except rare cyclic peptides) have a free amino group and a free carboxyl group that determine the direction of the amino acid sequence (from amino-end to carboxy-end).

Based on organic chemical principles, one would expect a very large number of possible conformations in a large polypeptide chain, but this is not the case. For the reasons, see below.

2. The Peptide Bond

O
C-N bond length = 0.132 nm (a peptide bond)
C=N bond length = 0.127 nm
C-N bond length = 0.146 nm

The C-N (peptide) bond has substantial double bond character.

It takes about 10 kJ/mol to form and is more stable than an ester bond.

The alpha-carbon bonds, on the other hand, are free to rotate.