Antioxidant benzylidene 2-aminoimidazolones from the Mediterranean sponge Phorbas topsenti

Three new benzylidene 2-aminoimidazolones, named phorbatopsins A-C, have been isolated from the Mediterranean marine sponge Phorbas topsenti, in addition to the known astaxanthin, adonirubin, taurine, and taurobetain. Their structures were elucidated through mass spectrometric and detailed 1D and 2D NMR spectroscopic data. The relative configuration of the molecules was determined on the basis of 1H and 13C chemical shifts and proton-proton coupling constants. The absolute configuration of the chiral carbon C-6 of phorbatopsin B was determined by modified Mosher's method. The antioxidant activity, evaluated through the Oxygen Radical Absorbance Capacity (ORAC) assay, was also reported for the seven isolated compounds and structure-activity relationships are discussed. Phorbatopsin A appeared as the most active with an ORAC value of 0.88. Further investigations to support the promising antioxidant activity of phorbatopsin A will be followed.