Examination of tryptamine-squaraine complexes as both colorimetric and fluorometric stains in gel electrophoresis

• Use of bis-tryptaminium dual-sulfonate bis(indolenine)squaraine salt.
• Ternary complexes indicate molecular interactions, observed in UV-vis.
• No enhanced fluorescence (above that of other squaraines) in protein solutions.
• X-ray crystal structures of two tryptamine proton-transfer salts are reported.
• Electrophoresis gels stained with bis-tryptaminium squaraine fluoresce.

A water-soluble dual pendent squaraine with tryptaminium cations has been mixed with mono-nitroaromatics in solution to create ternary complexes with the aim of creating indole-nitroaromatic charge-transfer pairs. Examination of the X-ray crystal structures of two tryptamine-nitroaromatic complexes, without the squaraine moiety, revealed that neither demonstrated charge-transfer pair or stacking associations between the indole and nitroaromatic rings. Examination of the fluorescence performance upon illumination with 300 nm light of each ternary squaraine complex, particularly in the presence of an equimolar amount of protein, showed that the levels across the series were no more than that observed for the base bis-tryptaminium squaraine, as was observed in a previous study. However, electrophoresis gels stained with the base bis-tryptaminium squaraine, whose X-ray crystal structure is also reported, were found to fluorescence; the only other squaraine dye of this type to do so in addition to the bis-piperidinium squaraine of previous studies.

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