Ergothioneine and related histidine derivatives in the gas phase: tautomer structures determined by IRMPD spectroscopy and theory.

L-Ergothioneine (ET) is a sulfur-containing derivative of the amino acid histidine that offers unique antioxidant properties. The enzyme independent redox-chemistry of ET relies on the availability of the thiol tautomer to allow oxidative formation of disulfide bridges, i.e., the tautomeric equilibrium. To study the intrinsic properties of ET the tautomeric equilibrium is studied in the gas-phase by infrared multiphoton dissociation (IRMPD) spectroscopy. The IR ion spectra of isolated molecular ions of ET and of the biosynthetic precursors of ET, i.e., hercynine and N-epsilon-methyl-hercynine are acquired. The analyte structures are independently investigated by density functional theory (DFT) and computed linear IR-spectra of tautomer ion structures are compared with the gas-phase spectra for identification. For the molecular ion of ET the simulated IR spectra of thione and thiol structures match the recorded IRMPD spectrum and that prevents an individual structure assignment. On the other hand, theory suggests that ET adopts a thione tautomer in MeOH solution which could be carried over from the condensed phase to gas phase and could be kinetically trapped after effective electrospray phase transfer and desolvation. Such a non-thermal behavior is also found for the molecular ions of protonated hercynine and N-epsilon-methylhercynine. Contrary to that, the sodium complex ions of ET, hercynine and N-epsilon-methyl-hercynine adopt the respective ground structures predicted by theory, which are reliably identified spectroscopically. For ET the thione tautomer is by far the most stable isomer in the sodium complex molecular ion.