DNA bending in solution: NMR studies on the structural roles of A/T tracts and the sequences at the junction

A summary of 1D/2D‐NMR studies on d(GA 4 T 4 C) 2 , d(GA 4 U 4 C) 2 , and d(GT 4 A 4 C) 2 in solution is reported in this article. Results of these studies indicate one important structural property of A ‐ T pairs, i.e., A ‐ T pairs are propeller twisted, which results in a series of interstrand bifurcated H‐bonds inside the A/T tract. The structural similarity of d(GA 4 T 4 C) 2 and d(GA 4 U 4 C) 2 suggests that the methyl group of thymine may be of little consequence as far as this structural peculiarity of the A/T tract is concerned. Comparison of d(GA 4 T 4 C) 2 (i.e., “junction B‐DNA model”) and d(GT 4 A 4 C) 2 (i.e., “straight B‐DNA model”) structures shows that not only the structural peculiarity of the A/T tract but also the sequence that joins two neighboring A/T tracts play crucial roles in DNA bending. In other words, when two A/T tracts are joined at the A → T sequence, a junction is created, whereas when two A/T tracts are connected at the T → A sequence, no discontinuity is observed.