Heteronuclear and homonuclear direct (D) and indirect (J) spin-spin interactions are important sources of structural information about nucleic acids (NAs). The Hamiltonians for the D and J interactions have the same functional form; thus, the experimentally measured apparent spin-spin coupling constant corresponds to a sum of J and D. In biomolecular NMR studies, it is commonly presumed that the dipolar contributions to Js are effectively canceled due to random molecular tumbling. However, in strong magnetic fields, such as those employed for NMR analysis, the tumbling of NA fragments is anisotropic because the inherent magnetic susceptibility of NAs causes an interaction with the external magnetic field. This motional anisotropy is responsible for non-zero D contributions to Js. Here, we calculated the field-induced D contributions to 33 structurally relevant scalar coupling constants as a function of magnetic field strength, temperature and NA fragment size. We identified two classes of Js, namely (1)JCH and (3)JHH couplings, whose quantitative interpretation is notably biased by NA motional anisotropy. For these couplings, the magnetic field-induced dipolar contributions were found to exceed the typical experimental error in J-coupling determinations by a factor of two or more and to produce considerable over- or under-estimations of the J coupling-related torsion angles, especially at magnetic field strengths >12 T and for NA fragments longer than 12 bp. We show that if the non-zero D contributions to J are not properly accounted for, they might cause structural artifacts/bias in NA studies that use solution NMR spectroscopy.

Bijvoet Center for Biomolecular Research Utrecht University Padualaan 8 3584 CH Utrecht The Netherlands

Central European Institute of Technology Masaryk University Kamenice 753 5 625 00 Brno Czech Republic

Faculty of Science Masaryk University Kotlářská 2 611 37 Brno Czech Republic

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic v v i Flemingovo náměstí 542 2 166 10 Praha 6 Czech Republic

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic v v i Flemingovo náměstí 542 2 166 10 Praha 6 Czech Republic Institute of Physics Academy of Sciences of the Czech Republic v v i Na Slovance 2 182 21 Prague 8 Czech Republic

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