Left-handed helices for DNA: studies on poly[d(I-C)]

Area of Science:

• Structural biology
• Biophysics
• Molecular genetics

Background:

• Previous work demonstrated satisfactory right- and left-handed double helical models for poly[d(A-T)] in D-form.
• The D-DNA and B-DNA models exhibit similar conformations, suggesting a D to B transition in solid phase.

Purpose of the Study:

• To investigate the structural possibilities of poly[d(I-C)] in D-form using fiber diffraction.
• To determine if right- or left-handed double helices are favored in poly[d(I-C)] D-form.

Main Methods:

• Fiber diffraction analysis of poly[d(I-C)] at different relative humidity levels.
• Stereochemical and theoretical energy calculations for DNA models.

Main Results:

• A crystalline D-form pattern (n = 8, h = 3.13 A) was obtained for poly[d(I-C)] at 75% relative humidity.
• The hydrated form (approx. 95% r.h.) yielded the classical B-form pattern (n = 10, h = 3.40 A).
• Both right- and left-handed double-helical models are consistent with the D-form fiber diffraction data for poly[d(I-C)].

Conclusions:

• The fiber diffraction data for poly[d(I-C)] in the D-form do not permit discrimination between right- and left-handed double helices.
• Theoretical energy calculations indicate similar stability for both right- and left-handed B- and D-DNA models.
• Right- and left-handed double-helical models of poly[d(I-C)] in a given form (B or D) are equally likely.