Chemical constituents of Morus alba (Sang-Bai-Pi), their tyrosinase inhibition, and UVA-induced photodamage protection.

Phytochemical investigations on the root bark of Morus alba (Sang-Bai-Pi) let to isolate twenty-five constituents (1-25), including two new flavonoids, (2S)-5,7,2',4'-tetrahydroxy-5'-C-β-D-glucopyranosylflavanone-7-O-glucoside (1) and cyclomethoxymoruol (2), and one new coumarin, esculetin 7-O-β-d-xylopyranosyl-(1 → 6)-β-D-glucopyranoside (3). The structures of these compounds were elucidated through a comprehensive approach involving HRESIMS, NMR, UV, CD, and TDDFT-ECD calculation methods. Biological studies revealed that compounds 3 and 18 exhibited significant tyrosinase inhibition, with IC50 values of 0.51 ± 0.01 μM and 5.12 ± 0.09 μM, respectively. In comparison, the positive control, phenethyl resorcinol (377), demonstrated stronger inhibitory activity with an IC50 value of 0.16 ± 0.01 μM. Furthermore, compounds 3, 8, 13, 17, 19, 22 and 23 showed notable protection against UVA-induced apoptosis in human dermal fibroblast-alpha (HDF-α) cells. Notably, the protective effects of 3 and 22 at a lower concentration of 10 μM surpassed the positive control, vitamin C (100 μM). Of particular interest, the new compound 3 possesses both tyrosinase inhibitory and anti-UV damage effects, suggesting potential applications in cosmetics and related industries.