*2.2. Fluorescence Measurements*

The spectrofluorometer FP-8200 (JASCO, Hachioji, Tokyo, Japan) was used to measure the fluorescence spectra. The instrument had a quartz cell, and the slit width was 5 nm. The inner filter effects were corrected for the reabsorption of emitted light using the equation [35]:

$$F\_{cor} = F\_{obs} \times e^{(A\_{ex} + A\_{em})/2}$$

*Fcor* and *Fobs* are the corrected and observed fluorescence in the above equation, respectively, and *Aex* and *Aem* are excitation and emission wavelengths absorbance values, respectively. The BSA spectra were recorded for the binary system, which consisted of BSA-ERL and BSA-QUR, and the ternary system consisted of BSA-QUR-ERL and BSA-ERL-QUR systems. For the measurement of spectra in the binary system, the concentration of BSA was held constant at 1.50 µM and the concentration of ERL was between 0.00–27.5 µM and QUR was between (0.00–35 µM) The spectra were recorded at three temperatures of 298, 303 and 310 K for BSA-ERL system and 298 K for BSA-QUR system. The concentration of BSA was 1.50 µM in the presence of ERL, and the spectra were recorded with ERL 0.00–27.50 µM in the presence of QUR 5.5 µM in the ternary system.

The site marker studies were undertaken to establish the binding Site for ERL with phenylbutazone for Site I and Ibuprofen for Site II as markers. The spectra for BSA were recorded with ERL in the presence of site markers phenylbutazone and ibuprofen.
