Chemical-shift ranges in proteins

The results of ab initio derivative Hartree-Fock calculations of the dipole and quadrupole shielding polarizabilities and hyperpolarizabilities of a number of small molecules are reported, together with estimates of the electric fields and field gradients present in proteins. It is argued that weak electrical interactions, mediated via these shielding polarizabilities, make major contributions to the chemical-shift nonequivalencies observed in proteins due to folding into their native conformations. The electric-field-induced shifts may be very large (≈5 ppm for ¹³C, ≈10 ppm for ¹⁷O and ¹⁷F), due to the low dielectric constants found in proteins, and in some cases they may dominate the experimentally observed spectral shifts.