Analysis of Nisin A and Some of Its Variants Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

The lantibiotic nisin and some of its variants and degradation products have been characterized, using a 9.4-T Fourier transform ion cyclotron resonance mass spectrometer and electrospray ionization. The abundances of all products in the sample (i.e., major component, variants, degradation products, and adducts) have been measured quantitatively. The mass resolution obtained in the electrospray ionisation mass spectra was approximately 100,000 over the measured range. The resulting mass accuracy, better than 0.7 ppm (or within 0.001 Da) allowed the molecular masses and in many cases chemical formulae of most components in the mixture to be identified unambiguously. Additionally, amino acid sequence information on nisin and a variant [nisin + 18 Da] was obtained using sustained off-resonance irradiation collisional activated decomposition (SORI–CAD) of mass-selected precursor ions. Even after introducing collision gas into the mass analyser for the SORI–CAD experiments, the mass accuracy in the fragment ion mass spectra was approximately 5 ppm. It was established that the [nisin + 18 Da] molecule, present as a minor component in the mixture, was a species formed predominantly via hydration of nisin at position 33, i.e., [Ser33]nisin, with a small contribution due to hydration at position 5,[2-hydroxy-Ala5]nisin.