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Cobalamins and the spectrochemical series
1School of Chemistry, Faculty of Science, University of the Witwatersrand, Johannesburg, South Africa. susan.chemaly@wits.ac.za
Dalton Transactions (Cambridge, England : 2003)
|October 23, 2008
Summary
This study reveals a new ligand-to-metal charge transfer (LMCT) band in cobalamins (cbl) using UV-visible-NIR spectroscopy. This band provides insights into electronic transitions and ordering of cobalamins.
Area of Science:
- Bioinorganic Chemistry
- Spectroscopy
- Cobalamin Chemistry
Background:
- Cobalamins (vitamin B12 and its analogs) are crucial biomolecules with complex coordination chemistry.
- Understanding their electronic transitions is key to elucidating their function and reactivity.
- Previous studies focused on pi-pi* transitions, leaving other electronic bands less explored.
Purpose of the Study:
- To systematically investigate and characterize a novel absorption band (band A) in the UV-visible-NIR spectra of various cobalamins.
- To determine the electronic origin and properties of band A and compare it with known transitions.
- To establish the relationship between band A and the spectrochemical and nephelauxetic series for cobalamins.
Main Methods:
- UV-visible-NIR spectroscopy was employed to analyze a diverse set of cobalamins in aqueous and methanolic solutions.
- Spectral deconvolution techniques were used to resolve and analyze overlapping absorption bands.
- A new cobalamin derivative, OCrO(3)-cbl(-), was synthesized and characterized.
Main Results:
- A broad absorption band (band A) was identified in the red and NIR regions for several cobalamins, with extinction coefficients lower than alpha/beta bands.
- Band A's position correlates with the spectrochemical series, distinct from the nephelauxetic series observed for pi-pi* transitions.
- The study provides the first systematic investigation of ligand-to-metal charge transfer (LMCT) bands in cobalamins.
Conclusions:
- Band A is attributed to a ligand-to-metal charge transfer (LMCT) transition from the corrin ring's pi orbital to Co(III).
- This finding expands the understanding of cobalamin electronic spectroscopy beyond pi-pi* transitions.
- The systematic study of LMCT bands offers new avenues for probing cobalamin structure-function relationships.
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