CRYSTAL-STRUCTURE, ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY, ELECTRON-PARAMAGNETIC-RESONANCE, AND MAGNETIC-SUSCEPTIBILITY STUDY OF [CU2(ASCIDH2)(1,2-MU-CO3)(H2O)2]CENTER-DOT-2H2O, THE BIS(COPPER(II)) COMPLEX OF ASCIDIACYCLAMIDE (ASCIDH4), A CYCLIC PEPTIDE ISOLATED FROM THE ASCIDIAN LISSOCLINUM-PATELLA

A bis(copper(II)) complex of the naturally occurring cyclic peptide as cidiacyclamide (ascidH-4), isolated from the ascidian Lissoclinum pate lla, has been characterized by X-ray crystallography, magnetic suscept ibility measurements, ion spray mass spectrometry, and EPR spectroscop y. The crystals are triclinic, space group P1, with a = 9.9420(10), an gstrom, b = 11.808(2), angstrom, c = 20.635(3) angstrom, alpha = 74.34 0(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 coordina ted by three nitrogen donors, one each from an oxazoline, a thiazole, and a deprotonated amide. A water molecule and the oxygen atom of a br idging carbonate anion complete the coordination sphere. The other cop per(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 ge ometry of the carbonate anion was considerably distorted from that in the free anion. The presence of the CO32- species was confirmed with i on spray mass spectral studies by comparing the mass spectra of isolat ed [Cu2(ascidH2)(1,2-mu-CO3)(H2O)2].2H2O with that generated in situ u sing (CO32-)-C-13 ([ascidH-2 + 2Cu2+ + CO32- + Na+]+: (CO32-)-C-12, m/ z 965.2; (CO32-)-C-13, m/z 966.3). Magnetic susceptibility measurement s (4-300 K) on the powdered sample show that weak ferromagnetic coupli ng (2J = +1.6 +/- 0.4 cm-1) occurs, probably via an intramolecular sup erexchange pathway across the 1,2-mu-CO32- bridge. EPR spectroscopy su ggests a structural reorganization of the binuclear copper site when t he binuclear copper ascidiacyclamide complex is dissolved in methanol and that the formation of [CU2(ascidH2)(1,2-mu-CO3)(solvent)2] is comp licated by a series of equilibria in which other binuclear and monomer ic species are involved.