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Growing interest in understanding the relationship between the global folding of nucleic acids and the sequence-dependent structure of individual base-pair steps has stimulated the development of new mathematical methods to define the geometry of the constituent base-pairs. Several approaches, designed to meet guidelines set by the nucleic acid community, permit rigorous comparative analyses of different three-dimensional structures, as well as allow for reconstruction of chain molecules at the base-pair level. The different computer programs, however, yield inconsistent descriptions of chain conformation. Here we report our own implementation of seven algorithms used to determine base-pair and dimer step parameters. Aside from reproducing the results of individual programs, we uncover the reasons why the different algorithms come to conflicting structural interpretations. The choice of mathematics has only a limited effect on the computed parameters, even in highly deformed duplexes. The results are much more sensitive to the choice of reference frame. The disparate schemes yield very similar conformational descriptions if the calculations are based on a common reference frame. The current positioning of reference frames at the inner and outer edges of complementary bases exaggerates the rise at distorted dimer steps, and points to the need for a carefully defined conformational standard. |
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