Axial Solvent Coordination in “Base-Off” Cob(II)alamin and Related Co(II)-Corrinates Revealed by 2D-EPR

Abstract

Detailed information on the structure of cobalt(II) corrinates is of interest in the context of studies on the coenzyme B(12) catalyzed enzymatic reactions, where cob(II)alamin has been identified as a reaction intermediate. Cob(II)ester (heptamethyl cobyrinate perchlorate) is found to be soluble in both polar and nonpolar solvents and is therefore very suitable to study solvent effects on Co(II) corrinates. In the literature, Co(II) corrinates in solution are often addressed as four-coordinated Co(II) corrins. However, using a combination of continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) and pulse ENDOR (electron nuclear double resonance) at different microwave frequencies we clearly prove axial ligation for Cob(II)ester and the base-off form of cob(II)alamin (B(12r)) in different solvents. This goal is achieved by the analysis of the g values, and the hyperfine couplings of cobalt, some corrin nitrogens and hydrogens, and solvent protons. These parameters are shown to be very sensitive to changes in the solvent ligation. Density functional computations (DFT) facilitate largely the interpretation of the EPR data. In the CW-EPR spectrum of Cob(II)ester in methanol, a second component appears below 100 K. Different cooling experiments suggest that this observation is related to the phase transition of methanol from the alpha-phase to the glassy state. A detailed analysis of the EPR parameters indicates that this transition induces a change from a five-coordinated (above 100 K) to a six-coordinated (below 100 K) Co(II) corrin. In a CH(3)OH:H(2)O mixture the phase-transition properties alter and only the five-coordinated form is detected for Cob(II)ester and for base-off B(12r) at all temperatures. Our study thus shows that the characteristics of the solvent can have a large influence on the structure of Co(II) corrinates and that comparison with the protein-embedded cofactor requires some caution. Finally, the spectral similarities between Cob(II)ester and base-off B(12r) prove the analogies in their electronic structure.

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

“…25,58,59 Both spectra exhibit a broad feature in the visible region with an absorption maximum at 470 nm (21,200 cm −1 ); however, the Co 2+ CFeSP spectrum possesses additional intensity near 420 nm (23,800 cm −1 ) that we attribute to S→Fe 3+ charge-transfer transitions from the oxidized [Fe 4 S 4 ] cluster present in this sample. In support of this assignment, taking the difference of the two Abs spectra presented in Figure 2 yields a trace ( Figure S1) that is very similar to the Abs spectrum reported for the [Fe 4 S 4 ] 2+ component of CFeSP lacking the corrinoid cofactor.…”

Reactions of Cobinamide with Glucose and Fructose

“…This observation supports the necessity of reductant coordination on Co 2+ for the further reduction stage since relatively tightly bound DMBI is present in Cbl(II) molecule [4] which probably can not be substituted by monosaccharide molecule during the process. Relatively weakly bound water molecule is present in Cbi(II), [31] that is likely to be easily substituted during the reaction of metallocomplex with monosaccharide. The coordination of reducing agent on Cbi(II) implies that the formation of Cbi(I) in this case is a result of inner sphere electron transfer.…”

26.5–40 GHz Ka-band pulsed EPR spectrometer

“…The inverting RF pulse for protons ( RF ϳ 45 MHz) at nominal 800W output of the RF amplifier is about 17 s long, which corresponds to the RF field B 2 ϳ 0.7 mT and a field-power conversion factor B 2 /͌P ϳ 25 T/͌W. For RF ϭ 11.23 MHz (the Zeeman frequency of 13 C at B o ϭ 1.049 T) B 2 /͌P ϳ 76 T/͌W was found. Thus, our conversion factor for the proton Zeeman frequency is considerably worse than that achieved in the Bruker Q-band resonator (14).…”

“…The several published applications were mostly ESEEM, demonstrating that the K a band is indeed suitable for ESEEM spectroscopy (11)(12)(13). Both of the described spectrometers employed a high-power (ϳ100 W) traveling wave tube amplifier (TWTA), which allows one to obtain microwave fields 2␤B 1 ϳ 25 MHz in conventional volume resonators accommodating high-volume sample tubes with the outer diameter (OD) of 2.5-3 mm.…”