Modeling Possible G-Quadruplexes and i-Motifs at DNA-DNA Contact Sites: Strategy, Classification, and Examples

Tetrahelical DNA structures, such as G-quadruplexes (G4s) or i-motifs (iMs), are adopted by sequences comprising several G/C tracts, exist in equilibria with respective duplexes, and may contribute to genomic instability upon helicase deficiency. To understand genomic rearrangements resulting from the juxtaposition of G/C-rich DNA duplexes, models of possible intermediate structures are needed. In this study, a general strategy for creating and verifying in silico 3D models of tetrahelical DNA was proposed. This strategy was used to investigate contacts of two or more duplexes with n G₃/C₃ tracts (n = 2-6) separated by T/A nucleotides. The revealed viable structures of DNA-DNA contacts include stacks of right-handed and left-handed G-quadruplexes (G4s), Holliday structure-resembling assemblies with the G4 and iM opposite each other on the borders of the central "hole", etc. Based on molecular dynamic simulations, the most probable variants were determined by estimating the contributions to the free energy. The results may be used to clarify the mechanisms of strand exchange and other rearrangements upon DNA breaks near prolonged G/C-rich sites in living systems. Additionally, they provide a balanced view on the polymorphic versus programmed DNA assemblies in artificial systems.