The Structure of Methylcob(III)alamin in Aqueous Solution - A Water Molecule as Structuring Element of the Nucleotide Loop

Spectroscopic and Computational Studies of Co1+Cobalamin: Spectral and Electronic Properties of the “Superreduced” B12 Cofactor

“…The cob(I)alamin (Co 1+ Cbl) “superreduced” B 12 cofactor state is accessed in enzymatic systems by heterolytic cleavage of the Co−C bond of the methylcobalamin cofactor (MeCbl, Figure ), which contains a low-spin Co ion in the +3 oxidation state that is equatorially ligated by the four nitrogens of a tetrapyrrole macrocycle, known as the corrin ring, and axially coordinated by a methyl group in the “upper” position and 5,6-dimethylbenzimidazole (DMB) in the “lower” position. The DMB base is the terminus of an intramolecular nucleotide loop bound to the corrin ring at C 17 . − At low pH, DMB dissociates from the Co to yield a “base-off” species with spectroscopic properties similar to those of methylcobinamide (MeCbi) in which the nucleotide loop is clipped off at the phosphodiester bond. Reduction of MeCbl to cob(II)alamin (Co 2+ Cbl) results in the dissociation of the upper axial methyl group. − Further reduction of Co 2+ Cbl to Co 1+ Cbl leads to the loss of the lower axial DMB ligand, providing an alternate pathway to the 4-coordinate, low-spin Co 1+ Cbl species ,, that is one of the most powerful nucleophiles known to date. , …”

B12‐retro‐Riboswitches: Guanosyl‐Induced Constitutional Switching of B12 Coenzymes

“…However, only insignificant shifts were seen at the cobalt‐coordinated methyl group CoCH 3 (Δ δ =−0.04 ppm) and at C2N and C4N (Δ δ =0.04 and 0.08 ppm, respectively) of the DMB base. The similarity of the chemical‐shift values associated with the signals of the hydrogen atoms attached to the methylcobalamin moieties of Na‐ 7 and 2 7, 45 support a structure in solution that is largely base on. Slightly shifted values of some propionic side‐chain methylene protons and the methyl group C1A are indicative of altered local (de)shielding effects as a result of the appended base.…”

“…At room temperature, the spectra were rather similar to those of a base‐on methylcorrinoid, such as methylcobalamin ( 2 ; e.g., see ref. 7) and exhibited an enhanced absorbance near 263 nm (a strongly positive Δ ε value at 271 nm) as a result of the appended guanosylate group (Figure 3).…”

“…In the NMR spectra of Na‐ 7 in D 2 O (measured at 26 °C), the signals of 69 carbon nuclei and 86 carbon‐bound hydrogen atoms could be unambiguously assigned (with the help of homo‐ and heteronuclear 2D spectra; see the http://www.wiley-vch.de/contents/jc_2111/2008/f701365_s.pdf). In the 1 H NMR spectrum of Na‐ 7 (26 °C, 500 MHz), the signals of the protons attached to C1A (Δ δ =0.36 ppm), C32 (Δ δ =−0.39/−0.35 ppm), and C172 (Δ δ =−0.11/0.46 ppm) atoms of the corrin ligand experienced the largest shift differences compared with 2 ,7, 45 as was the proton attached to the ribose C2R atom (Δ δ =0.69 ppm; see Figure 4 (top) and the http://www.wiley-vch.de/contents/jc_2111/2008/f701365_s.pdf). However, only insignificant shifts were seen at the cobalt‐coordinated methyl group CoCH 3 (Δ δ =−0.04 ppm) and at C2N and C4N (Δ δ =0.04 and 0.08 ppm, respectively) of the DMB base.…”

“…5 In 1 , an exceptional 5,6‐dimethylbenzimidazolyl (DMB) α‐pseudonucleotide is appended to a no less unique cobalt–corrin unit 5. 6 The DMB base features an intramolecular cobalt coordination to the lower (α) axial position of the cobalt center in 1 , as in the “base‐on” form of the two mammalian B 12 cofactors methylcobalamin ( 2 )6, 7 and adenosylcobalamin (coenzyme B 12 ; 3 ) 6. 8…”