Structural examination of enantioselective intercalation: 1H NMR of Rh(en)2phi3+ isomers bound to d(GTGCAC)2.

Abstract

The enantioselective recognition of d(GTGCAC)2 by delta- and lambda-Rh(en)2phi3+ (en = ethylenediamine; phi = 9,10-phenanthrenequinone diimine) has been examined in a series of one-dimensional (1D) and two-dimensional (2D) 500 MHz 1H NMR experiments both to extend our understanding of the basis for the enantioselective DNA binding and to gain structural information concerning intercalation by the octahedral metal complexes. delta-Rh(en)2phi3+ forms a symmetric 1:1 complex with d(GTGCAC)2, and the metal complex is in slow exchange with the oligodeoxynucleotide bound form at 295 K. The strong upfield shifts of the phi ligand's aromatic protons (0.6-1.3 ppm) are consistent with full intercalation of the phi ligand into the DNA base stack. 2D-NOESY experiments reveal a loss in internucleotide connectivity between G3 and C4 bases, while new NOE cross peaks are observed between the phi ligand and the G3 deoxyribose sugar. In contrast to binding by delta-Rh(en)2phi3+, the 1:1 lambda-Rh(en)2phi(3+)-d(GTGCAC)2 complex shows much broader resonances, and both metal complex and DNA protons appear to be in the intermediate exchange regime. The loss of C2 symmetry in the 1:1 complex is consistent with binding by lambda-Rh(en)2phi3+ at the T2G3 step. Although the enantiomeric metal complexes display different sequence selectivities and exchange characteristics, lambda- and delta-Rh(en)2phi3+ interact with the oligonucleotide duplex in a fundamentally similar manner, through the full intercalation of the phi ligand. Upfield movements in chemical shifts of phi protons are nearly identical for the two enantiomers, and both lambda- and delta-Rh(en)2phi3+ stabilize the duplex to melting by 5-10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)