Flexibility of A.T pairs in left-handed DNA helices

We have analysed by various approaches the structure of cloned synthetic sequences in supercoiled plasmids. Individual inserts were formed by d(C-G)n blocks interrupted by the presence of A.T pairs positioned either in phase or out of phase of pur-pyr alternation. Based on the thermodynamic analysis we obtained results confirming that A.T pairs are easily incorporated into left-handed helices without significant energetic penalty. Sequences GTAC which are known to form cruciform structures in multiple repetition underwent a B-Z transition. In the case of plasmids containing AA/TT code words and substantial discontinuities in purine-pyrimidine alternation our analysis indicates that Z-Z junctions formed by A.T pairs contributed little to the overall energetic demands of the B-Z transition probably thanks to their high conformational flexibility.