| Abstract: | The compaction of poly[d(A–T)] · poly[d(A–T)] by Co(III) is accompanied by the formation of ψ(+)- and ψ(-)-structures. The chirality of the ψ-structure depends on the Co(III) concentration, ionic strength, temperature, pH, and the chain length of the polymer. The two forms can be readily interconverted by manipulating these factors. Phase diagrams have been constructed that demonstrate the regions of stability of the enantiomers as a function of two variables, while other factors are held constant. At critical points in the phase diagram the two forms are in such unstable equilibrium that mechanical motion will cause ψ(+) ? ψ(-) interconversion. The formation of both ψ(+)- and ψ(-)-structures by the action of Co(III) on poly[d(A–T)] · poly[d(A–T)] contrasts markedly with the behavior of poly[d(G–C)] · poly[d(G–C)] in similar circumstances by forming only the ψ(+)-structure and that of native DNA to produce no ψ at all. Thus the base sequence is important in determining the structure of chirally associated DNA molecules. |
