CONFORMER-DEPENDENT PROTON-TRANSFER REACTIONS OF UBIQUITIN IONS

The conformations of ubiquitin ions before and after being exposed to proton transfer reagents have been studied by using ion mobility/mass spectrometry techniques. Ions were produced by electrospray ionization and exposed to acetone, acetophenone, n-butylamine, and 7-methyl-1,5, 7-triazabicyclo[4.4.0]dec-5-ene. Under the conditions employed, the +4 to +13 charge states were formed and a variety of conformations, whic h we have characterized as compact, partially folded, and elongated, h ave been observed. The low charge state ions have cross sections that are similar to those calculated for the crystal conformation. High cha rge states favor unfolded conformations. The ion mobility distribution s recorded after ions have been exposed to each base show that the low est charge state that is formed during proton-transfer reactions favor s a compact conformation. More open conformations are observed for the higher charge states that remain after reaction. The results show tha t for a given charge state, the apparent gas-phase acidities of the di fferent conformations are ordered as compact < partially folded < elon gated. (C) 1997 American Society for Mass Spectrometry.