Hydration-induced modulation of aromaticity and reactivity in anthocyanidins: a quantum mechanical study

This study explores the hydration effects on aromaticity, orbital delocalization, and local and global reactivity descriptors of anthocyanidins. Using the Harmonic Oscillator Model of Aromaticity (HOMA) and para-delocalization index (PDI), the quantitative assessment of the aromaticity of the three rings in anthocyanidins is performed. It revealed that the hydration generally increases their aromaticity, though the extent of change varies across different rings in the skeletal structure. The HOMA and PDI indices show consistent trends in aromaticity, with ring B exhibiting the highest aromaticity, followed by fused rings, i.e., ring A and ring C. The study also highlights the role of orbital delocalization indices (ODI) in understanding the hydration effects on the electronic structure of anthocyanidins. A significant increase in the delocalization of electrons in the HOMO post-hydration is observed. The global reactivity descriptors computed under conceptual density functional theory (CDFT) show that the hydration of anthocyanidins affects their reactivity towards electron-rich species. The acidic forms of anthocyanidins are characterized as superelectrophiles whose nature is retained while the electrophilic strength decreases slightly upon hydration. The decrease in global chemical hardness (η), increase in global nucleophilicity (NI), and changes in vertical ionization potential (VIP) and vertical electron affinity (VEA) indicate the differences in reactivity arising from the hydration of anhydrous forms of anthocyanidins. This should have implications for their antioxidant properties and potential applications in food packaging and smart sensors for quality monitoring of protein-rich foods such as meat and fish, which are perishable in nature. The atomic level attribution over the 2D chemical structures created a heatmap, which assisted the local reactivity analysis using condensed dual descriptors (CDDs). The heatmap overlapped with 2D chemical structures highlighted the specific sites, mostly in the fused ring C, suggesting it to be more favourable for nucleophilic attack after hydration. This observation reinforces the alteration in reactivity as predicted from the several global descriptors of the anthocyanidin molecules. Overall, this comprehensive analysis provides a deeper understanding of the hydration effects on the chemical behaviour of anthocyanidins, offering valuable insights into their applications.