BINDING OF CO(III) TO A DNA OLIGOMER VIA REACTION OF [CO(NH3)(5)(OH2)](3+) WITH (5MEDC-DG)(4)

The interaction specificities of cobalt(III) ammines with the self-complementary eight-base pair DNA oligomer (5medC-dG)(4) have been investigated. Standard protocol for preparing DNA samples calls for heat annealing the DNA oligomer in phosphate buffer in the absence or presence of cobalt(III) ammine complex for 2 min at 80 degrees C, followed by slow cooling to 25 degrees C. An alternative method for DNA preparation is incubation of the oligomer in the presence of the cobalt(III) complex at 37 degrees C followed by exhaustive dialysis. The conformational properties of the thus-treated DNA oligomer were determined by inspection of the UV and CD spectra at 25 and 95 degrees C and thermal denaturation studies. With heat annealing in the absence of any cobalt(III) complex, (5medC-dG)(4) assumes a double-stranded, right-handed B conformation at 25 degrees C. Upon heat annealing in the presence of 200 mu M [Co(NH3)(6)](3+), (SmedC-dG)(4) assumes a double-stranded, left-handed Z conformation at 25 degrees C. In contrast, the CD and UV spectra of (5medC-dG)(4) heat annealed in the presence of 200 mu M [Co(NH3)(5)(OH2)](3+) are consistent with a distorted B-like conformation at 25 degrees C. Incubation of the oligomer in the presence of [Co(NH3)(5)(OH2)](3+) results in modification of the conformational properties of the oligomer at both 25 and 95 degrees C relative to the untreated oligomer. The extent of modification depends upon the incubation concentration of [CO(NH3)(5)-(OH2)](3+) and the reaction time. Atomic absorption (AA) analyses of these treated DNA samples indicate a high degree of cobalt association to the oligomer. These studies suggest that [Co(NH3)(5)(OH2)](3+) reacts with the oligomer, resulting in tight binding of the cobalt(III) metal center to the DNA lattice. The combined results are interpreted in terms of coordination of the cobalt(III) to N7 of guanine with subsequent loss of the aquo group from [Co(NH3)(5)(OH2)](3+).