2.2.6. Circular Dichroism Spectra Changes in HSA upon QTP Binding

µM, QTP = 10 µM, C = 0-30 μM), λex = 295 nm, T = 295 K.

2.2.6. Circular Dichroism Spectra Changes in HSA upon QTP Binding Circular dichroism (CD) spectroscopy is a versatile technique mainly used to detect structural and conformational changes in protein structure. The CD spectra of HSA have two negative peaks in the UV region, which reflect α-helix at around 208 and 222 nm of the protein [42]. Figure 6 represents the CD spectra of HSA alone and the HSA-QTP system at different molar ratios of 1:0–1:2. The addition of QTP leads to a decrease in the ellipticity of HSA, suggesting the loss of α-helical content. The CD results showed that the α-helix content of the HSA and QTP-HSA system was 55.92% and 48.88%, respectively. Therefore, these results suggest that the addition of QTP leads to secondary structure Circular dichroism (CD) spectroscopy is a versatile technique mainly used to detect structural and conformational changes in protein structure. The CD spectra of HSA have two negative peaks in the UV region, which reflect α-helix at around 208 and 222 nm of the protein [42]. Figure 6 represents the CD spectra of HSA alone and the HSA-QTP system at different molar ratios of 1:0–1:2. The addition of QTP leads to a decrease in the ellipticity of HSA, suggesting the loss of α-helical content. The CD results showed that the α-helix content of the HSA and QTP-HSA system was 55.92% and 48.88%, respectively. Therefore, these results suggest that the addition of QTP leads to secondary structure change of HSA α-helix content.

### change of HSA α-helix content. 2.2.7. QTP-Induced Thermal Stabilization of HSA

The binding of drugs to plasma proteins can increase the protein's thermal stability [43]. Various studies have shown that drugs induced thermal stabilization to HSA [44,45]. Therefore, the thermal stability measurements of HSA were carried out at different temperatures in the absence and presence of QTP binding. The temperature-dependent titrations measurements were performed on HSA (5 µM) without or with QTP (50 µM) in different temperature range, 25–80 ◦C (5 ◦C intervals). Figure 7 shows the influence of temperature on the fluorescence intensity of the HSA and HSA-QTP system at 343 nm. In the presence of QTP at 45 ◦C, the decrease in FI of the HSA-QTP system was lesser than HSA alone. However, our thermal stability results demonstrated QTP-induced stability to HSA via QTP-HSA system formation (coupling of binding and unfolding equilibrium) [46].

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**Figure 6.** Circular dichroism spectra of HSA (5 µM) in the absence and presence of QTP (10 µM).

**Figure 6.** Circular dichroism spectra of HSA (5 μM) in the absence and presence of QTP (10 μM).

**Figure 7.** Thermal stability profiles of HSA and the QTP-HSA (1:10) system in the temperature range, 25–80 ◦C, as monitored by fluorescence intensity measurements at 343 nm (FI 343 nm) using a protein concentration of 5 µM in 60 mM sodium phosphate buffer, pH 7.4.
