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The water soluble porphyrin tetrakis(4-N-methylpyridyl)porphine (H2TMpyP) and its copper(II) derivative (CuTMpyP) convert Z-poly(dG-dC) to the B-form. For H2TMpyP, the fraction Z character (fr-Z) is given by fr-Z = 1.0 - 21 rO and for CuTMpyP, fr-Z = .94 - 12 rO where rO identical to [Porphyrin]O/[DNA]O. Neither the manganese(III) derivative of of this porphyrin (MnTMpyP) nor tetrakis(2-N-methylpyridyl)porphine (H2TMpyP-2) is nearly as effective at causing the conversion. The former two porphyrins have been shown to intercalate into B-poly(dG-dC) whereas the latter two porphyrins do not. The kinetics of the Z----B conversion are independent of porphyrin or poly(dG-dC) concentration for 1/rO greater than 6. At smaller values of 1/rO, the conversion rate is greatly increased for H2TMpyP and CuTMpyP. The interaction of these porphyrins with Z-poly(dG-dC) follows simple first order kinetics in this latter concentration range. It is proposed that for small values of 1/rO the sequence of events begins with a porphyrin-unassisted distortion of the Z-duplex (with a rate constant of 0.6 s-1) followed by a rapid uptake of porphyrin in what may be an intercalative mode. The porphyrin thus located in Z-regions brings about rapid conversion to the B-form. Binding of H2TMpyP or CuTMpyP to B-regions of a predominantly Z-strand leads to conversion of Z to B. However, this conversion process is considerably slower than when the porphyrins bind directly to Z-regions.

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