A spectroscopic and computational exploration of tryptophan–water cluster structures in the gas phase

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Abstract
Mass-selected R2PI spectra of hydrated tryptophan clusters, generated through thermal evaporation into an expanding moist argon jet, have been recorded and analysed using ultra-violet hole-burn and infra-red ion dip spectroscopy, and interpreted in the light of a series of ab initio calculations for singly, doubly and triply hydrated structures. Despite the wide range of conformers populated in the monomer, the majority of the hydrated tryptophan clusters populate two, very similar structures only. Analysis of their infrared ion-dip spectra does not favour their assignment to either single or double hydrates, but does support their assignment to a pair of near degenerate, triply hydrated clusters. In contrast to other, related molecules, the most stable monomer tryptophan conformation is altered by the hydration. Time of flight mass spectra, reflecting the population of hydrated tryptophan clusters prepared through thermal evaporation into an expanding moist argon jet, differ strikingly from the equivalent spectra generated under laser desorption conditions, where a propensity for generating signal in the Trp·W(3)+ mass channel is observed.