Left-handed Z form in superhelical DNA: a theoretical study.

This is a comprehensive statistical mechanical treatment of the Z form formation in purinepyrimidine stretches of different length inserted into superhelical DNA. The B-Z transition for short inserts is shown to follow the "all-or-none" principle. Over some critical value of the insert length n, the B-Z transition in the insert proceeds in two stages. The flipping of m base pairs into the Z form is followed by a gradual growth of the Z-form stretch until it occupies the whole insert. By fitting the theoretical transition curves to experimental ones the fundamental thermodynamic parameters of the B-Z transition have been determined: the B-Z junction energy Fj = 4-5kcal.mol-1 and the free energy change delta FBZ = 0.5-7.0 kcal.mol-1 under standard salt conditions. Calculations show that the B-Z transition in short purinepyrimidine inserts may be seriously affected by cruciform formation in the carrier DNA.