Role of H-bonding in the size-dependent formation of zein-geraniol complexes: Effect on anti-fungal activity

Geraniol is an essential oil known for its anti-fungal activity, however, it is susceptible to environmental conditions which decreases its activity. Therefore, in this study, different concentrations of geraniol (C_G) were complexed with zein to study the interaction mechanism and improve the anti-fungal activity of geraniol. The light scattering technique revealed that the hydrodynamic diameter (D_H) of the zein–geraniol (ZG) complexes increased from 400 to 1200 nm when C_G ≤ 9 mg/mL and remained constant around 1100 nm when 9 < C_G ≤ 18 mg/mL. However, at higher C_G (>18 mg/mL), D_H reduced to about 400 nm. Further, the CD data indicated a transition in the secondary structure of zein from α–helix to β–sheet with the addition of increasing C_G. Molecular dynamics simulations and FTIR analysis suggested these transitions were due to the prevalence of H–bond between zein and geraniol molecules, where geraniol molecules acted as bridges to form intramolecular H-bonds in zein. Moreover, thermodynamic parameters obtained from the fluorescence spectroscopy studies indicated dynamic quenching of zein in the presence of geraniol. The negative values of ΔH (–157.62 kJ/mol) and ΔS (–480.79 J/mol⋅K) validated the hypothesis that H-bonds and van der Waals interactions were responsible for the formation of ZG complexes. In addition, the MIC₉₀ of the ZG complexes (C_G = 22.5 mg/mL) against Candida albicans was slightly lower than pure geraniol (C_G = 25 mg/mL). The FICI value (0.375) of ZG complexes with fluconazole indicated synergistic activity and provided a lead towards the development of combinatorial anti-fungal treatments.