Conformation of protonated glutamic acid at room and cryogenic temperatures

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 and 2-cc) with roughly equal population (ΔG₂₉₈ = 0.0 kJ mol^-1). Both have bridging C[double bond, length as m-dash]O(HNH)⁺O[double bond, length as m-dash]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 than 2-cc, in line with its slightly lower calculated energy (ΔE₀ = 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.