## 3 Results and Discussion
### Thermodynamic Parameters and Binding Modes  
Essentially, there are four types of non-covalent interactions between ligand and protein, i.e., the hydrophobic effect, hydrogen bonding, van der Waals force and electrostatic interactions [42]. Generally, the signs and magnitudes of the thermodynamic parameters of the enthalpy change (\(\Delta H\)) and entropy change (\(\Delta S\)) are important for identifying the main forces involved in the binding process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & **T (K)** & \(K_{\mathbf{a}}\) (\(\times\)**10\({}^{\mathbf{4}}\) M\({}^{-\mathbf{1}}\))** & \(n\) & \(R\)a  & \(SD\)b  \\ \hline \multirow{3}{*}{R = H} & 298 & 2.15794 & 0.99949 & 0.26651 \\  & 307 & 1.87645 & 0.99962 & 0.2702 \\  & FA & 1.04135 & 0.99948 & 0.25962 \\  & Dig & 2.17623 & 0.99938 & 0.24163 \\ \hline \multirow{3}{*}{R = CF\({}_{3}\)} & 298 & 3.52395 & 0.99985 & 0.12641 \\  & PB & 3.41465 & 0.99971 & 0.18126 \\  & FA & 3.03507 & 0.99985 & 0.18132 \\  & Dig & 4.02741 & 0.99976 & 0.15165 \\ \hline \multirow{3}{*}{R = OCH\({}_{3}\)} & Blank & 3.57039 & 0.99965 & 0.15519 \\  & PB & 2.89855 & 0.99941 & 0.22308 \\  & FA & 2.45095 & 0.99977 & 0.15596 \\  & Dig & 3.27478 & 0.99936 & 0.20352 \\ \hline \multirow{3}{*}{R = CH\({}_{3}\)} & Blank & 5.16790 & 0.99977 & 0.13434 \\  & PB & 4.50395 & 0.99994 & 0.07291 \\ \cline{1-1}  & FA & 2.91938 & 0.99953 & 0.22246 \\ \cline{1-1}  & Dig & 5.08774 & 0.99995 & 0.05945 \\ \hline \multirow{3}{*}{R = CH\({}_{3}\)} & Blank & 3.58307 & 0.99954 & 0.16563 \\  & PB & 3.43302 & 0.99964 & 0.16227 \\ \cline{1-1}  & FA & 2.95264 & 0.99987 & 0.11889 \\ \cline{1-1}  & Dig & 3.75253 & 0.99943 & 0.18557 \\ \hline \multirow{3}{*}{R = CN} & Blank & 4.79585 & 0.99987 & 0.08076 \\ \cline{1-1}  & PB & 4.79839 & 0.99990 & 0.07751 \\ \cline{1-1}  & FA & 3.28633 & 0.99934 & 0.19871 \\ \cline{1-1}  & Dig & 5.00248 & 0.99952 & 0.14806 \\ \hline \hline \end{tabular}  
* \(R\) is the correlation coefficient; b \(SD\) is the standard deviation for the \(K_{\mathbf{a}}\) values.  
\end{table} 
Table 2: Binding constants of PDQL to HSA. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Compound** & **Site Marker** & \(K_{\mathbf{a}}\) (\(\times\)**10\({}^{\mathbf{4}}\) M\({}^{-\mathbf{1}}\))** & \(R\)a  & \(SD\)b  \\ \hline \multirow{3}{*}{R = H} & Blank & 2.15794 & 0.99949 & 0.26651 \\  & PB & 1.87645 & 0.99962 & 0.2702 \\  & FA & 1.04135 & 0.99948 & 0.25962 \\  & Dig & 2.17623 & 0.99938 & 0.24163 \\ \hline \multirow{3}{*}{R = CF\({}_{3}\)} & Blank & 3.52395 & 0.99985 & 0.12641 \\  & PB & 3.41465 & 0.99971 & 0.18126 \\  & FA & 3.03507 & 0.99985 & 0.18132 \\  & Dig & 4.02741 & 0.99976 & 0.15165 \\ \hline \multirow{3}{*}{R = OCH\({}_{3}\)} & Blank & 3.57039 & 0.99965 & 0.15519 \\  & PB & 2.8985 & 0.99941 & 0.22308 \\  & PA & 2.45095 & 0.99977 & 0.15596 \\  & Dig & 3.27478 & 0.99936 & 0.20352 \\ \hline \multirow{3}{*}{R = CH\({}_{3}\)} & Blank & 5.16790 & 0.99977 & 0.13434 \\  & PB & 4.50395 & 0.99994 & 0.07291 \\ \cline{1-1}  & FA & 2.91938 & 0.99953 & 0.22246 \\ \cline{1-1}  & Dig & 5.08774 & 0.99995 & 0.05945 \\ \hline \hline \end{tabular}  
* \(R\) is the correlation coefficient; b \(SD\) is the standard deviation for the \(K_{\mathbf{a}}\) values.  
\end{table} 
Table 3: Effects of the site probes on the binding constants of PDQL to HSA. [PERSON] and [PERSON] [43] used the laws of thermodynamics to evaluate the primary mode of binding forces between drugs and biological molecules: (1) \(\Delta H>0\) and \(\Delta S>0\) indicate a hydrophobic interaction; (2) \(\Delta H<0\) and \(\Delta S<0\) suggest that hydrogen bonding and van der Waals force are the dominant forces; and (3) \(\Delta H\cong 0\) and \(\Delta S>0\) imply that electrostatic interactions are dominant.