Characterisation of genetically modified nisin molecules by Fourier transform ion cyclotron resonance mass spectrometry

Modified nisin molecules, synthesised by strains of Lactococcus lactis with deliberately mutated nisA genes, have been characterised using Fourier transform ion cyclotron resonance mass spectrometry and electrospray ionisation. The predicted substitutions in the three nisin variants synthesised were first confirmed by precise measurement of the molecular mass (precision +/-0.1 Da). Analysis of the lower intensity peaks in the mass spectra showed the presence of some minor components, notably hydrated molecules, in the first two samples. The third sample contained a major hydrated component that could be isolated in pure form by high-performance liquid chromatography. The engineered nisin molecules were further characterised by tandem mass spectrometry using the sustained off-resonance irradiation collisionally-activated decomposition technique. This yielded a number of sequence ions that were compared with those measured in a previous study of nisin A itself. The location of each substituent was deduced from the observed mass shifts of the sequence ions. This permitted definitive confirmation of the predicted substitutions. Out of several possible sites it was confirmed that position 33 contained the additional water molecule in the major hydrated form of one of the nisin variants.