Figure 1.

NMR investigations of cytosine NH₂ rotations in double-stranded DNA. (A) 15-bp DNA used in this study. The residue numbering is based on that of Fernandez et al.¹⁴ Cytosine and guanine nucleotides in this DNA were labeled by ¹³C and ¹⁵N isotopes. The sample solution contained 0.3 mM DNA, 20 mM potassium succinate (pH 5.8), 100 mM KCl, and 0.4 mM NaF. (B) ¹³C-decoupled ¹H-¹⁵N HSQC spectrum for cytosine NH₂ groups in the 15-bp DNA duplex at 25°C. The resonances were assigned as described in the Supporting Information. (C)Examples of ODDS-zz-EXSY sub-spectra for cytosine NH₂ groups of the ¹³C/¹⁵N-labeled 15-bp DNA at 25°C (with a mixing time of 47 ms). In the second sub-spectrum, off-diagonal and diagonal cross peaks are swapped through coherence transfer between 2H_z^aN_z and 2H_z^bN_z terms via two ¹J_NH couplings.¹³ (D) Ratios of intensities of off-diagonal exchange cross peaks and off-diagonal auto cross peaks for the NH₂ groups of C1 and C24 at 25°C. Solid curves represent the best-fit curves obtained through the fitting with Eq. 1. The k_rot constant represents a rate constant for a 180° rotation and is a half of the exchange rate constant k_ex.