Vitamin B12 as an allosteric cofactor; dual fluorescence, hysteresis, oscillations and the selection of corrin over porphyrin

Dalton Transactions : an International Journal of Inorganic Chemistry
Susan M ChemalyJohn M Pratt

Abstract

Studies of the emission spectra of four Co(III) cobinamides (diaquo-, aquohydroxo-, dihydroxo- and dicyano-) show (1) that the excited states corresponding to the alphabeta and epsilon absorption bands behave like the S(1) and S(2) levels in the non-alternant hydrocarbon azulene (with emission from S(2)> S(1) in violation of Kasha's rule) and (2) that the excited states include a TICT (twisted intramolecular charge transfer) mechanism, as in the simpler cyanines, but where the TICT state gives rise to dual fluorescence instead of cis-trans isomerisation. Combined with the previously reported dual fluorescence from the S(1) level in synthetic metal corrinoids and in the naturally-occurring metal-free corrin, this provides evidence that the existence of an additional (metastable) ground state with a significantly different vibronic splitting and nuclear configuration is an intrinsic property of the basic corrin ligand (irrespective of the nature of the side-chains and the metal ion or even the absence of a metal) which distinguishes it from porphyrin. The occurrence of hysteresis (and its associated oscillations) in redox reactions of the cobinamides involving both the Co(III/II) and Co(II/I) couples indicates that the corrin lig...Continue Reading

Citations

Sep 28, 2007·Natural Product Reports·Gemma L HollidayMartin J Warren
Nov 18, 2011·The Journal of the Acoustical Society of America·Michael A Ainslie, Timothy G Leighton
Nov 9, 2019·Angewandte Chemie·Anant S BalijepalliMark W Grinstaff

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