13C chemical shift tensors in hypoxanthine and 6-mercaptopurine: effects of substitution, tautomerism, and intermolecular interactions.

Principal values of the (13)C chemical shift tensor (CST) are measured for two biologically interesting and structurally related compounds, hypoxanthine and 6-mercaptopurine, and differences in the values are discussed with an attempt to reveal chemical shifts sensitive to substitution and prototropic tautomerism in the purine ring. Furthermore, methods of density-functional theory (DFT) are used to calculate principal values of the (13)C chemical shift tensor and orientations of the principal components. Values calculated for isolated molecules are compared to those for several supramolecular clusters and then to experimental data to investigate the degree of modulation of the (13)C CSTs by molecular packing. Focusing on the protonated carbons, C2 and C8, which are crucial for relaxation measurements, we show that neglecting intermolecular interactions can lead to errors as large as 30 ppm in the delta(22) principal component. This has significant implications for the studies of molecular dynamics, employing spin relaxation, in large fragments of nucleic acids at high magnetic fields.