Hexammine cobalt(lIl) chloride (Co(NH3)63+) provokes a B-DNA -4 Z-DNA -+ P-DNA conformational transition in poly(dG-dC).poly(dG-dC) and poly(dG-m5dC).poly(dG-m5dC). The circular dichroism spectrum of '-DNA is characterized by a manyfold increase of positive ellipticity in the range of 300-225 nm and the complete absence of a negative peak. In order to ascertain the helical handedness of '-DNA, we used a recently developed enzyme immunoassay technique. This method consisted of treating the polynucleotides with Co(NH3)63+ to convert them to the Z-or '-DNA forms and immobilizing these conformations on a microtiter plate.The plates were subsequently treated with a monoclonal anti-Z-DNA antibody Z22, alkaline phosphatase conjugated, affinity purified immunoglobulins, and the phosphatase substrate. The enzyme-substrate reaction was monitored by reading the absorbance at 405 nm with a microplate autoreader. The monoclonal anti-Z-DNA antibody had no reactivity to the B-DNA form, but bound strongly to both the Z-and '-DNA forms, showing that Co(NH3)63+-induced '-DNA form of the polynucleotides exists in the left-handed Z-DNA conformation.
INTRODQUCTIONMultivalent ions like Co(NH3)63+ and Ru(NH3)63+ are excellent promoters of the transition of poly(dG-dC).poly(dG-dC) and poly(dG-m5dC).poly(dG-m5dC) from their usual right-handed B-DNA form to the left-handed Z-DNA conformation at micromolar concentrations of these counterions (1-4). Increasing concentrations of Co(NH3)63+ are also known to provoke a supramolecular organization of the polynucleotides from the Z-DNA form to a '-DNA form (5-6). The '-DNA form is a believed to be a twisted, tightly packaged assembly of DNA.Several agents, including polylysine, spermine, ethylene glycol, and salt are known to convert native DNAs to the '-DNA form (7-9). The formation of this structural form of DNA is characterized by a large (10-to 100-fold) increase in the intensity of the molar ellipticity of DNA in circular dichroism (CD) spectroscopic measurements. This property has presented a basic problem in interpreting the CD spectrum of '-DNA and assigning its helical handedness. This uncertainty has recently led to the conclusion that '-DNA is the end product in the