Spectroscopic and computational studies of reversible O₂ binding by a cobalt complex of relevance to cysteine dioxygenase

Abstract

The substitution of non-native metal ions into metalloenzyme active sites is a common strategy for gaining insights into enzymatic structure and function. For some nonheme iron dioxygenases, replacement of the Fe(ii) center with a redox-active, divalent transition metal (e.g., Mn, Co, Ni, Cu) gives rise to an enzyme with equal or greater activity than the wild-type enzyme. In this manuscript, we apply this metal-substitution approach to synthetic models of the enzyme cysteine dioxygenase (CDO). CDO is a nonheme iron dioxygenase that initiates the catabolism of l-cysteine by converting this amino acid to the corresponding sulfinic acid. Two mononuclear Co(ii) complexes (3 and 4) have been prepared with the general formula [Co(Tp)(CysOEt)] (R = Ph (3) or Me (4); Tp = hydrotris(pyrazol-1-yl)borate substituted with R-groups at the 3- and 5-positions, and CysOEt is the anion of l-cysteine ethyl ester). These Co(ii) complexes mimic the active-site structure of substrate-bound CDO and are analogous to functional iron-based CDO models previously reported in the literature. Characterization with X-ray crystallography and/or H NMR spectroscopy revealed that 3 and 4 possess five-coordinate structures featuring facially-coordinating Tp and S,N-bidentate CysOEt ligands. The electronic properties of these high-spin (S = 3/2) complexes were interrogated with UV-visible absorption and X-band electron paramagnetic resonance (EPR) spectroscopies. The air-stable nature of complex 3 replicates the inactivity of cobalt-substituted CDO. In contrast, complex 4 reversibly binds O at reduced temperatures to yield an orange chromophore (4-O). Spectroscopic (EPR, resonance Raman) and computational (density functional theory, DFT) analyses indicate that 4-O is a S = 1/2 species featuring a low-spin Co(iii) center bound to an end-on (η) superoxo ligand. DFT calculations were used to evaluate the energetics of key steps in the reaction mechanism. Collectively, these results have elucidated the role of electronic factors (e.g., spin-state, d-electron count, metal-ligand covalency) in facilitating O activation and S-dioxygenation in CDO and related models.

Dioxygenation of metal(II)‐cysteinato complexes in CDO biomimetic models: Can ruthenium and osmium reach iron performances?

“…Analogously to Ru‐catalyst, the reaction proceeds through a singlet transition state ts2 , in which the second oxygen atom approaches the sulfur one to form the ring structure 1 int3 . Our results identified this transformation as the rate‐limiting step, in agreement with other CDO mimetics having other divalent metals in place of iron . In both cases, the reaction proceed along the singlet spin state after the spin crossing just described.…”

“…On the contrary, metals such Ru and Os, which instead adopt a low spin configuration, are less efficient in providing a low barrier for the oxygen‐sulfur binding process. The presence of a paramagnetic Fe ( S = 2) allows the unpaired spin density to delocalize onto the thiolate ligand via metal–sulfur covalency, while low spin metals failed in that, as was also recently described for a low‐spin Co variant of a CDO mimetic …”

A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity

“…Cyclic voltammetry of 1 reveals a single quasi-reversible wave ( E 1/2 = −0.81 V vs Fc + /Fc; Δ E pp = 200 mV) (Figure c). The Co II center in 1 is thus highly reducing compared to other Co II complexes that bind O 2 , ,,, likely because of the strongly donating, dianionic dithiolato ligand set.…”

“… a n.r. indicates not reported. b Reference . c Reference . d Reference . e Reference . f Reference . g Reference . h Reference . …”

A synthetic model of the nonheme iron–superoxo intermediate of cysteine dioxygenase

“…Since then, we surmised that it might be possible to extend the lifetime of reactive species by preparing CDO models with alternative substrates that are similar to, but not identical with, the native substrate. Moreover, our experience with cobalt-substituted CDO models, 14 as well as a report from the Hikichi group, 12b suggested that the hydrotris(3,5-dimethylpyrazolyl)borate ligand (Tp Me2 ) facilitates formation of metal-superoxo species due to its small steric profile. With these ideas in mind, we prepared the complex, [Fe(Tp Me2 )(2-ATP)] ( 1 ), where 2-ATP is 2-aminothiophenolate (Figure 1).…”