Long-wavelength dyes for fluorescent detection of proteins

Albumins are the major plasma proteins circulating in the bloodstream. Since serum albumin is a reliable prognostic indicator for morbidity and mortality, liver and other diseases and acts as carrier for nutritional factors and drugs, analytical methods of albumin study are in high demand. Squaraine dyes have been reported as efficient noncovalent fluorescent labels for albumins, exhibiting high fluorescence quantum yields when bound to these proteins.

A wide series of squaraine dyes based on indolenine, benzoxazole, benzothiazole and benzoselenazole heterocycles (Fig. 5) were tested for their sensitivity to various albumins. It is shown that benzothiazole squaraine dyes containing long N-alkyl substituents are highly selective to albumins and increase the fluorescence intensity up to 400 times in the presence of these proteins. Using of some of these benzothiazole dyes allows quantification of HSA in the range from 0.2 µg/ml to 500 µg/ml that is comparable with commercially used dyes such as CBB and Pyrogallol Red Protein.

Figure 5. Structures of studied squaraine dyes.

It was shown for HSA-sensitive benzothiazole squaranines that nature of the substituent in squaraine cycle modulates affinity of the dyes to binding site I or II of human serum albumin.
We consider these researches to result in the development of squaraine - based fluorometric assay for the study of pharmacokinetics and pharmacodynamics of potential drugs and characterization of their binding to HSA.

• K. D. Volkova, V. B. Kovalska, M. Yu. Losytskyy, L. V. Reis, P. F. Santos, P. Almeida, D. E. Lynch, S. M. Yarmoluk Aza-substituted squaraines for the fluorescent detection of albumins // Dyes and Pigments. – 2011. – Vol. 90. – P. 41–47.
• K.D. Volkova, V.B. Kovalska, M.Y. Losytskyy, A. Bento, L.V. Reis, P.F. Santos, P. Almeida, S.M. Yarmoluk. Studies of Benzothiazole and Benzoselenazole Squaraines as Fluorescent Probes for Albumins Detection.// J. Fluorescence. – 2008. – Vol. 18, №5. – P. 877-882.
• K.D. Volkova, V.B. Kovalska, A.L. Tatarets, L.D. Patsenker, D.V. Kryvorotenko, S.M. Yarmoluk. Spectroscopic study of squaraines as protein-sensitive fluorescent dyes // Dyes and Pigments. – 2007. – Vol. 72, No. 3 – P. 285–292.