Probing Hydrogen-Bonding Interactions within Phenol-Benzimidazole Proton-Coupled Electron Transfer Model Complexes...

Hydrogen-bonding interactions within a series of phenol-benzimidazole model proton-coupled electron transfer (PCET) dyad complexes are characterized using cryogenic ion vibrational spectroscopy. A highly red-shifted and surprisingly broad (>1000 cm^-1) transition is observed in one of the models and assigned to the phenolic OH stretch strongly H-bonded to the N₍₃₎ benzimidazole atom. The breadth is attributed to a combination of anharmonic Fermi-resonance coupling between the OH stretch and background doorway states involving OH bending modes and strong coupling of the OH stretch frequency to structural deformations along the proton-transfer coordinate accessible at the vibrational zero-point level. The other models show unexpected protonation of the benzimidazole group upon electrospray ionization instead of at more basic remote amine/amide groups. This leads to the formation of HO-⁺HN₍₃₎ H-bond motifs that are much weaker than the OH-N₍₃₎ H-bond arrangement. H-bonding between the N₍₁₎H⁺ benzimidazole group and the carbonyl on the tyrosine backbone is the stronger and preferred interaction in these complexes. The results show that conjugation effects, secondary H-bond interactions, and H-bond soft modes strongly influence the OH-N₍₃₎ interaction and highlight the importance of the direct monitoring of proton stretch transitions in characterizing the proton-transfer reaction coordinate in PCET systems.