Peptide cleavage-mediated aggregation-enhanced emission from metal nanoclusters for detecting trypsin and screen its inhibitors from foods

Herein, a novel peptidyl probe is first synthesized by the amide reaction between glutathione-capped gold nanoclusters (GSH-AuNCs) and positive charged arginine-rich peptide (Arg9), and it is called Arg9-GSH-AuNCs. Polyuridylic acid (polyU), as a polyanion, is firstly used to induce the aggregation-induced emission enhancement (AEE) of positive charged Arg9-GSH-AuNCs through electrostatic attraction. The peptide bond on the carboxyl terminal of the arginine residues can be recognized and cleaved by trypsin. Hence, polyU is hard to induce the aggregation of peptidyl probe again, and the AEE from GSH-AuNCs becomes weak. It is an ideal platform to detect trypsin and screen trypsin inhibitors. The fluorescent peptidyl probe exhibits excellent sensitivity for trypsin. A good linear range of trypsin is obtained from 1 to 50 ng·mL−1 with the limit of detection (LOD) 0.31 ng·mL−1, which is lower than those of most reported works. Furthermore, the inhibitory effects of the extracts from foods (i.e., soybean seed flour, black soybean seed flour, mung bean seed flour, and azuki bean seed flour) on the trypsin activity have been studied. The half maximal inhibitory concentration (IC50) values of extracts from soybean seed flour, black soybean seed flour, mung bean seed flour, and azuki bean seed flour are 4.5, 9.3, 11.6, 20.9 μg·mL−1, respectively. The screening results show that these extracts from foods all have inhibitory effects on trypsin. Among them, the extract from soybean seed flour exhibits significant inhibitory effect.