Abstract
Determination of a single “fingerprint” mass spectrum of a biologically active compound, required for compound identification in mixtures and biological materials in such applied tasks as ecological monitoring and biological imaging, may become a nontrivial problem for labile compounds whose mass spectra depend strongly on the applied experimental conditions. In the present communication, qualitatively different mass spectral patterns obtained for the bisquaternary ammonium salt ethonium Cat2+•2Cl– under varied conditions of electrospray ionization and matrixassisted laser desorption/ionization (MALDI) are described and systematized. It is shown that qualitative changes in the electrospray mass spectra of ethonium occur upon a cone voltage increase from 10 V to 100 V, which are caused by the subsequent appearance and destruction of several primary ions: the intact dication Cat2+, the dication-counterion cluster Cat2+•Cl– and[Cat ‑ H]+ and [Cat ‑ CH3]+ ions. Novel experimental evidence of survival of the gas-phase dication Cat2+ with the shortest possible distance of ca 4 Co between the quaternary nitrogen atoms provided by two CH2 groups and Cat2+ fragmentation with preservation of the doubly charged state of the fragments are revealed under soft electrospray conditions. The interpretation of the MALDI mass spectrum is made taking into account the formation of a salt of the bisquaternary ammonium base and 2,5-dihydroxybenzoic acid, which results in the absence of any chlorine-containing ions in the mass spectra and a fragment ion distribution that is different from that observed under electrospray ionization conditions. The main mass spectral features revealed for ethonium may aid in the identification of other types of bisquaternary ammonium compounds.
Highlights
Widening of the scope of mass spectrometry from straightforward identification of compounds to studies of interactions in supramolecular assemblies of fragile biomolecules and to biological imaging brings to life new requirements to the mass spectra obtained by soft ionization techniques
At low cone voltage (CV) values below 10 V (Figure 1a) the peak of the intact dication Cat2+ (m/z 257) is the most abundant one in the mass spectrum. This behavior is consistent with the known preference of preservation of multiply charged species at low CV [28], but it is unexpected taking into account the small distance between the quaternary nitrogen atoms, which is believed to be the cause of a dramatic destabilization of dications
It is shown that the distinctions of electrospray mass spectral patterns in different cone voltage ranges are caused by subsequent appearance and destruction of several primary ions: the intact dication Cat2+, the dication-counterion cluster Cat2+Cl, and two ions [Cat – H]+ and [Cat – CH3]+ (30 V-100 V)
Summary
Widening of the scope of mass spectrometry from straightforward identification of compounds to studies of interactions in supramolecular assemblies of fragile biomolecules and to biological imaging brings to life new requirements to the mass spectra obtained by soft ionization techniques. The trivial task of determining the “fingerprint” mass spectrum characteristic of an individual compound can be significantly complicated by the chemical reactivity, lability, and degradability of some types of compounds, which are reflected in their mass spectra. High sensitivity of such compounds to variation of external conditions leads to noticeable qualitative differences in their mass spectral patterns obtained by different ionization/desorption methods, but on variation of experimental parameters in the framework of a single ionization technique as well.
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