LUMINESCENT SPECTRAL PROPERTIES OF RHODAMINE DERIVATIVES WHILE BINDING TO SERUM ALBUMIN ** N. Nizomov, 1* Z. F. Ismailov, 1 É. N. Kurtaliev, 1 Sh. N. Nizamov, 1 G. Khodzhaev, 2 and L. D. Patsenker 3 UDC 535.37:(547.633.6+547.962.3) We have studied the effect of blood serum albumin on the absorption and fluorescence spectra of rhodamine C (RC), rhodamine 6G (R6G), and rhodamine 3B (R3B). Interaction of the dye with protein is assessed using the binding parameters: binding constants and concentrations of binding sites. We have studied the effect of temperature on the binding parameters. We have observed that heating a mixture of the dye solution with protein for 30 min leads to an increase in the binding constant for rhodamine 3B with protein by a factor of 2, while the concentration of binding sites increases by a factor of 2.3. This is explained by features of the globular protein structure and a change in its conformation when heated. We have shown that rhodamine 3B at a concentration of 10 –5 M is the most effective among the studied rhodamine dyes for application as a fluorescent probe when studying conformational changes in blood serum protein. Key words: rhodamine dyes, fluorescence, serum albumin, binding constant, effect of temperature. Introduction. Fluorescent probes have a number of features which make them useful for studying proteins and membranes. First of all, the probe itself "selects" the binding area and can respond to a change not only in this moiety of the protein but also at other sites where new binding areas may appear, to which the probe is redistributed. This distinguishes the probe from labels covalently bonded to the protein. In order to study the behavior of fluorescent probes binding to biomolecules, it is desirable to have no overlap between the absorption regions of the probes and the biomolecules; i.e., the fluorescent probe should have an absorption band in the region above 300 nm. Fluorescent probes meeting this requirement include rhodamine dyes, which are widely used as the working substance in liquid la- sers [1, 2], silver-free photography [3], analytical chemistry [4, 5], in dosimetry of ionizing radiation [6–8], and for other purposes. Rhodamines are among a number of traditional dyes widely used in biomedical research and clinical diagnostics [9–11]. The aim of this work was to study the interaction between rhodamine dyes and biological systems by lumi- nescent spectral analysis methods. Experimental procedure. We used rhodamines (quantum electronics grade) and bovine serum albumin (BSA) (Medpreparat, Konotop, Ukraine). Triton X-100 was used as the nonionic detergent (ethoxylated n-1,1,3,3-tetramethyl- butylphenol (9–10 EO), Serva, Germany). Triton X-100 concentration in the solutions: 8 mM. All the solutions were prepared in twice-distilled water. The absorption spectra were measured on an SF-14 prism spectrophotometer, and the fluorescence spectra were measured on an ISP-51 spectrograph with FE ′ P-1 photoelectric attachment. As the detector, we used an FE ′ U-79 photoelectron multiplier. Fluorescence was excited by a 100-watt MBGN halogen lamp, from the spectrum of which we isolated light with wavelength 550 nm by means of a light filter. We used a light filter with halfwidth 4 nm from * To whom correspondence should be addressed. ** Report given at the Third International Conference on Liquid State Physics: Current Problems, May 27–31, 2005, Kiev, Ukraine. 1 Samarkand State University, 15 Universitetskii bul’v., Samarkand 703004, Uzbekistan. E-mail: nnizamov@ yandex.ru. 2 Samarkand Agricultural Institute, Uzbekistan. 3 Institute of Scintillation Materials, National Academy of Sci- ences of Ukraine, Kharkov. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 380–384, May–June, 2006. Original article submitted August 8, 2005. Journal of Applied Spectroscopy, Vol. 73, No. 3, 2006 0021-9037/06/7303-0432 ©2006 Springer Science+Business Media, Inc. 432