Elucidating Protomer Structures and Fragmentation Dynamics of Protonated Aminobenzoic and Aminocinnamic Acids via Cryo-Ion Mobility and Laser Spectroscopy

Biological and synthetic systems featuring coexisting amino and carboxy groups, such as amino acids and peptides, serve as vital models for understanding hydrogen-bonded proton transport networks. Unambiguously identifying protonation isomers (protomers) is crucial yet challenging, as their structures are heavily modulated by both intrinsic stabilities and extrinsic solvent environments. Here, we present a comprehensive investigation into the protonation sites and geometric structures of protonated 4-aminobenzoic acid (H⁺(4ABA)) and 4-aminocinnamic acid (H⁺(4ACA)) utilizing a synergistic combination of cryogenic ion mobility-mass spectrometry (Cryo-IM-MS), UV photodissociation (UVPD) and IR-UV double-resonance (IR-UV DR) spectroscopy, and quantum chemical calculations. Cryo-IM-MS demonstrates a pronounced solvent-dependent protomer distribution, where electrospray ionization from H₂O/CH₃OH = 1:1 and CH₃CN solution predominantly yields the O- and N-protomers, respectively. Isomer-selective IR-UV DR spectroscopy successfully pins down the precise spatial orientations of the functional groups. Crucially, H⁺(4ABA) and H⁺(4ACA) exhibit markedly distinct photofragmentation pathways driven by the unsaturated alkenyl link in H⁺(4ACA). While the H⁺(4ABA) O-protomer undergoes competitive decarboxylation (CO₂ loss) and dehydration (H₂O loss), the extended π-conjugation in H⁺(4ACA) introduces a substantial kinetic bottleneck that completely suppresses CO₂ elimination. In the case of the H⁺(4ACA) N-protomer, the alkenyl group acts as an active, structurally preorganized hydrogen source, facilitating highly efficient H₂O loss via a tightly constrained transition state. These results provide profound fundamental insights into how structural modifications and local environments dictate competitive deactivation pathways, offering broader implications for the design and understanding of functional charge-transport materials.