Figure 3.

Hydrogen-bond scalar ¹⁵N-³¹P coupling ^h3J_NP between ¹⁵N and ³¹P nuclei as evidence for the presence of CIP. (a, b, c) 2D H₃(N)P spectra that give heteronuclear ¹H-³¹P correlation cross peaks via ^h3J_NP coupling. ³¹P chemical shifts are relative to trimethylphosphate (TMP). (d, e, f) Difference spectra recorded by the spin-echo ^h3J_NP–modulation constant-time HISQC experiment. Measured absolute values of ^h3J_NP are also indicated. Percentage errors were estimated to be less than 20%. Values in parentheses are ^h3J_NP values taking account of the correction due to partial self-decoupling arising from ³¹P longitudinal relaxation (see SI). ³¹P carrier positions were set to −3 ppm for panels a, b, d, and e (for DNA phosphate); and 107 ppm for panels c and f (for DNA phosphorodithioate). Spectra in panels a and d were recorded for Complex I, and spectra in panels b, c, e, and f for Complex II. The pulse sequences and other relevant details are given in Figure S1 in SI. (g) Phosphorodithioate regions of 1D ³¹P spectra recorded for Duplex II in the free state and a mixture of Duplex II and Complex II. ³¹P chemical shift of the cross peak in the panel c agrees with ³¹P chemical shift of the signal from Complex II in the panel g.