Base Pair Buckling Can Eliminate the Interstrand Purine Clash at the CpG Steps in B-DNA Caused by the Base Pair Propeller Twisting

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Abstract
Results of calculations using various empirical potentials suggest that base pair buckling, which commonly occurs in DNA crystal structures, is sufficient to eliminate the steric clash at CpG steps in B-DNA, originating from the base pair propeller twisting. The buckling is formed by an inclination of cytosines while deviations of guanines from a plane perpendicular to the double helix axis are unfavorable. The buckling is accompanied by an increased vertical separation of the base pair centers but the buckled arrangement of base pairs is at least as stable as when the vertical separation is normal and buckle zero. In addition, room is created by the increased vertical separation for the bases to propeller twist as is observed in DNA crystal structures. Further stabilization of base stacking is introduced into the buckled base pair arrangement by roll opening the base pairs into the double helix minor groove. The roll may lead to the double helix bending and liberation of guanines from the strictly perpendicular orientation to the double helix axis. The liberated guanines further contribute to the base pair buckling and stacking improvement. This work also suggests a characteristic very stable DNA structure promoted by nucleotide sequences in which runs of purines follow runs of pyrimidine bases.