Three new meroterpenoids from Sargassum macrocarpum and their inhibitory activity against amyloid β aggregation.

Amyloid β (Aβ) is thought to be involved in the pathogenesis of Alzheimer's disease (AD). Aβ aggregation in the brain is considered the cause of AD. Therefore, inhibiting Aβ aggregation and degrading existing Aβ aggregates is a promising approach for the treatment and prevention of the disease. In searching for inhibitors of Aβ42 aggregation, we found that meroterpenoids isolated from Sargassum macrocarpum possess potent inhibitory activities. Therefore, we searched for active compounds from this brown alga and isolated 16 meroterpenoids, which contain three new compounds. The structures of these new compounds were elucidated using two-dimensional nuclear magnetic resonance techniques. Thioflavin-T assay and transmission electron microscopy were used to reveal the inhibitory activity of these compounds against Aβ42 aggregation. All the isolated meroterpenoids were found to be active, and compounds with a hydroquinone structure tended to have stronger activity than those with a quinone structure.