Detection and localisation of disulphide bonds in a synthetic peptide reproducing the sequence 1-30 of Par j 1.0101 by electrospray ionisation mass spectrometry

The structural characterisation of a synthetic peptide reproducing the sequ ence 1-30 of Par j 1.0101, a major allergenic protein present in the pollen of Parietaria judaica, by combined use of chemical and enzymatic cleavage, reversed-phase high-performance liquid chromatography and electrospray ion isation mass spectrometry (ESI-MS), is described. Direct ESI-MS of the synt hetic peptide after reaction with methyl iodide showed that the product is a mixture of two peptides: one form in which two out of the four cysteine r esidues present in the sequence are oxidised and a minor amount of another form in which all the cysteines are fully reduced. It was ascertained, usin g the combined procedure indicated above and without prior separation of th e two species, that the disulphide bond in the partially oxidised form is l ocated between cysteines 29 and 30. These results show the usefulness of th is approach for characterising synthetic peptides containing multiple cyste ine residues in the sequence.