Vibrational signatures of zwitterionic and charge-solvated structures for alkaline earth-tryptophan dimer complexes in the gas phase

Abstract

Infrared multiple-photon dissociation (IR-MPD) spectra in the N–H and O–H stretching region (3000–3700 cm −1) are reported for gas-phase monomeric M 2+Trp and dimeric M 2+Trp 2 complexes (where M = Mg, Ca, Sr, and Ba). The spectra are obtained by irradiating the complexes in the Penning trap of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, using a tunable continuous-wave (cw) optical parametric oscillator (OPO) laser in combination with a fixed wavelength CO 2 laser (10.6 μm). These spectra are compared with previously recorded mid-IR-MPD spectra, using the free electron laser FELIX, and are interpreted based on harmonic frequency calculations performed with density-functional theory (DFT). The experimental spectra show that a simple assignment of bands can be made to distinguish zwitterionic (ZW) from charge solvation (CS) complexes. In particular, the carboxylic acid O–H stretch at ∼3550 cm −1 identifies the presence of a CS structure, whereas the NH 3 + antisymmetric stretching mode in the 3150–3375 cm −1 range is diagnostic of a ZW structure. For the monomeric Ba 2+Trp complex, exclusively the ZW structure is observed. Conversely, for the dimeric complexes of M 2+Trp 2 (M = Sr and Ba) merely CS/CS geometries are confirmed. Surprisingly, for smaller alkaline earth metal dications (M = Mg and Ca), the IR-MPD spectra are consistent with the presence of mixed CS/ZW dimers. This is contrary to most previous trends for alkali metals, where larger cations typically favor ZW stabilization.