Modeling the interactions between peptide functions and Sr2+: formamide–Sr2+ reactions in the gas phase

Abstract

The interactions between formamide, which can be considered a prototype of a peptide function, and Sr²⁺ have been investigated by combining nanoelectrospray ionization/mass spectrometry techniques and G96LYP DFT calculations. For Sr an extended LANL2DZ basis set was employed, together with a 6-311+G(3df,2p) basis set expansion for the remaining atoms of the system. The observed reactivity seems to be dominated by the Coulomb explosion process yielding [SrOH]⁺ + [HNCH]⁺, which are the most intense peaks in the MS/MS spectra. Nevertheless, additional peaks corresponding to the loss of HNC and CO indicate that the association of Sr²⁺ to water or to ammonia leads to long-lived doubly charged species detectable in the timescale of these experimental techniques. The topology of the calculated potential energy surface permits us to establish the mechanisms behind these processes. Although the interaction between the neutral base and Sr²⁺ is essentially electrostatic, the polarization triggered by the doubly charged metal ion results in the activation of several bonds, and favors different proton transfer mechanisms required for the formation of the [SrOH]⁺, [SrOH₂]²⁺ and [SrNH₃]²⁺ products.