Tuning thiol addition to squaraines by ortho-substitution and the use of serum albumin

Tuning the reactivity of squaraine dyes toward nucleophilic addition of thiols was investigated. A series of water soluble, aniline-derived squaraines were synthesized with various ortho substitutions to the squaraine ring. As hypothesized, we found that placing moderately electron donating groups in the ortho position conveyed intermediate reactivity to thiols between the essentially non-reactive hydroxyl-substituted squaraines and very reactive non-substituted squaraines. Furthermore, serum albumin was tested for its influence on the addition of thiols to the squaraines. The dyes bind in the hydrophobic cavities of the protein, and thus we expected serum albumin to affect the squaraines' reactivity. Rather than a protective effect by the protein, we found a cooperative effect for thiol addition.