Antifungal Activity and Molecular Orbital Energies of Aldehyde Compounds from Oils of Higher Plants

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Abstract
Mechanisms of antifungal actions of cinnamaldehyde, citral, perillaldehyde and citronellal were investigated. The growth inhibitions by the aldehydes were reduced or abolished in the presence of cysteine or glutathione in some cases, indicating that the inhibitions were mainly due to reactions of the aldehydes with SH groups involved in the fungal growth. In other cases, the SH compounds were ineffective on the inhibitory actions of the aldehydes. By calculating energies of molecular orbitals of the aldehydes, it was found that the antifungal activity was related to the energy of the lowest empty molecular orbital, i.e., the lower the energy, the higher is the antifungal activity. The energy values of the molecular orbitals indicate that the aldehydes except citronellal are good electron acceptors. It was further demonstrated, by studies of difference spectra, that cinnamaldehyde, citral and perillaldehyde are capable of forming charge transfer complexes with tryptophan, a good electron donor. These results strongly suggest that the antifungal actions of the aldehydes are possibly due to their abilities to form charge transfer complexes with electron donors in addition to their reactivity with SH groups.

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