The structures and properties of anionic tryptophan complexes

Abstract

The physicochemical properties of [Trp_n–H]⁻ and [Trp_n⋯Cl]⁻ (n = 1, 2) have been investigated in a combined computational and experimental infrared multiple dissociation (IRMPD) study. IRMPD spectra within the 850–1900 cm⁻¹ region indicate that deprotonation is localized on the carboxylic acid moiety in [Trp_n–H]⁻ clusters. A combination of hydrogen bonding and higher order charge–quadrupole interactions appear to influence cluster geometries for all investigated systems. Calculated global minimum and low energy geometries of [Trp⋯Cl]⁻ and [Trp₂⋯Cl]⁻ clusters favour coordination of the halide by the indole NH. [Trp₂–H]⁻ and [Trp₂⋯Cl]⁻ exhibit additional π–π interactions between the heterocyclic side chains.