Title: Structure and Biological Activity of Chemically Modified Nisin A Species
Abstract: Nisin, a 34‐residue peptide bacteriocin, contains the less common amino acids lanthionine, β‐methyl‐lanthionine, dehydroalanine (Dha), and dehydrobutyrine (Dhb). Several chemically modified nisin A species were purified by reverse‐phase HPLC and characterized by two‐dimensional NMR and electrospray mass spectrometry. Five constituents, [2‐hydroxy‐Ala5]nisin, [Ile4‐amide,pyruvyl‐Leu6]des‐Dha5‐nisin, [Met(O)21]nisin, [Ser33]nisin, and nisin‐(1–32)‐peptide amide, were found in a commercial nisin sample. A further species, [2‐hydroxy‐Ala5]nisin‐(1–32)‐peptide amide, was obtained by freeze drying an acidic nisin solution. These compounds are formed by chemical modification of nisin: the addition of a water molecule to the dehydroalanine residues, which can lead to the cleavage of the polypeptide chain, or the oxidation of methionine residues. The 2‐hydroxyalanine‐containing products have a limited stability; they are spontaneously converted into the corresponding des‐dehydroalanine derivatives. The growth‐inhibiting activity of the modified nisins towards different bacteria was determined. The 2‐hydroxyalanine‐containing species and the des‐dehydroalanine derivative show a strong reduction in biological activity as compared to native nisin. [Met(O)21]nisin and [Ser33]nisin show moderate or no reduction in biological activity.