Carbethoxylation of coordinated histidine by diethylpyrocarbonate

The metastable, reversible carbethoxylation of histidine in the cobalt(III) complexes (ethylene-diamine)(histidine)chlorocobalt(III) chloride, [Co(III)(en)ClHis]Cl, and (diethylenetriamine) (histidine)cobalt(III) dichloride, [Co(III)(dien)His]Cl2, following reaction with diethylpyrocarbonate, was observed using UV spectroscopy. These observations indicate that diethylpyrocarbonate can react with a coordinated imidazole ring. CD transitions associated with coordinated carbethoxyhistidine in [Co(III)(en)ClHis]Cl and [Co(III)(dien)His]Cl2 were also observed. The molar ellipticities of the CD bands observed for carbethoxy [Co(III)(en)ClHis]Cl or carbethoxy [Co(III) (dien)His]Cl2 are much larger than the molar ellipticities of the CD transition associated with carbethoxy N alpha-acetyl-L-histidine, indicating that loss of rotational freedom of the imidazole ring could be a major factor in the enhanced CD.