Marine metabolites and metal ion chelation. Circular dichroism studies of metal binding to Lissoclinum cyclopeptides

Abstract

Variable temperature circular dichroism (CD) spectra of the patellamides A (3), B (4) and E (5), isolated from the ascidian (‘sea squirt’) Lissoclinum patella, show that they have very similar thermodynamically preferred macrocyclic ring conformations. In addition, the CD profile of the ‘figure eight like’ conformation 10 in the patellamides has been defined, and CD spectroscopy is shown to provide an insight into the interconversions between the limiting conformations, 9 and 10, in this family of cyclopeptides. The cyclopeptides 3–5 bind both Cu²⁺ and Zn²⁺ and CD studies show that as a family, in line with previous studies, they can bind more than one metal ion per molecule. Thus, the first binding domain for the three patellamides shows a binding constant in the range 2 × 10⁴ to 2 × 10⁵, and a second binding site for patellamide B (4) has K = 230 (Cu²⁺) and K = 16–20 (Zn²⁺). The CD spectra of the patellamide–metal conjugates can be correlated with the ‘square form’ conformation 9 of the cyclopeptides. This best fit situation vis-à-vis metal chelation, could have important implications regarding the biological activity and modus operandi of the cyclopeptides in vivo.