Structure of metal adducts of anthracyclines probed by absorption, circular dichroism and paramagnetic NMR

A structural study of metal ion adducts of a new anthracycline disaccharide (MEN 10755) was undertaken. The trivalent lanthanide ion Yb(III) was employed as paramagnetic structural probe for 1H NMR analysis. Through a comparative spectroscopic investigation [UV–Vis absorption and circular dichroism (CD), 1H NMR], the isomorphism between its adducts with lanthanide ions (La3+, Yb3+, Lu3+) and calcium (one of the most representative biological cations) was verified. Solution behavior and cation binding were also investigated by means of optical titrations. In agreement with other anthracyclines, MEN 10755 was found to dimerize in aqueous solution [estimated Kdim (pH7.6) = 7 × 103], but not in methanol. A prevalent complex Yb3+–MEN 10755 (1:1) in both buffered aqueous and methanolic solutions (estimated Kcompl = 2100 M−1) was observed. A numerical analysis of the LIR and LIS 1H NMR literature data for a similar adduct (Yb3+–daunorubicin) was performed using newly developed software, PERSEUS (Paramagnetic Enhanced Relaxation and Shifts for Eliciting Ultimate Structures), and the structure of the complex was characterized, locating definitely the binding site on the O-11, O-12 quinone system. The components of the anisotropic part of the magnetic susceptibility tensor were also determined. Finally, a study of the time-dependent formation of an Yb3+–MEN 10755 complex through 1H NMR, UV–Vis CD and induced NIR CD was carried out. Copyright © 2002 John Wiley & Sons, Ltd.