Interpretation of Indirect Nuclear Spin−Spin Couplings in Isomers of Adenine: Novel Approach to Analyze Coupling Electron Deformation Density Using Localized Molecular Orbitals

Abstract

Adenine, an essential building block of nucleic acids present in all living systems, can occur in several tautomeric forms. The phenomenon of tautomerism can be investigated by several experimental methods, including nuclear magnetic resonance. In this study, long-range (1)H-(13)C and (1)H-(15)N coupling constants for N-alkyl derivatives related to four tautomers of adenine are investigated in DMSO and DMF solutions. To investigate the structural dependence of the coupling constants and to understand how polarization propagates in the system, Fermi contact (FC) terms were calculated for the individual isomers and analyzed by using density functional theory (DFT), and the coupling pathways were visualized using real-space functions. The coupling electron deformation densities (CDD) of several (1)H-X (X = (13)C, (15)N) pairs are evaluated and compared. In order to analyze the CDD in more detail, a new approach to break down the CDD into contributions from Boys or Pipek-Mezey localized molecular orbitals (LMOs) has been developed. A similar approach has been applied to split the value of the FC contribution to the J coupling into the LMO contributions. On the basis of chemical concepts, the contributions of sigma-bonds, pi-electrons, and lone pairs of electrons are discussed. The lone pair of electrons at the nitrogen atom contributes significantly to the (1)H-C horizontal line(15)N coupling, whereas the (1)H-C=N-(13)C coupling is affected in a somewhat different way. Surprisingly, the contribution of the intervening C horizontal lineN bond to the FC term for (1)H-C=(15)N coupling originates exclusively in sigma-electrons, with a vanishingly small contribution calculated for the pi-electrons of this fragment. This behavior is rationalized by introducing the concept of "hard and soft J elements" derived from the polarizability of the individual components.