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 (CG) 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 (DH) of the zein-geraniol (ZG) complexes increased from 400 to 1200 nm when CG ≤ 9 mg/mL and remained constant around 1100 nm when 9 < CG ≤ 18 mg/mL. However, at higher CG (>18 mg/mL), DH 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 CG. 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 (CG = 22.5 mg/mL) against Candida albicans was slightly lower than pure geraniol (CG = 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.