Cancer of Unknown Primary Site: Not All is Lost!

Abstract

Ab initio shielding tensor calculations were carried out on residues in human ubiquitin. Reported experimental data on isotropic and anisotropic components of the amide proton chemical shifts were used as benchmarks to test the validity of the chosen basis sets as well as methods in structure optimization and shielding calculations. The best agreement with the experimental values was observed when the 6-311**G and 6-311++G(2d,2p) basis sets were used to optimize the structure and to calculate the shielding tensor, respectively. The same method was employed in subsequent model calculations to characterize the dependence of amide proton shielding to the local structure. Both the isotropic and the anisotropic components of the symmetric tensor were found to depend very strongly on the hydrogen bond length. A weaker dependence can also be observed for the hydrogen bond angle. Antisymmetric tensor elements were found to be relatively small. This study permits separation of various local structure contributions to the amide proton shielding tensor that complements scarce experimental data.