Conformation of protonated glutamic acid at room and cryogenic temperatures. Issue 17 (27th January 2017) - Bouchet, Aude
Abstract : Linear infrared spectroscopy of protonated glutamic acid in a cryogenic ion trap allows for the clear-cut and quantitative identification of the two conformers of this fundamental biomolecule. Abstract : Recognition properties of biologically relevant molecules depend on their conformation. Herein, the conformation of protonated glutamic acid (H + Glu) isolated in quadruple ion traps is characterized by vibrational spectroscopy at room and cryogenic temperatures and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. The infrared multiple photon dissociation (IRMPD) spectrum recorded in the fingerprint range at room temperature using an IR free electron laser is attributed to the two most stable and nearly isoenergetic conformations (1-cc and2-cc ) with roughly equal population (Δ G 298 = 0.0 kJ mol −1 ). Both have bridging CO⋯(HNH) + ⋯OC ionic H-bonds of rather different strengths but cannot be distinguished by their similar IRMPD spectra. In contrast, the higher-resolution single-photon IRPD spectrum of H2 -tagged H + Glu recorded in the conformation-sensitive X–H stretch range in a trap held at 10 K distinguishes both conformers. At low temperature, 1-cc is roughly twice more abundant than2-cc, in line with its slightly lower calculated energy (Δ E 0 = 0.5 kJ mol −1 ). This example illustrates the importance of cryogenic cooling, single-photon absorption conditions, and the consideration of the X–H stretch range for the identification of biomolecular conformations involving hydrogen bonds.