Stability, Bioavailability, and Structure–Activity Relationship of Casein-Derived Peptide YPVEPF with a Sleep-Enhancing Effect

YPVEPF (Tyr-Pro-Val-Glu-Pro-Phe) is an outstanding sleep-enhancing peptide derived from casein. This study aimed to evaluate the bioavailability of YPVEPF in vitro and in vivo and to explore its structure–activity relationship through a sleep test and cheminformatics. Our results showed that YPVEPF was unstable against gastrointestinal enzymes and almost totally degraded to YPVEP in vitro. However, the pharmaco-kinetics results in vivo showed that the C_max of YPVEPF was 10.38 ± 4.01 ng/mL at 5 min, and YPVEPF could be detected in the stomach, intestine, and brain at 12.89 ± 0.55, 10.26 ± 0.23, and 2.47 ± 0.55 ng/g, respectively. The main metabolites including YPVEP, YP, PVEPF, and PVEP were identified. We first explored whether the fragment YPVEP also had a strong sleep-enhancing effect, and the sleep-enhancing effects of PVEPF and PVEP (lacking a Tyr residue) significantly decreased compared with those of YPVEPF and YPVEP. Moreover, molecular docking and quantum calculations revealed that the N-terminus Tyr played a dominant role in YPVEPF and YPVEP. They had distinctive self-folding structures and varying electron-withdrawing properties of the groups at the N terminus, allowing different binding modes and electron/proton transfer.