Quantifying hydrogen-bonding strength: the measurement of 2hJNN couplings in self-assembled guanosines by solid-state 15N spin-echo MAS NMR.

2hJNN hydrogen-bond mediated J couplings are measured in the solid state for two synthetic deoxyguanosine derivs. by 15N MAS NMR spin-echo expts. The use of rotor-synchronized Hahn-echo pulse train (RS-HEPT) 1H decoupling, with a duty cycle of 6%, allows spin-echo durations out to 200 ms, hence enabling the accurate detn. of J couplings as small as 3.8 Hz. A single-crystal X-ray diffraction structure exists for the shorter alkyl chain deriv. dG(C3)2: the observation of significantly different 2hJNN couplings, 6.2  0.4 and 7.4  0.4 Hz, for the two resolved N7 resonances is to be expected given the NHN hydrogen-bonding distances of 2.91 and 2.83 .ANG. for the two distinct mols. in the asym. unit cell. For the longer alkyl chain deriv., dG(C10)2, for which there is no single-crystal diffraction structure, a 15N refocused INADEQUATE spectrum (Pham et al., J. Am. Chem. Soc., 2005, 127, 16018-16019) has demonstrated the presence of N2-HN7 intermol. hydrogen-bonds indicative of a quartet-like structure. The 2hJNN hydrogen-bond mediated J coupling of 5.9  0.2 Hz is at the lower end of the range (5.9-8.2 Hz) of 2hJNN couplings detd. from soln.-state NMR of guanosine quartets in quadruplex DNA. A full discussion of the detn. of error bars on the fitted parameters is given; specifically, error bars detd. by a non-linear fitting (using the covariance matrix) or in a Monte-Carlo fashion are found to give effectively identical results.