Electrospray ionization (ESI) with its isolation of biomolecules in the gas phase eliminates interactions with the solvent. A stepwise examination of these interactions by gradually increasing the number of bound water molecules allows for studying these solvent effects in detail. This approach effectively bridges the gap between isolated molecules and aqueous environments. Remarkably, the addition of just one water molecule can already cause significant structural changes, such as altering the protonation site. Over the past three decades, considerable efforts have been dedicated to developing experimental techniques for studying hydrated species in the gas phase, primarily through the use of electrospray ionization sources. We have designed and built a hydration ESI source based on Shinji Nonose et al. (2005), which will be further commissioned. Meanwhile we investigated already the effects of single hydration on protonated phosphotyrosine.
