Abstract
This chapter discusses the structure of polypeptide. The basic unit of polypeptide or protein structure is the amino acid. Amino acids are carboxylic acids with an amino group attached to the carbon next to the carboxyl group. This carbon atom is called the 2 α-carbon. Amino acids form polypeptides when the amino group of one amino acid condenses with the carboxyl group of another amino acid to form an amide, or peptide linkage. The polypeptide backbone is comprised of a repeating –C–C–N– unit. That is, peptide linkages alternate with α-carbons. The polypeptides have an amino end or N-terminus and a carboxyl end or C-terminus. In many natural polypeptides, the peptide linkage is planar, that is, the torsion angle of the C–N bond is either 0° or 180°. Most commonly, the oxygen and hydrogen atoms of this linkage aretrans to one another (torsion angle is 180°). This is called the trans-peptide linkage. In the cis-peptide linkage the torsion angle is 0°.
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