title: Thermodynamic parameters are sequence-dependent for the supercoil-induced B to Z transition in recombinant plasmids.
abstract: The entropy and enthalpy changes which contribute to the thermodynamics of the B to Z transition were determined for three recombinant plasmids containing a (dC-dG)16 tract and for a plasmid containing a pair of (dT-dG)20 regions. For each base pair which adopts a left-handed conformation in the plasmids with (dC-dG)16 sequences, the delta HBZ and delta SBZ are -2.1 kcal/mol bp and -8.8 cal/K-mol bp, respectively. In the plasmid containing the (dT-dG)20 tracts, however, the delta HBZ and delta SBZ values are 0.58 kcal/mol bp and -0.76 cal/K-mol bp, respectively. Also, these determinations show that for each B-Z junction that forms in the plasmids containing the (dC-dG), the enthalpy and entropy changes are 24 kcal/mol junction and 65 cal/K-mol junction, whereas for the (dT-dG) plasmid, the enthalpy and entropy changes are -1.8 kcal/mol junction and -22 cal/K-mol junction, respectively. Those values for the enthalpy and entropy changes for the formation of a BZ junction in (dC-dG) and (dT-dG) plasmids suggest that the properties and possibly the structures of the junctions are different. Calculations using the enthalpy and entropy changes determined in this study reveal that the B to Z transition in plasmids containing (dC-dG) blocks are more temperature-dependent than the transitions in plasmids with (dT-dG) blocks. Surprisingly, at temperatures above 60 degrees C, calculations indicate that the B to Z transitions in (dT-dG) plasmids should be energetically favored over that transition in (dC-dG) plasmids.
mesh_terms: D001483:Base Sequence; D004274:DNA, Recombinant / Q000032:analysis*; D004278:DNA, Superhelical / Q000032:analysis*; D008433:Mathematics; D009690:Nucleic Acid Conformation; D010957:Plasmids; D011089:Polydeoxyribonucleotides / Q000032:analysis; D013816:Thermodynamics
chemical_list: D004274:DNA, Recombinant; D004278:DNA, Superhelical; D011089:Polydeoxyribonucleotides; C016681:poly(dG).poly(dC); C017650:polydeoxy(guanine-thymine) nucleotide