On the structure of poly d(A-S4T).

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.