| Abstract
| - A comparison between three different methods commonly used to estimate ring current effects on chemicalshifts is presented. Haigh−Mallion, Johnson−Bovey, and classical point-dipole approximations were usedto estimate the ring current contribution to chemical shifts for protons in several proteins for which bothdetailed X-ray crystal structures and chemical shift assignments were available. For the classical point-dipole model, new proportionality constants were calculated by fitting to ring current estimations from boththe quantum-mechanical Haigh−Mallion and semiclassical Johnson−Bovey methods and compared withthe previously used point-dipole constant of Perkins and Dwek. Statistical analysis of the predictions obtainedby all methods indicates that the point-dipole approximation parametrized against quantum-mechanical datais superior to the previously used classical model, comparable to Johnson−Bovey calculations, and slightlypoorer than predictions from the Haigh−Mallion theory. The implementation of a pseudoenergy penaltyterm for use in structure refinement from chemical shift data based on the classical point-dipole model isdescribed, and its usefulness in cases where other NMR information is limited is discussed with a specificexample.
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