Pseudocoenzyme B12 and adenosyl-factor A: Electrochemical synthesis and spectroscopic analysis of two natural B12 coenzymes with predominantly 'base-off' constitution

Wolfgang Fieber, Bernd Hoffmann, Wolfgang Schmidt, Erhard Stupperich, Robert Konrat, Bernhard Kräutler

Veröffentlichungen: Beitrag in FachzeitschriftArtikelPeer Reviewed

Abstract

Pseudocoenzyme B12 (=Coß-(5'-deoxy-5'-adenosyl)-(adenin-7-yl)cobamide; 1) and adenosyl-factor A (=Coß-(5'-deoxy-5'-adenosyl)-(2-methyladenin-7-yl)cobamide; 3) are two natural analogues of coenzyme B12 (­enosylcobalamin-Coß-(5'-deoxy-5'-adenosyl)- (5,6-dimethyl-1H-benzimidazolyl)cobamide; 2), where the Co-coordinating 5,6-dimethyl-1H-benzimidazole nucleotide base of 2 is replaced by the purine bases adenine and 2-methyladenine. In contrast to 2, which exists solely in the 'base-on' form, UV/VIS spectroscopy qualitatively indicates 'base-off' constitution for 1 and 3 in aqueous solution. (cf. the established 'base-off' form as unexpected binding mode of B12 cofactors in several B12-dependent enzymes, such as in methionine synthase from Escherichia coli and in glutamate mutase from Clostridium cochlearium). In the present work, pseudocoenzyme B12 (1) was synthesized in 85% yield by alkylation with 5'-O-tosyladenosine of (adenin-7-yl)cob(I)amide, which was produced electrochemically from pseudovitamin B12 (Coß-cyano-(adenin-7-yl)cobamide). Likewise, adenosyl-factor A (3) was prepared in ca. 70% yield from factor A (=Coß-cyano-(2-methyladenin-7-yl)cobamide; 5). All the spectroscopic properties of 1 and 3 in aqueous solution indicated that these two Coß-(5'-deoxy-5'-adenosyl)-(adenin-7-yl) cobamides exist predominantly in a 'base-off' constitution, with minor but significant contributions of the 'base-on' form. From the UV/VIS spectra, the temperature-dependent equilibrium constants of the 'base-off'/'base-on' reconstitution reaction were determined as Kon (1)=0.30 and Kon (3)=0.48 at 25°, corresponding to a contribution of the 'base-on' forms of 23% for 1 and of 32% for 3.
OriginalspracheEnglisch
Seiten (von - bis)927-944
Seitenumfang18
FachzeitschriftHelvetica Chimica Acta
Jahrgang85
Ausgabenummer3
DOIs
PublikationsstatusVeröffentlicht - 2002
Extern publiziertJa

ÖFOS 2012

  • 104021 Strukturchemie

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