The effective field which a nucleus sees is dependent on the electron environment of the atom. The external magnetic field will polarize the electron spins such that the external field is reduced. In theory can be calculated from the electron density surrounding the nucleus. In practice shielding constants are difficult to calculate because of bonding and non bonding interactions. The contributions to shielding can be divided into intrinsic contributions and extrinsic contributions.
Intrinsic shielding depends on the charge density and electronegativity of the bonded groups.
A positive charge will deshield while a negative charge will shield. In general, the electron density on the nuclear spin depends on the relative electronegativities of the bonded atoms. In addition, bonds will deshield atoms in the plane of the bond and shield atoms above and below the plane of the ring. Tyr., Phe, His, trp, A, G, C, T, U, FAD, NAD, Heme. The presence of these groups result in ring current shift .