NRVS investigation of ascorbate peroxidase compound II: Observation of Iron(IV)oxo stretching.

Abstract The protonation state of ascorbate peroxidase compound II (APX-II) has been a subject of debate. A combined X-ray/neutron crystallographic study reported that APX-II is best described as an iron(IV)hydroxide species with an Fe O distance of 1.88 A (Kwon, et al. Nat Commun 2016, 7, 13,445), while X-ray absorption spectroscopy (XAS) experiments (utilizing extended X-ray absorption fine structure (EXAFS) and pre-edge analyses) indicate APX-II is an authentic iron(IV)oxo species with an Fe O distance 1.68 A (Ledray, et al. Journal of the American Chemical Society 2020, 142, 20,419). Previous debates concerning ferryl protonation states have been resolved through the application of Badger's rule, which correlates Fe O bond distances with Fe O vibrational frequencies. To obtain the required vibrational data, we have collected Nuclear Resonance Vibrational Spectroscopy (NRVS) data for APX-II. We observe a broad vibrational feature in the range associated with iron(IV)oxo stretching (700–800 cm−1). This feature appears to have two peaks at 732 cm−1 and 770 cm−1, corresponding to Fe O distances of 1.69 and 1.67 A, respectively. The broad vibrational envelope and the presence of multiple resonances could reflect a distribution of hydrogen bonding interactions within the active-site pocket.