Abstract
Standard nuclear magnetic resonance (NMR) spectroscopy exploits the nuclear Overhauser effect (NOE) to obtain interproton distance restraints. Structures of RNA and DNA are relatively imprecisely determined, due to the shortage of medium and long-range NOEs compared to those of proteins. Longrange constraints are ideally needed to define the structures more precisely. We report here on the NMR study of a ternary paramagnetic complex consisting of a DNA oligomer, a drug, and a divalent cation. The purpose of the study is to utilize pseudocontact shifts as structural restraints in solution NMR of macromolecules.1,2 By assigning the paramagnetic spectrum of the DNA complex, we have obtained long-range constraints out to 25 A from the metal ion. The complex consists of d(TTGGCCAA)2 bound to two molecules of chromomycin-A3 (I) in the minor groove at the G4C5 step.3-5 A single Co2+ cation is strongly bound to the O1 and O9 atoms of two chromomycins and to two water molecules, forming a pseudo-octahedral coordination site.6 The
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