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

Pham, Tran N. and Griffin, John M. and Masiero, Stefano and Lena, Stefano and Gottarelli, Giovanni and Hodgkinson, Paul and Fillip, Claudiu and Brown, Steven P. (2007) Quantifying hydrogen-bonding strength:The measurement of 2hJNN couplings in self-assembled guanosines by solid-state 15N spin-echo MAS NMR. Physical Chemistry Chemical Physics, 9 (26). pp. 3416-3423. ISSN 1463-9076

Abstract

2hJNN hydrogen-bond mediated J couplings are measured in the solid state for two synthetic deoxyguanosine derivatives by 15N MAS NMR spin-echo experiments. The use of rotor-synchronised 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 determination of J couplings as small as 3.8 Hz. A single-crystal X-ray diffraction structure exists for the shorter alkyl chain derivative 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 Å for the two distinct molecules in the asymmetric unit cell. For the longer alkyl chain derivative, 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 intermolecular 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 determined from solution-state NMR of guanosine quartets in quadruplex DNA. A full discussion of the determination of error bars on the fitted parameters is given; specifically, error bars determined by a non-linear fitting (using the covariance matrix) or in a Monte-Carlo fashion are found to give effectively identical results.