Ab initio quantum mechanical calculations of the variation of the 1H and 13C nuclear magnetic shielding constants in proline as a function of the angle psi.

The variation of the nuclear magnetic shielding constant of the different protons and carbons of trans HCO-L-Pro-NH2 with the value of the angle psi is calculated by a non-empirical method for three conformations of the proline ring. The results concerning the CH protons show that the chemical shift of the alpha, beta and gamma endo hydrogens can vary by more than 1 ppm when psi goes from -30 degrees to 180 degrees. The theoretical variation of the chemical shift difference between alpha and gamma or beta and gamma carbons is found to be sensitive to the puckering of the proline ring. For the second of these differences the theoretical results are in agreement with Siemion's relation only for a limited range of molecular conformations. Additional calculations show that the variations of the proton shifts with the value of psi are due to the magnetic anisotropy of the proline carbonyl group and to the polarization of the CH bonds by the multipolar charge distribution carried by this carbonyl. The results are discussed in relation to experiment and the possibility of using 1H and 13C chemical shifts for the determination of the value of the torsion angle about the C alpha C' bond.