Comparison of the helix–coil conformational changes of poly(deoxyadenylate‐deoxytymidylate) and poly(deoxyadenylate‐deoxythymidylate) induced by variations of hydrogen‐ion concentration

Abstract

AbstractDouble‐helical poly(dG‐dC) and poly(dA‐dT) are DNA analogs in which the interactions between the two strands of the helix are, respectively, either the stronger G/C type or the weaker A/T type along the entire length of macromolecules. Thus, these synthetic polynucleotides can be considered as representatives of the most stable and the least stable DNA. In the investigations presented here, potentiometric titrations and stopped‐flow kinetic experiments were carried out in order to compare the pH‐induced helix–coil conformations (10°C and 150mM [Na+]) the pH of the helix–coil transition (pHm) is 12.81 for poly(dG‐dC) and 11.76 for poly(dA‐dT). The unwinding of double‐helical poly(dG‐dC) initiated by a sudden change in pH was found to be a simple exponential process with rate constants in the range of 200–600 sec−1, depending on the final value of the pH jump. The intramolecular double‐helix formation of poly(dG‐dC) was studied by lowering the pH of the solutions from a value above pHm to that below pHm in dilute solutions (15.5 ug/ml [polymer]). Under these conditions, the observed rewinding reactions displayed a major and two exponential phases, all of which were independent of polymer concentration. From the comparison of the results of poly(dA‐dT) and poly(dG‐dT) would unwind faster than poly(dG‐dC). However, if the pH jumps are such that they present the same perturbation of these polymers relative to their pHm values, no significant differences exist between the rates of helix–coil conformation changes of poly(dA‐dT) and poly(dG‐dC).