THE SELECTIVE B-A CONFORMATIONAL TRANSITION OF THE CENTRAL DINUCLEOTIDE (CPG) SEGMENT OF (CAATCCGGATTG)(2) INDUCED BY DELTA-CO(EN)(3)(3+)

The binding of Co(NH3)(6)(3+), Delta-Co(en)(3)(3+) and Delta-Rh(en)(3) (3+) to the dodecanucleotide d(CAATCCGGATTG)(2) has been studied by H- 1 NMR. Addition of Co(NH3)(6)(3+), Delta-Co(en)(3)(3+) or Delta-Rh(en) (3)(3+) to d(CAATCCGGATTG)(2) induce a similar pattern of chemical shi ft movements of the dodecanucleotide base protons. Analysis of the che mical shift changes as a function of added metal-ion complex shows tha t the binding affinity for d(CAATCCGGATTG)(2) decreases in the order: Delta-Co(en)(3)(3+) > Delta-Rh(en)(3)(3+) > Co(NH3)(6)(3+). Addition o f a saturating ratio of Delta-Co(en)(3)(3+) to d(CAATCCGGATTG)(2) indu ces a selective B to A type-DNA transition for the central CpG segment of the dodecanucleotide, with all other nucleotide residues maintaini ng the B-type conformation.