Revealing Spectral Characteristics of Shared Protons in Deprotonated Glutamic Acid Using Cryogenic Ion Trap Spectroscopy and Anharmonic Calculations

Shared protons in dicarboxylate motifs are essential biological components. Infrared (IR) spectroscopy is the premier tool to probe the OHO vibrations defining these structures, as vibrational frequencies directly reflect the underlying potential surface. Previously, infrared multiple photon dissociation (IRMPD) studies of deprotonated glutamic acid showed broad features, but their origin─thermal effects, multiple conformers, or intrinsic anharmonicity─remained unknown. To gain a better understanding, we utilized cryogenic ion spectroscopy at 4 K. We discovered that despite a single shared-proton conformer being dominant, broadband features persist, disappearing only upon deuteration. High-dimensional anharmonic calculations (27 modes) show that the OHO vibration is extensively coupled to combination bands and overtones of numerous low-frequency modes (700–1200 cm–1). This demonstrates that shared proton motion is intrinsically linked to the dynamics of the entire molecule rather than being an isolated vibration.