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 sequence and a dangling T residue at the 5'- end has been used as a model for studying conformational characteristics of DNA duplexes with double-single strand junctions that occur during replication and transcription. The proton resonances of the molecule have been assign ed by NOESY and TOCSY/HOHAHA experiments. The NOE intensities at seven different mixing times were quantified for calculating distance const raints with the two-spin approximation. The experiments have uncovered the interesting structural features of the molecule. The sugar proton coupling constants were determined by the P. E. COSY experiment for e ach residue of the duplex to examine the sugar conformations of the DN A heptamer. The structural features of the heptamer in solution have b een obtained. The overall structure of the DNA duplex is basically of a B-type DNA, and substantial changes from a standard B-DNA have been observed, especially in the terminal region of the molecule. The dangl ing T residue was found in an unusual syn-conformation.