Abstract
AbstractThe (formal) replacement of Co in cobalamin (Cbl) by NiII generates nibalamin (Nibl), a new transition‐metal analogue of vitamin B12. Described here is Nibl, synthesized by incorporation of a NiII ion into the metal‐free B12 ligand hydrogenobalamin (Hbl), itself prepared from hydrogenobyric acid (Hby). The related NiII corrin nibyric acid (Niby) was similarly synthesized from Hby, the metal‐free cobyric acid ligand. The solution structures of Hbl, and Niby and Nibl, were characterized by spectroscopic studies. Hbl features two inner protons bound at N2 and N4 of the corrin ligand, as discovered in Hby. X‐ray analysis of Niby shows the structural adaptation of the corrin ligand to NiII ions and the coordination behavior of NiII. The diamagnetic Niby and Nibl, and corresponding isoelectronic CoI corrins, were deduced to be isostructural. Nibl is a structural mimic of four‐coordinate base‐off Cbls, as verified by its ability to act as a strong inhibitor of bacterial adenosyltransferase.
Highlights
Active vitamin B12 derivatives exclusively utilize cobalt as their specific transition metal center, which is bound and activated exquisitely by a helical corrin macrocycle.[1]
We report on the first nickel-complexes of natural corrin ligands, including nibalamin (Nibl)
We describe the syntheses of crystalline nibyric acid (Niby), the novel NiII complex of hydrogenobyric acid (Hby),[3] and hydrogenobalamin (Hbl), the metalfree complete B12 ligand
Summary
Active vitamin B12 derivatives exclusively utilize cobalt as their specific transition metal center, which is bound and activated exquisitely by a helical corrin macrocycle.[1]. Koppenhagen and co-workers, back in the 1970s, reported the isolation of Hbl from a Chromatium strain supplemented with 5,6-dimethylbenz-imidazole (DMB) They were able to characterize Hbl by UV/Vis-spectroscopy and demonstrated that it could be converted into vitamin B12 by insertion of cobalt,[4e,7] and later reported its mass spectrum.[8]. The UV/Vis absorption spectrum of an aqueous solution of Niby displayed bands at 464 nm (shoulder), 448 nm and 334 nm (Figure 1), and exhibited similar gross features to those observed in an absorption spectrum of the Ni-corrin NiCor (but with a slightly red-shifted maxima).[6a,c] The solution structure of the diamagnetic NiII-corrin Niby (molecular formula C45H64N10O8Ni, for HR mass spectra see SI, Figure S3) was analyzed by NMR spectroscopy, providing assignment of all 52 non-exchangeable H atoms and 44 C atoms (see SI, Figure S4 and Table S1). I) 0.5 m Ni(OAc) pH 6, 1 h, 90 8C, Ar. ii) 3 equiv B12 nucleotide moiety,[1a,12] HOBt, EDC*HCl, H2O, 0 8C, 4 d. iii) 0.5 m Ni(OAc) pH 6, 1 h, 90 8C, Ar (see the SI for details)
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