The Electronic Effects of 3-Methoxycarbonylcoumarin Substituents on Spectral, Antioxidant, and Protein Binding Properties

Abstract

Coumarin derivatives are a class of compounds with pronounced biological activities that depend primarily on the present substituents. Four 3-methoxycarbonylcoumarin derivatives with substituents of different electron-donating/electron-withdrawing abilities (Br, NO₂, OH, and OMe) were investigated structurally by NMR, IR, and UV-VIS spectroscopies and density functional theory methods. The appropriate level of theory (B3LYP-D3BJ/6-311++G(d,p) was selected after comparing similar compounds' experimental and theoretical structural parameters. The natural bond orbital and quantum theory of atoms in molecules were employed to investigate the intramolecular interactions governing stability. The electronic effects of substituents mostly affected the aromatic ring that the substituents are directly attached to. The antioxidant properties were investigated by electron paramagnetic resonance spectroscopy towards HO^•, and the percentages of reduction were between 13% (6-Br) and 23% (6-OMe). The protein binding properties towards transport proteins were assessed by spectrofluorimetry, molecular docking, and molecular dynamics (MD). The experimentally determined binding energies were well reproduced by molecular docking, showing that the spontaneity of ibuprofen binding was comparable to the investigated compounds. The flexibility of HSA in MD simulations depended on the substituents. These results proved the importance of electronic effects for the protein binding affinities and antioxidant properties of coumarin derivatives.

Keywords: BSA; DFT; EPR; coumarin; molecular dynamics.

Figure 1

Structures of (a) coumarin and (b) 3-methoxycarbonylcoumarin.

Figure 2

Optimized structures of coumarin derivatives at the B3LYP-D3BJ/6-311++G(d,p) level of theory.

Figure 3

IR spectra of (a) 6-Br, (b) 6-NO₂, (c) 6-OH, and (d) 6-OMe (experimental—black line, calculated at B3LYP-D3BJ/6-311G(d,p) level of theory—blue peaks).

Figure 4

UV-Vis spectra of (a) 6-Br, (b) 6-NO₂, (c) 6-OH, and (d) 6-OMe (experimental—black line, calculated at B3LYP-D3BJ/6-311G(d,p) level of theory—blue peaks).

Figure 5

The experimental EPR spectra of DEPMPO/HO^• adducts with and without compounds (a) 6-Br, (b) 6-NO₂, (c) 6-OH, and (d) 6-OMe.

Figure 6

The BSA-binding fluorescence curves for 1-Br at (a) 27 °C, (b) 32 °C, and (c) 37 °C, and (d) Van’t Hoff’s plot for binding.

Figure 7

Interactions of WF and four coumarin derivatives with HSA at Sudlow I binding site.

Figure 8

Interactions of IP and four coumarin derivatives with HSA at Sudlow I binding site.

Figure 9

RMSF diagrams describing interactions of (a) 6-Br, (b) 6-OH, (c) 6-OMe, and (d) 6-NO₂ with HSA compared to ibuprofen.

Figure 10

Rg diagrams describing interactions of (a) 6-Br, (b) 6-OH, (c) 6-OMe, and (d) 6-NO₂ with HSA compared to ibuprofen.