Mass Spec Ion Structure Controlled By Solvent Effects

By employing different solvent systems, Zhixin Tian and Steven R. Kass of the University of Minnesota, Minneapolis, used electrospray ionization (ESI) mass spectrometry to selectively form and analyze two ionic structures of tyrosine (J. Am. Chem. Soc., DOI: 10.1021/ja802088u). The advance could point the way to performing similar chemical analyses on more complex biomolecules, such as proteins. ESI transports molecules of interest from solution to the gas phase for MS analysis. Many biomolecules can be ionized at different sites, and sometimes the most stable ions in the liquid and gas phases are different, Kass says. For researchers who are attempting to identify protein fragments or study biomolecule reactivity by MS, it’s important to know whether ESI produces the gas-phase or the liquid-phase structure. The Minnesota team used trimethylsilyl azide to derivatize tyrosine carboxylate, which is the most stable ion in solution, and tyrosine phenoxide, which is the most stable ion in the gas phase. They analyzed the MS fragmentation patterns to differentiate between the two anions, which have the same mass. By using methanol/water or acetonitrile solvent systems, the researchers controlled the relative amounts of carboxylate and phenoxide anions they observed.