Synthesis, Biotransformation, Characterization, and DFT Study of Organic Azachalcone Dyes and Secondary Metabolites with Biological and Conformation Dependence of Dipolar-Octupolar NLO Responses

Abstract

Chalcones are organic chromophores with diverse biological applications and potential for use in various electronic devices due to their recognized optical properties. This research focuses on the organic synthesis, FT-NMR characterization, and biotransformation of three azachalcones (1-3) using the Exserohilum rostratum fungus, yielding novel compounds (1a-3a). In vitro biological assays against Gram-positive and Gram-negative bacteria revealed promising pharmacological potential for these new chromophores. A key structural difference, the interchange of an HC = CH bond by a H₂C-CH₂ bond, significantly impacts biological and electronic properties. For instance, while biotransformed 1a exhibits similar activity to tetracycline and amoxicillin, compounds 2a and 3a demonstrate a 4-fold and thirty-fold increase in inhibitory activity against Gram-negative E. coli, respectively, compared to their parent compounds. Density functional theory calculations suggest that the biotransformation reaction reduces the refractive index (n), which may limit its applicability in certain light-handling applications. However, Hyper-Rayleigh scattering calculations indicate that these chromophores exhibit higher nonlinear optical (NLO) responses compared to standard NLO materials such as urea and p-nitroaniline, making them promising candidates for photonic and optoelectronic devices, such as nanostructured circuits. Interestingly, while the original molecules exhibit a dominant dipolar (Φ _J=1) NLO response, the biotransformed compounds, as stable isomers, display a predominant octupolar (Φ _J=3) architecture. These findings highlight the potential of these novel compounds for biotechnological and optoelectronic applications.

Figure 1

Compounds 1, 2, and 3 azachalcones synthesized (top), and their respective bioreduction products 1a, 2a, and 3a obtained (bottom).

Figure 2

Route of the azachalcone synthesis.

Figure 3

Structural representation of the compounds under study. The chemical elements are colored white (hydrogen), gray (carbon), red (oxygen), and blue (nitrogen).

Figure 4

Highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) plotted for 1 (left side) and 1a (right side) using the M06-2X/6-311++G(d,p) level of quantum mechanics.

Figure 5

Octupolar (Φ_J=3) and dipolar (Φ_J=1) contributions plotted the (a) azachalcones (1–3) and their (b) derivatives (1a–3a).