Crystal Structure and Electrospray Ionization Mass Spectrometry, Electron Paramagnetic Resonance, and Magnetic Susceptibility Study of [Cu2(ascidH2)(1,2-.mu.-CO3)(H2O)2].cntdot.2H2O, the Bis(copper(II)) Complex of Ascidiacyclamide (ascidH4), a Cyclic Peptide Isolated from the Ascidian Lissoclinum patella

Abstract

A bis(copper(II)) complex of the naturally occurring cyclic peptide ascidiacyclamide (ascidH-4), isolated from the ascidian Lissoclinum patella, has been characterized by X-ray crystallography, magnetic susceptibility measurements, ion spray mass spectrometry, and EPR spectroscopy. The crystals are triclinic, space group P1, with a = 9.9420(10), angstrom, b = 11.808(2), angstrom, c = 20.635(3) angstrom, alpha = 74.340(10)-degrees, beta = 87.520(10)-degrees, gamma = 89.460(10)-degrees, V = 2330.3(6) angstrom3, Z = 2, and R = 0.058. The geometry around one copper(II) atom is distorted square pyramidal, the metal ion coordinated by three nitrogen donors, one each from an oxazoline, a thiazole, and a deprotonated amide. A water molecule and the oxygen atom of a bridging carbonate anion complete the coordination sphere. The other copper(II) atom exhibits a similar coordination environment with an added distant Cu-O interaction making up a distorted octahedral environment. The Cu(I) ... Cu(2) distance was determined as 4.43 angstrom. The geometry of the carbonate anion was considerably distorted from that in the free anion. The presence of the CO32- species was confirmed with ion spray mass spectral studies by comparing the mass spectra of isolated [Cu2(ascidH2)(1,2-mu-CO3)(H2O)2].2H2O with that generated in situ using (CO32-)-C-13 ([ascidH-2 + 2Cu2+ + CO32- + Na+]+: (CO32-)-C-12, m/z 965.2; (CO32-)-C-13, m/z 966.3). Magnetic susceptibility measurements (4-300 K) on the powdered sample show that weak ferromagnetic coupling (2J = +1.6 +/- 0.4 cm-1) occurs, probably via an intramolecular superexchange pathway across the 1,2-mu-CO32- bridge. EPR spectroscopy suggests a structural reorganization of the binuclear copper site when the binuclear copper ascidiacyclamide complex is dissolved in methanol and that the formation of [CU2(ascidH2)(1,2-mu-CO3)(solvent)2] is complicated by a series of equilibria in which other binuclear and monomeric species are involved.