*3.4. UV Absorption Studies*

The two critical aspects that are studied by UV absorption are changes in the protein structure and protein–ligand complex formation [50]. The absorption spectral changes for BSA occur in protein–ligand complex formation and therefore infers a static quenching mechanism, whereas no spectral changes will be recorded in dynamic quenching [51]. Furthermore, the intensity of the BSA absorption peak at 280 nm was higher on interaction with ERL (Figure 3a), suggesting a static quenching mechanism between BSA and ERL.

Spectra for BSA was compared to spectra of the studied ligands ERL and QUR, BSA-ERL complex, and BSA-QUR complex. An increase in absorption spectra was observed for the BSA-ERL system compared to BSA alone. The ERL showed different absorption peaks compared to both BSA and BSA-ERL systems. The higher absorption peak for the BSA-QUR system was observed compared to BSA alone, and the spectra were different from the QUR absorption spectra.

The absorption spectra for the ternary system (BSA-QUR)-ERL was also studied, and it was different from both the binary system spectra (Figure 3b).

ment regimen.

*3.4. UV Absorption Studies* 

**Figure 3.** Ultraviolet–visible spectra for BSA (1.5 μM) in presence and absence of ERL (0.00–27.5 μM) (**a**); UV-visible spectra comparison for BSA, ERL, BSA-ERL, QUR, BSA-QUR, and BSA-ERL in the presence of QUR (**b**). **Figure 3.** Ultraviolet–visible spectra for BSA (1.5 µM) in presence and absence of ERL (0.00–27.5 µM) (**a**); UV-visible spectra comparison for BSA, ERL, BSA-ERL, QUR, BSA-QUR, and BSA-ERL in the presence of QUR (**b**).

with higher plasma area under the curve (AUC) for ERL. One of the most common adverse events related to the ERL treatment regimen is skin disorders, including acneiform rash, xeroderma (dry skin), pruritus, and paronychia. Sometimes, these side effects are so severe and necessitate treatment interruption or cessation [49]. One of the reasons attributed to the severity of these side effects for ERL is its plasma concentrations. Therefore, the severity of side effects influences patients' adherence to the treatment regimen. Since QUR may affect the AUC of ERL in plasma, increasing the plasma concentrations of ERL which might lead to unwanted adverse events affecting patients' adherence to the treat-

The two critical aspects that are studied by UV absorption are changes in the protein structure and protein–ligand complex formation [50]. The absorption spectral changes for BSA occur in protein–ligand complex formation and therefore infers a static quenching mechanism, whereas no spectral changes will be recorded in dynamic quenching [51]. Furthermore, the intensity of the BSA absorption peak at 280 nm was higher on interaction with ERL (Figure 3a), suggesting a static quenching mechanism between BSA and ERL.
