Indirect NMR spin-spin coupling constants (3)J(P, C) and (2)J(P, H) across the P-O center dot center dot center dot H-C link can be used for structure determination of nucleic acids
Calculated indirect NMR spin-spin coupling constants (3)J(P,C) and (2)J(P,H) were correlated with the local structure of the P-O center dot center dot center dot H-C linkage between the nucleic acid (NA) backbone phosphate and the H-C group(s) of a nucleic acid base. The calculations were carried out for selected nucleotides from the large ribosomal subunit (Ban et al. Science 2000, 289, 905) with the aim of identifying NMR parameters suitable for detection of certain noncanonical RNA structures. As calculations in the model system, dimethyl-phosphate-guanine, suggest, the calculated indirect spin-spin couplings across the linkage are sensitive to the mutual orientation and distance between the phosphate and nucleic acid base. A short distance between the nucleic acid base and phosphate group and the angles C center dot center dot center dot P-O and P center dot center dot center dot C-H smaller than 50 are prerequisites for a measurable spin- spin interaction of either coupling (vertical bar J vertical bar >1 Hz). A less favorable arrangement of the P-O center dot center dot center dot H-C motif, e. g., in nucleotides of the canonical A-RNA, results in an effective dumping of both spin-spin interactions and insignificant values of the NMR coupling constants. The present work indicates that quantum chemical calculations of the indirect spin-spin couplings across the P-O center dot center dot center dot H-C motif can help detect some rare but important backbone topologies, as seen for example in the reverse kink-turn. Measuring of (3)J(P,C) and (2)J(P,H) couplings can therefore provide critical constraints on the NA base and phosphate geometry and help to determine the structure of NAs.