Amino acid
In short Amino acids are the building blocks of peptides and proteins. Twenty of them occur in the genetic code; modified forms are also used in synthesis.
Every amino acid carries an amino group, a carboxyl group and a side chain. The side chain determines whether the building block tends to get on with water or not, and whether it carries a charge. These properties decide how readily a finished peptide dissolves.
Amino acids are linked by the peptide bond between the carboxyl and amino groups. A sequence is written in the same direction: from the free amino end (N-terminus) to the free carboxyl end (C-terminus).
In product names, amino acids appear as abbreviations. KPV, for example, stands for lysine, proline and valine: three building blocks, three letters.
For practical laboratory work the side chains matter most. They decide whether a peptide is hydrophilic or hydrophobic and in which pH range it dissolves best. Plenty of arginine, lysine and histidine makes a molecule basic; plenty of aspartic acid and glutamic acid makes it acidic.
Two amino acids deserve particular attention because they are sensitive to oxygen: cysteine and methionine. If a sequence contains either of them, dimethyl sulfoxide is practically ruled out as a solvent. The other options are set out under solubility.
In solid-phase peptide synthesis exactly one amino acid is added per cycle. The number of links is therefore also the number of working steps, and one of the reasons why long sequences cost more than short ones. The process is described in how a peptide is made.