抄録
This paper deals with so called red-yellow shift, the color change of corrinoid compounds caused by pH change. The relation between absorption spectrum and electronic charge in cyanocobalamin, monocyano-monoaquocobinamide, methylcobalamin and monomethyl-monoaquocobinamide was examined in a range of pH 1〜11. A spectral change accompanied with a charge change was found to occur at pH 2.5〜3.5 and at pH 8〜9 respectively in methylcobalamin and in monocyano-monoaquocobinamide, while cyanocobalamin was always neutral and red in the whole pH range, and monomethyl-monoaquocobinamide was always positive and yellow above pH 3. Although the results described above suggested that the yellow color was due to the protonation, monomethyl-monothiocyanatocobinamide was found to be neutral in spite of its yellow color, the absorption maximum being at 475mμ. Considering that the absorption maximum of monomethyl-monopyridinocobinamide and monomethyl-monocyanocobinamide was at 523mμ and at 580mμ respectively, it is concluded that the visible absorption spectrum of a corrinoid compound with a certain upper ligand depends upon the lower ligand very much. The color change caused by protonation can be thus explained as a result of a replacement of the lower ligand.
