NMR Studies on Structural Features of DNA[d（TCGCGCG）]2 With a Dangling Residue T

The DNA heptamer [d（TCGCGCG）]2,which has an alternating pyrimidine-purine se-quence and a dangling T residue at the 5′-end has been used as a model for studying conformational charac-teristics of DNA duplexes with double-single strand junctions that occur during replication and transcription.The proton resonances of the molecule have been assigned by NOESY and TOCSY/HOHAHA experiments.The NOE intensities at seven different mixing times were quantified for calculating distance constraints withthe two-spin approximation.The experiments have uncovered the interesting structural features of themolecule.The sugar proton coupling constants were determined by the P.E.COSY experiment for eachresidue of the duplex to examine the sugar conformations of the DNA heptamer.The structural features ofthe heptamer in solution have been obtained.The overall structure of the DNA duplex is basically of a B-typeDNA,and substantial changes from a standard B-DNA have been observed,especially in the terminal regionof the molecule.The dangling T residue was found in an unusual syn-conformation.