Base pair fluctuations in helical models for nucleic acids

A statistical method is developed to estimate the maximum amplitude of base pair fluctuations in a three dimensional mesoscopic model for nucleic acids. The thermal vibrations around helix diameter are viewed as Brownian motion particle embedded stable helical structure. probability return initial position computed, function time, by integrating over paths consistent with physical properties potential. zero time condition first-passage defines constraint select integral cutoff various macroscopic conformations, obtained tuning twist, bending and slide between adjacent pairs along molecule stack. Applying short homogeneous chain at room temperature, we obtain meaningful estimates twist conformation $\sim 10.5$ per turn, typical double stranded DNA helices. Untwisting helix, broaden grows. found increase also presence sliding which shortens contour length, situation peculiar dsRNA molecules.