## 3 Results
### Density Functional Theory (DFT) Calculations  
As per FMO theory, the HOMO and LUMO of the analyzed molecules are fundamental for determining molecular transport properties and anticipating the chemical reactivity of the conjugated system [50]. In addition, the energy gap (\(\Delta\)E) signifies the fundamental energy requirement for initiating covalent bond formation. The HOMO indicates a molecule's reactivity with electrophiles, acting as an electron donor. A higher energy HOMO typically suggests a greater electrophilic interaction. In contrast, a lower energy level corresponds to a reduced tendency to react with nucleophiles, making the molecules less reactive in that regard. The calculated energy values for **4d** show a HOMO of \(-5.618\) eV and a LUMO of \(-1.104\) eV. In comparison, the Aspirin molecule has a HOMO of \(-7.785\) eV and a LUMO of \(-2.146\) eV (Figure 9A). These values indicate the energy levels associated with electron donation and acceptance for each molecule. The difference between the HOMO and LUMO helps to assess the molecule's reactivity. Molecules with a smaller energy gap are typically more polarizable and exhibit higher chemical reactivity. The \(\Delta\)E values indicate that the **4d** molecule, with a AE of 4.513 eV, is more reactive than Aspirin, which has a \(\Delta\)E of 5.635 eV. This enhanced reactivity of **4d** allows it to form hydrogen bonds with the BSA protein more  
Figure 8: Free energy profile derived from RMSD and Rg (in nm), with the **4d_BSA complex shown on the left and the Aspirin_BSA complex on the right. The structures corresponding to the minimum energy regions were obtained. efficiently and rapidly compared to Aspirin, suggesting stronger and faster interactions between **4d** and the protein. The HOMO orbitals of the **4d** molecule are predominantly situated on the thiazole group thione and (-CH\({}_{2}\)CH\({}_{3}\)) groups, while the LUMO is spread across the majority of the molecule. These results are in agreement with docking studies, which revealed interactions between these specific groups and the BSA-binding site, suggesting that these regions play a key role in the binding process. The MEP surface is crucial for studying how a substrate interacts with its protein. It reveals the distribution of electric charge across a molecule, which is essential for understanding hydrogen bonding, reactivity and the overall polarity. Additionally, MEP helps in identifying molecular interactions, including polarizability and structure-activity relationships, offering valuable insights into how molecules behave and interact with one another. The areas with greater nucleophilic potential, shown in red in Figure 9B, are found around the thione (C=S) and carbonyl (C=O) groups in both molecules, indicating these regions as possible sites for electrophilic reactions, highlighting their reactivity in chemical interactions. On the other hand, the regions with higher electrophilic potential are located in the aromatic ring and thiazole group of both molecules, marking these sites as likely targets for nucleophilic reactions. This is validated by molecular docking, which reveals that these groups form hydrogen bonds with the amino acids of the BSA protein. The ELF (electron localization function) is a useful method for evaluating the likelihood of electron pair localization and gaining insights into electron behavior in multielectronic systems. Similarly, the LOL is widely used to characterize the molecular bonding, reactivity and chemical structure. Both ELF (localized orbital locator) and LOL rely on kinetic energy density to examine the electron distribution [51; 52; 53]. Nevertheless, the LOL map presents a more straightforward and clearer visualization than ELF. While ELF emphasizes the density  
Figure 9: FMO analysis. (**A**) HOMO and LUMO energies obtained for **4d (left)** and Aspirin (right). (**B**) MEP surface of both molecules. of electron pairs, LOL highlights the gradient of localized orbitals, making it especially beneficial when orbitals overlap.