Length and sequence effect on the B-Z transition of [d(A-T)n]2 oligonucleotide induced by a cationic porphyrin

•Formation of the Z-form requires at least 14 alternated AT base-pairs.•The B-Z transition of the oligonucleotide couples with the formation of extensive porphyrin stacking.•The time-dependent B-Z transition and porphyrin stacking can be elucidated by the sum of two exponential curves.•The presence of even one GC base-pair in the middle of alternated AT sequence prevents the B-Z transition.

trans-BMPyP induced the B-Z transition for alternated AT oligonucleotides as it was evident by inversed CD spectrum. The transition occurred simultaneously with appearance of the extensive stacking of porphyrin. Complete B-Z transition required at least 14 base-pairs long. Insertion of one or two GC base pairs prevented the B-Z transition.

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