Synthesis of squaraine-N-hydroxysuccinimide esters and their biological application as long-wavelength fluorescent labels.

We describe the synthesis and the fluorescence spectral characterization of two conjugatable long-wavelength fluorescence probes. These probes consist of a squaraine moiety, which is a cyanine-type chromophore with a central squarate bridge, and a reactive N-hydroxysuccinimide group for coupling with amino functions. One form is water soluble due to the presence of a sulfobutyl group; the other is water insoluble. The water-insoluble form was reacted with taurine to achieve water solubility and this squaraine-taurine conjugate displayed a very high affinity for bovine serum albumin. The squaraines exhibit short lifetimes and low quantum yields in water, with a significant increase in lifetime and quantum yield when bound to proteins. Their absorption maxima around 635 nm in water and 640 nm when bound to proteins allow excitation with the newly commercially available diode laser sources at 635, 645, and 650 nm. The spectral properties and photostabilities of the water-soluble squaraine probes are compared with those of the commercially available CY5-NHS ester.