Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". General structure In the structure shown at the top of the page, R represents a side chain specific to each amino acid. The carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the α-carbon are referred to as α-amino acids. These include proline and hydroxyproline, which are secondary amines. In the past they were often called imino acids, a misnomer because they do not contain an imine grouping . Isomerism The common natural forms of amino acids have a zwitterionic structure, with ( in the case of proline) and functional groups attached to the same C atom, and are thus α-amino acids. With the exception of achiral glycine, natural amino acids have the L configuration, and are the only ones found in proteins during translation in the ribosome. The L and D convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of glyceraldehyde from which that amino acid can, in theory, be synthesized (D-glyceraldehyde is dextrorotatory; L-glyceraldehyde is levorotatory). An alternative convention is to use the (S) and (R) designators to specify the absolute configuration. Almost all of the amino acids in proteins are (S) at the α carbon, with cysteine being (R) and glycine non-chiral. Cysteine has its side chain in the same geometric location as the other amino acids, but the R/S terminology is reversed because sulfur has higher atomic number compared to the carboxyl oxygen which gives the side chain a higher priority by the Cahn-Ingold-Prelog sequence rules, whereas the atoms in most other side chains give them lower priority compared to the carboxyl group. Rarely, D-amino acid residues are found in proteins, and are converted from the -amino acid as a post-translational modification. Side chains Amino acids are designated as α- when the amino nitrogen atom is attached to the α-carbon, the carbon atom adjacent to the carboxylate group. There are several ways to classify amino acids; however, they are often grouped by the polarity of their side chains, as depicted in the graphic at the head