Metal binding of Lissoclinum patella metabolites. Part 1: Patellamides A, C and ulithiacyclamide A

Studies on three Thz, and Oxn containing cyclic peptides, patellamide A and C (1, 3) and ulithiacyclamide A (9) isolated from the Indo-Pacific ascidian (seasquirt) Lissoclinum patella have delineated their metal binding selectivity. Patellamide C (3) shows extreme selectivity for Cu2+ even in the presence of an excess of Zn2+ and shows no binding at all to Co2+, Ni2+ and Hg2+. Patellamide A (1) is less selective for Cu2+, whereas ulithiacyclamide A (9) shows selectivity similar to that of patellamide C (3). The selectivity was studied by circular dichroism spectroscopy and mass spectrometry. The CD spectra obtained whilst patellamide C was slowly titrated with Cu2+ show one isosbestic point indicating the Cu2+ binding involves only two conformations. These studies indicate that Cu2+, not Zn2+ is the biologically relevant metal for these compounds and point towards a potential ecological function of these complexes.