Journal of The American Society for Mass Spectrometry

, Volume 29, Issue 9, pp 1835–1847 | Cite as

Structural Investigation of the Hormone Melatonin and Its Alkali and Alkaline Earth Metal Complexes in the Gas Phase

  • Satrajit ChakrabartyEmail author
  • Matthew J. DiTucci
  • Giel Berden
  • Jos Oomens
  • Evan R. WilliamsEmail author
Focus: Application of Photons and Radicals for MS: Research Article


Gas phase infrared dissociation spectra of the radical cation, deprotonated and protonated forms of the hormone melatonin, and its complexes with alkali (Li+, Na+, and K+) and alkaline earth metal ions (Mg2+, Ca2+, and Sr2+) are measured in the spectral range 800–1800 cm−1. Minimum energy geometries calculated at the B3LYP/LACVP++** level are used to assign structural motifs to absorption bands in the experimental spectra. The melatonin anion is deprotonated at the indole-N. The indole-C linking the amide chain is the most favored protonation site. Comparisons between the experimental and calculated spectra for alkali and alkaline earth metal ion complexes reveal that the metal ions interact similarly with the amide and methoxy oxygen atoms. The amide I band undergoes a red shift with increasing charge density of the metal ion and the amide II band shows a concomitant blue shift. Another binding motif in which the metal ions interact with the amide-O and the π-electron cloud of the aromatic group is identified but is higher in energy by at least 18 kJ/mol. Melatonin is deprotonated at the amide-N with Mg2+ and the metal ion coordinates to the amide-N and an indole-C or the methoxy-O. These results provide information about the intrinsic binding of metal ions to melatonin and combined with future studies on solvated melatonin-metal ion complexes may help elucidate the solvent effects on metal ion binding in solution and the biochemistry of melatonin. These results also serve as benchmarks for future theoretical studies on melatonin-metal ion interactions.

Graphical Abstract


Melatonin-metal ion interaction Ion spectroscopy Free electron laser Infrared spectroscopy Infrared multiphoton dissociation 



This material is based upon work supported by the National Science Foundation Division of Chemistry under grant number CHE-1609866 and the Swiss National Science Foundation under grant number P2BSP2_148624. We gratefully acknowledge the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) for the support of the FELIX Laboratory. We thank Prof. Ryan R. Julian for useful discussions and his innovative contributions to studies of ion-photon interactions.

Supplementary material

13361_2018_2020_MOESM1_ESM.docx (24 kb)
ESM 1 (DOCX 24 kb)


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Copyright information

© American Society for Mass Spectrometry 2018

Authors and Affiliations

  1. 1.Department of ChemistryUniversity of CaliforniaBerkeleyUSA
  2. 2.Laboratory of Physical ChemistryZurichSwitzerland
  3. 3.Institute for Molecules and Materials, FELIX LaboratoryRadboud University NijmegenNijmegenThe Netherlands
  4. 4.Van’t Hoff Institute for Molecular SciencesUniversity of AmsterdamAmsterdamThe Netherlands

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