Little is known about the influence of multiple consecutive ‘non-standard’ (Z, 6-amino-5-nitro-2(1H)-pyridone, and P, 2-amino-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one) nucleobase pairs on the structural parameters of duplex DNA. P:Z nucleobase pairs follow standard rules for Watson–Crick base pairing but have rearranged hydrogen bonding donor and acceptor groups. Using the X-ray crystal structure as a starting point, we have modeled the motions of a DNA duplex built from a self-complementary oligonucleotide (5΄-CTTATPPPZZZATAAG-3΄) in water over a period of 50 μs and calculated DNA local parameters, step parameters, helix parameters, and major/minor groove widths to examine how the presence of multiple, consecutive P:Z nucleobase pairs might impact helical structure. In these simulations, the PZ-containing DNA duplex exhibits a significantly wider major groove and greater average values of stagger, slide, rise, twist and h-rise than observed for a ‘control’ oligonucleotide in which P:Z nucleobase pairs are replaced by G:C. The molecular origins of these structural changes are likely associated with at least two differences between P:Z and G:C. First, the electrostatic properties of P:Z differ from G:C in terms of density distribution and dipole moment. Second, differences are seen in the base stacking of P:Z pairs in dinucleotide steps, arising from energetically favorable stacking of the nitro group in Z with π–electrons of the adjacent base
