Comparisons of ring-current shifts calculated from the crystal structure of egg white lysozyme of hen with the proton nuclear magnetic resonance spectrum of lysozyme in solution.

Detailed ring-current calculations with some consideration for protein flexibility are described for egg white lysozyme of hen. These are based on 15 sets of crystal coordinates of lysozyme and over 60 assigned resonances in the 1H nuclear magnetic resonance (NMR) spectrum of lysozyme. These analyses evaluate two basic assumptions: that the refined protein crystal structure is similar to the protein structure in solution and that the present ring-current theories in application to the aromatic amino acids offer a good description of the observed shifts in the protein NMR spectrum. The dipolar, Johnson-Bovey, and Haigh-Mallion equations are tested. The conventional ring-current intensity factors for the tyrosine and phenylalanine rings of 0.94 and 1.00 are found to be satisfactory, but those for the tryptophan ring need to be increased. With this, ring-current effects offer good explanations for the chemical shifts of the aliphatic and aromatic protons in lysozyme for which the NMR signals have to date been resolved and assigned. Ring-current effects do not, however, explain directly the shifts of the six tryptophan N-H protons nor those of the alpha protons. Applications of these calculations are described to compare the tetragonal and triclinic forms of native lysozyme, to propose several assignments of 1H NMR signals in the spectrum of lysozyme, and to analyze some of the conformational changes which occur on the binding of Gd(III) and GlcNAc sugars to lysozyme. The uses and limitations of the calculations for protein NMR are briefly discussed.