Balance of radicalar, cationazed and protonated ions of nonpolar compounds in MALDI mass spectrometry: Perspectives for volatile secondary metabolites dynamic analysis and imaging generation

Our group has been investigating the ionization reaction in mass spectrometry and its significance on natural products structures elucidation. Systematic investigations performed on the effect of pH, cone potential energies, solvent system on the ionization reactions and significant differences were observed in ESI-MS [1]. Analysis of nonpolar compound with extensive polyene chain (β-carotene) showed an unexpected radical molecular ion formation in ESI and MALDI [2]. Correlations between the radical formation process and the ionization energy, calculated by molecular orbital methods or obtained by electrochemical techniques has suggested the importance of radical ion resonances to define the balance between protonated and molecular ion in ESI source and its significance on metabolomics studies [3]. In the other hand, continuous investigation on MALDI ionization processes presented the formation of radical molecular ions for molecules with no conjugation systems. To confirm these phenomena, we have analyzed in this work, erythralin (spirocyclic alkaloid), grandisin derivatives (tetrahydrofuran lignans), lapachol (quinone structure) and a collection of monoterpenes, which was used as the more nonpolar example of our standard collection. In ESI, no molecular ions were observed. However, MALDI afforded radical molecular ions for almost all analyzed compounds and in some cases, a high amount of in source fragmentation. Trying to elucidate the influence of ionization reactions balance, five nonpolar terpene standards were analyzed by using MALDI UltrafleXtreme Bruker^®, applying LiDHB matrix. This matrix induces the cationized ion as the most intensive signal, suppressing radical and protonated species. Finally, this methodology allowed us to obtain, for the first time, essential oil fingerprints of Lychnophora (Asteraceae) and Eremanthus (Asteraceae) and open the perspective to investigate metabolites dynamics or imaging generation by MALDI-MS of nonpolar compounds.

Acknowledgements: CNPq, FAPESP (2014/20302 – 4, 2014/50265 – 3) Lychnoflora are acknowledged for financial support.

Keywords: MALDI, grandisin, erytralin, lapachol, terpenes, Asteraceae.

References:

[1] Lopes NP, Stark CBW, Hong H, Gates PJ, Staunton J. A study of the effect of pH, solvent system, cone potencial and the addition of crown ethers on the formation of the monensin protoneted parents ion in electrospray mass spectrometry. Analyst 2001; 126: 1630 – 1632

[2] Guaratini T, Vessecchi RL, Lavarda FC, Maia PMBG, Naal Z, Gates PJ, Lopes NP. New chemical evidence for the ability to generate radical molecular ions of polyenes from ESI and HR-MALDI mass spectrometry. Analyst 2004; 129: 1223 – 1226

[3] Ernst M, Silva DB, Silva RR, Vêncio RZN, Lopes, NP. Mass spectrometry in plant metabolomics strategies: from analytical platforms to data acquisition and processing. Nat Prod Rep 2014; 31: 784 – 806