Porphyrin induced Z to B conversion of poly(dG-dC) 2 in ethanol

Abstract

The water soluble porphyrins H 2TMpyP-2, H 2TMpyP-4, and CuTMpyP-4 are found to bind to Z-form poly(dG-dC) 2 in 60% ethanol (v/v) and to facilitate the conversion of the polymer to the B form. Metalloporphyrins with axial ligands (MnTMpyP-4, ZnTMpyP-4) interact to some degree with the Z form, but do not lead to extensive conversion to the B form. The conversion of the Z form into the B form was determined by CD titration experiments, which were used to quantitate the fraction of poly(dG-dC) 2 present in each conformation. Under all conditions each bound porphyrin molecule converts multiple base pairs from Z to B. The kinetics of porphyrin reactions with Z-poly(dG-dC) 2 in 60% ethanol were measured using two different detection techniques. Stopped flow spectrophotometry was used to observe the time-dependent spectral changes associated with the porphyrins during the reaction. Time-dependent changes in the poly(dG-dC) 2 conformation were observed directly using CD. The porphyrin absorbance changes under the conditions of these experiments have a much shorter half time (t 1 2 ~ 0.1 to 2 sec) than the CD changes (t 1 2 ~ 10 sec). Thus it could be determined that a complex with spectral characteristics similar to those of the porphyrin intercalated into B-form poly(dG-dC) 2 is produced while the polymer is predominantly in the Z form.