New Structural Insights into Saraines A, B, and C, Macrocyclic Alkaloids from the Mediterranean Sponge Reniera (Haliclona) sarai

The structural peculiarity of saraines A, B and C, wherein a zwitterionic-like form is present due to the coordination of a nitrogen atom to the C-2 aldehyde by a strong proximity effect, has been further extended by experimental and theoretical evidences. They include the detection of [2M + H](+) clusters in electrospray ionization mass spectrometry in the positive ion mode of a water/acetonitrile solution, whereas only signals corresponding to [M + H](+) ions are detectable after addition of trifluoroacetic acid to the same solution. The zwitterionic form can be trapped to give O-methyl derivatives only by reaction with a strong alkylating agent such as Meerwein's reagent. DFT calculations carried out on the core structural model suggest that the nucleophilic N-1 atom in the minimized conformation is at a suitable distance from the carbonyl group and also defines an approach angle in good agreement with the Burgi-Dunitz model. Antibacterial and hemolytic activities and inhibition of acetylcholinesterase (AChE) assays have been evaluated for saraines A-C in comparison to those of saraines 1-3 and isosaraine 1, all the metabolites isolated from the sponge Reniera (Haliclona) sarai collected in the Northern Adriatic sea.