The Screening Of Potential Antifungal In Citrus Sour Rot Disease And Structure-activity Analysis

Sour disease is one of the most serious postharvest diseases of citrus fruits caused by Geotrichum citri-aurantii.Currently,there was few fungicides in the market can effectively inhibit Geotrichum citri-aurantii.Therefore,it is great practical significance and huge market potential to develop antifungal methods which can effectively inhibit the growth of Geotrichum citri-aurantii.In this paper,35 kinds of compounds were screened from food additives,pharmaceutical intermediates and other fields,the relationship between antifungal activity and structure of compounds was described,and the antifunal activity of thymol inclusion complex was determined.The main results were as follow:(1)The antifungal test of 35 kinds of compounds:in vitro,22 kinds of compounds,such as salicylic aldehyde,thymol,can inhibit the growth of Geotrichum citri-aurantii mycelium at low concentration,and the better antifungal effect with the increase of compounds concentration.Among these compounds,the antifungal activity of salicylic aldehyde is the strongest,the minimum inhibitory concentration(MIC)and the minimum antifungal concentration(MFC)are both 0.25 mg/mL.The antifungal activity of 10 kinds of compounds,such as hydroxyl benzaldehyde,ethyl benzoate and fennel aldehyde,can inhibit the growth of Geotrichum citri-aurantii mycelium in the concentration range of 4.00 mg/ml-16.00 mg/ml in different degree.The growth of Geotrichum citri-aurantii could not be inhibited completely in the experimental concentration of 4 kinds of compounds,flower hydrocarbon,decyl aldehyde,amyl benzoate and hexyl benzoate.(2)Structure-activity analysis:The 35 compounds were analysed by by DFT(density functional theory)method using Gaussian09 software.The results were as follow:benzene derivatives containing aldehyde or hydroxyl groups,when the molecules are in the adjacent position,such as hydroxy benzaldehyde,can be formed in the intermolecular hydrogen bonding,meanwhile the stability and the antifungal activity of compounds were increased.The results of molecular electrostatic potential and Millikan constants showed that the oxygen atoms in aldehydes,carboxyl groups and esters were susceptible to electrical reactions and become receptors that bind to biomolecules.Moreover,the amphiphilic power of the double bond oxygen atom was stronger than the unit oxygen atom.The aldehyde group is an antifungal active group to Geotrichum citri-aurantii,and the ester group is an inactive radical.The stability,active group and molecular activity of the compounds can affect the antifungal of the compounds.Finally,the QSAR regression equation is analyzed,and the linear equation is:y=0.2989x+0.8580 R2=0.9646.(3)The inclusion complex of thymol and its antifungal activity against Geotrichum citri-aurantii:the inclusion complex of thymol was prepared by aqueous solution method,the structure of the inclusion complex of thymol was determined by SEM,X-ray diffraction(XRD),1D and 2D NMR(NMR).In vitro assay indicated that the inclusion complex of thymol could reduce the sour rot of citrus fruit and maintained the fruit quality(water loss rate,coloring index,hardness,VC contents,PH,soluble solids).After 5 d of culture,the incidence rate was 37%and 27%in the inclusion complex of thymol treated(4.00 mg/mL and 8.00 mg/mL).Above all,in this search various potentially antifungal substances were screened out to inhibit the growth of Geotrichum citri-aurantii.The result of structure-activity analysis showed that add the aldehyde group to the compounds could increase the intramolecular hydrogen bond and molecular activity,and the antifungal activity of the compound was increased.The result of the inclusion complex of thymol showed the inclusion complex of thymol can reduce the sour rot and maintained the fruit quality.