In vitro antioxidant activities of the novel pentapeptides Ser‐His‐Glu‐Cys‐Asn and Leu‐Pro‐Phe‐Ala‐Met and the relationship between activity and peptide secondary structure

BACKGROUND Using high‐performance liquid chromatography/tandem mass spectrometry, two novel antioxidant pentapeptides [Ser‐His‐Glu‐Cys‐Asn ( SHECN ) and Leu‐Pro‐Phe‐Ala‐Met ( LPFAM )] were identified from 1–3‐ kDa soybean protein hydrolysates ( SPH ). The MTT [3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide] assay was used to evaluate cytotoxicity in HepG2 cells. Antioxidant activity was measured using in vitro assays, including the cellular antioxidant activity assay ( CAA ), 2,2‐diphenyl‐1‐picrylhydrazyl or 2,2‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) diammonium salt ( ABTS ) inhibition, and oxygen radical absorbance capacity ( ORAC ) assays. Finally, the secondary structure was determined using circular dichroism ( CD ). RESULTS The results revealed that two novel peptides were nontoxic and possessed antioxidant activity. SHECN had significantly higher antioxidant activity than LPFAM ( P  < 0.05). The CAA value of SHECN was 776.22 µmol QE  100 g −1 . SHECN also showed significant DPPH inhibition (70.18 ± 4.06%) and ABTS inhibition (88.16 ± 0.76%). It had normalized ORAC values of 0.3000 ± 0.0070 µmol  GE  mg −1 and 0.0900 ± 0.0020 µmol  TE  mg −1 , respectively. The results of the CD analysis demonstrated that, compared to LPFAM , which had much lower antioxidant activity, SHECN had a high β‐sheet content and reduced α‐helix content. CONCLUSION The results indicated that SHECN possessed high antioxidant activity. A higher β‐sheet content and lower content levels of α‐helix appear to be correlated with antioxidant activity. © 2016 Society of Chemical Industry