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

• Published in: Journal of the American Society for Mass Spectrometry 2018-09, Vol.29 (9), p.1835-1847
• Main Authors: Chakrabarty, Satrajit, DiTucci, Matthew J., Berden, Giel, Oomens, Jos, Williams, Evan R.
• Format: Article
• Language: English
• Subjects: Absorption spectra; Alkali metals; Alkaline earth metals; Analytical Chemistry; Banded structure; Binding; Bioinformatics; Biotechnology; Blue shift; Calcium ions; Charge density; Chemistry; Chemistry and Materials Science; Coordination compounds; Doppler effect; Earth; Focus: Application of Photons and Radicals for MS: Research Article; Infrared spectra; Ions; Mass spectrometry; Mathematical analysis; Melatonin; Metal ions; Metals; Organic Chemistry; Oxygen atoms; Proteomics; Protonation; Red shift
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1007/s13361-018-2020-0

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 (Mg 2+ , Ca 2+ , and Sr 2+ ) 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 Mg 2+ 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 ᅟ