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Abstract:
The helical twist of poly d(A-s4T) was determined from the periodicity of the cleavage patterns of the double stranded polydeoxynucleotide adsorbed on calcium phosphate and found to be 14 bp per turn. Both cleavage patterns and 31P NMR spectra indicate a mononucleotide structure rather than an alternating B DNA like poly d(A-T). The failure of nucleosome formation excludes a B type structure. The discrepancy of the mononucleotide structure found in 31P NMR spectra and the dinucleotide structure given by X ray fiber diffraction is explained by an alternating tilt of the planes of the base pairs (base roll) as a consequence of a strong propeller twist. The importance of interstrand stacking interactions of adjacent 4-thiothymidines for the helical stability is discussed.
Insights
The helical twist of poly d(A-s4T) DNA was found to be 14 base pairs per turn. This study reveals a unique mononucleotide structure, differing from typical B DNA, due to base pair tilting and propeller twist.
Area of Science:
- Biochemistry
- Structural Biology
- Nucleic Acid Chemistry
Background:
- Polydeoxynucleotides exhibit diverse helical structures influencing their biological functions.
- Understanding DNA conformation is crucial for molecular biology and drug design.
Purpose of the Study:
- To determine the helical twist and structural characteristics of poly d(A-s4T).
- To investigate the discrepancy between NMR and X-ray diffraction data for this DNA analog.
Main Methods:
- Analysis of cleavage patterns of poly d(A-s4T) adsorbed on calcium phosphate.
- 31P Nuclear Magnetic Resonance (NMR) spectroscopy.
- X-ray fiber diffraction analysis (for comparison).
Main Results:
- The helical twist of poly d(A-s4T) was determined to be 14 base pairs (bp) per turn.
- Cleavage patterns and 31P NMR spectra indicated a mononucleotide structure, not an alternating B DNA.
- Failure of nucleosome formation further excluded a B-type structure.
- Discrepancy between NMR and X-ray data explained by base pair tilting (base roll) and propeller twist.
Conclusions:
- Poly d(A-s4T) adopts a unique helical structure distinct from canonical B DNA.
- Base roll and propeller twist are key factors in its structural conformation and stability.
- Interstrand stacking of 4-thiothymidines contributes significantly to helical stability.