Alcian Blue Stain - Chemistry

Chemistry

It is a tetravalent basic (cationic) dye with a copper (Cu2+, coordination 4 of 6, orbital configuration d9 with Jahn–Teller distortion) phthalocyanine nucleus (CuPc) with three or four pendent isothiouronium side chains imparting its bulkiness and positive charges. In order to qualify as an alcian blye family member there has to be at least 2 side chains and the mixtures often have 3 chains in average to qualify as 8G. Four tetramethylisothiouronium groups per molecule are shown in the picture. ICI had claimed an average of about three side chains per molecule, but analyses by Prof Scotts lab suggested between three and four. Most of them are at the 2(3) positions, as in the formula and sometimes a cartoon representation uses the methylene bridge criss crossing across the bond between these two positions to indicate that it could bind either of these two positions. A large number of isomers, differing in the positions of the cationic groups, are possible. Alcian blue 7GX carries fewer isothiouronium groups than 8GX. Similarly 5GX and 2GX may have even fewer side groups but it was not rigorously proven.

The phthalocyanine aromatic nucleus has a large Conjugated system which has a CBN (Conjugated bond number) of 48. However it is the charges on the isothiouronium side groups that still keeps it water soluble. These side groups can carry bulkier alkyl or aryl substituents rather than the 8x2 methyl groups as in the image given. These groups split off from the macrocyclic ring during the washing at the end of staining or by rather mind conditions (e.g. pH above 5.6) or during spontaneous degradation.

The metals in the Phthallocyanine nucleus and substituted groups directly attached to the aromatic nucleus determine colors of the members of the metal phthallocyanine family e.g. Alcian Blue and the copper phthalocyanine itself are blue, but brominated or chlorinated copper phthalocyanine and sulfonated copper phthalocyanine are green.

Alcian Blue has a relatively high solubility in salt solutions and stains slower than other dyes. By changing pH or ambient salt concentrations characteristic staining patterns can be obtained.