Water soluble squaraine dyes for use as colorimetric stains in gel electrophoresis. Part 2

•Synthesis of dual-sulfonate bis(indolenine)squaraine metal salts by ion exchange.•Dyes are suitable as colorimetric stains for proteins in gel electrophoresis.•No improvement in stain quality observed over gels stained with piperidinium salt.•None of the gels stained with the metal salts could be imaged fluorimetrically.•The X-ray crystal structures of six squaraine variants in this series are reported.

In this study we demonstrate that it is possible to ion exchange the small organic cations used to charge balance the dual-pendant sulfonate groups on the particular squaraine system under investigation with metal ions, although we also report that problems with low solubility in water may arise. The ion exchanged squaraine dyes that were water soluble include the lithium, sodium and potassium salts, as well as the sulfonic acid variant, were examined as colorimetric stains for protein separation in sodium dodecylsulphate polyacrylamide gel electrophoresis. Although all four performed well as stains, none were better than the previously reported bis-piperidinium variant, which can also be imaged fluorimetrically. None of the gels stained with the ion exchanged dyes exhibited any similar fluorescence ability. Also reported here are the X-ray crystal structures of the bis-morpholinium, bis-piperidinium, bis-(3-hydroxypyridinium), bis-(2-amino-1,3-thiazolium), and bis-(2-aminopyridinium) variants as well as the bis-piperidinium variant of the dual-pendant sulfonate bis(benz[e]indolenine)squaraine.

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