Isolation And Identification Of Bound Phenolic Compounds From Pearl Millet And Evaluate Its Antioxidant Activities

Pearl millet is not only one of the most important minor cereals,but also a resource riched a variety of bioactivity agents,especially phenolic compounds.It is usually difficult to extrac the insoluble bound phenolic compounds from pearl millet directly due to their conjugation with cell wall materials.Most of the published literatures on pearl millet phenolic compounds focused on the crude extract of free phenolic compounds and its antioxidant activity.It was believed that most of the phenolic compounds in pearl millet could be obtained by using fine powder and long extraction time.However,the total phenolic content of pearl millet was underestimated due to the inefficient extraction of bound compounds.Currently there are no reports involved in the pure bound phenolics and their structures identification.In this investigation,the bound phenolics from pearl millet were isolated and identified and their antioxidant activities were evaluated preliminarily so as to provide a theoretical basis for the processing and product development of pearl millet.The extraction technology of bound phenolics was established and the extract parameters were optimized.According to the phenolic content,a response surface methodology was introduced to optimize the alkaline digestion conditions for bound compounds releasing on the basis of single factor experiments,and the optimum parameters were obtained as below:digestion time of 5 h,digestion temperature of 67?,the ratio of solvent to materials 1:15 and alkaline concentration of 5 mol/L.Under these conditions,the content of bound phenolics extracted was 9.297?M GAE/g.Pure bound phenolic compounds from Pearl millet were characterized.Afer removing the free compounds of Pearl millet using organic solvent,the remaining residue was digested under the optimum conditions,and the digested solution was extracted by organic solvent and a crude extract containing various bound compounds was yielded.The crude extract underwent solvent extraction,vacuum filtration,vacuum concentration,and various column chromatography methods,such as reduced pressure silica gel chromatography,open column silica gel chromatography,and gel column chromatography Sephadex LH-20,and 4 pure compounds were obtained,their structures were identified as trans ferulic acid(1),trans ferulic acid methylester(2),cis ferulic acid(3)and cis ferulic acid methylester(4)by mass spectrum and nuclear magnetic resonance(NMR)technology including 1D-NMR,1H-,13C-,DEPT and 2D-NMR,HMQC,HMBC,NOSY spectrum,and compared with the literature data.This is the fist time to isolate and identify compounds 2?4 from pearl millet.The antioxidant activity of pure bound phenolics was evaluated.The pure phenolic compounds were evaluated preliminarily by DPPH radical scavenging method.The results revealed that all the compounds showed strong DPPH radical scavenging activity with IC50 vaules of 17.19?M,26.48?M,21.19?M and 32.48?M,respectively,comparing with the positive control.