Binding of Copper(II) to the Cyclic Octapeptide Patellamide D

Patellamide D (patH4), a cyclic octapeptide from the ascidian Lissoclinum patella, with two oxazoline and two thiazole rings, has the 24-azacrown-8 macrocyclic structure. The cyclic octapeptide forms multiple mononuclear copper(II) complexes in which there are three nitrogen ligating atoms, arising from a deprotonated amide and oxazoline and thiazole rings, coordinated to the copper(II) ion. The multiple mononuclear copper(II) complexes can be ascribed to different metal binding sites in patH4; there are four in the asymmetric patH4 molecule and the binding of different ligands (Cl- or H2O) to the fourth equatorial coordination site. Patellamide D also forms three binuclear copper(II) complexes ([Cu2(patH2)]2+, [Cu2(patH2)(OH)]+, and [CU2(patH2)(CO3)]) in which each copper(II) ion is coordinated by three nitrogen ligating atoms from a deprotonated amide and oxazoline and thiazole rings. The remaining coordination sites may be occupied by either Cl- or H2O in the first two complexes, while carbonate is assumed, on the basis of X-my crystallography presented in a future paper, to bridge the two copper(II) ions in the third complex. Electronic absorption, circular dichroism, electron paramagnetic resonance, and mass spectra are reported as a function of the concentration of base added to the copper(II) patellamide solutions.