New structural insights into saraines A, B and C,macrocyclic alkaloids from the Mediterranean sponge Reniera (Haliclona)sarai

Saraines A, B and C are structurally complex diamine alkaloids first isolated from the sponge Reniera sarai collected in the bay of Naples. Their peculiar core structure induces an uncommon behaviour responsible of chromatographic and spectroscopic anomalies, and that was explained by a strong "proximity effect" between a tertiary ammine and aldehyde groups, able to be involved into a cyclic zwitterionic form. On saraines A-C recently isolated from the same sponge collected in the Northern Adriatic Sea, we report now on new structural evidences including: a) NMR analysis in different solvents, b) electrospray ionization (ESI)-MS spectra showing signals for mixed [2M+H]+ clusters in neutral conditions and [M+H]+ ions in acidic media, c) zwitterionic form trapped by conversion to a stable O-methyl ammonium salt derivative, d) density functional theory (DFT) calculations on the amine-aldehyde form, resulting in good agreement with Bürgi–Dunitz angle, a model able to forecast the preferred spatial approach in the nucleophilic addition to a carbonyl group. In addition, biological activities including antibacterial, haemolytic and anti-acetilcholinesterase assays were investigated for saraines A-C, in order to clarify their ecological role.