CHEMICAL-SHIFTS IN PROTEINS - A SHIELDING TRAJECTORY ANALYSIS OF THE FLUORINE NUCLEAR-MAGNETIC-RESONANCE SPECTRUM OF THE ESCHERICHIA-COLI GALACTOSE BINDING-PROTEIN USING A MULTIPOLE SHIELDING POLARIZABILITY LOCAL REACTION FIELD MOLECULAR-DYNAMICS APPROACH

We report the first successful calculation of the fluorine nuclear mag netic resonance spectrum of a protein, the galactose binding protein f rom Escherichia coli, labeled with [5-F-19]tryptophan. Our results ind icate that the experimental F-19 chemical shifts are dominated by weak , or long-range, electrical interactions, which can be calculated by u sing the responses of the shielding tensor elements to the uniform fie ld components (partial derivative sigma(alphabeta)/partial derivative E(x)) and the nonuniform or gradient terms (partial derivative sigma(a lphabeta)/partial derivative V(ii)), together with the average values of the fields (E(x)) and field gradients (V(ii)) obtained from molecul ar dynamics (MD) trajectories. A series of ''shielding trajectories'', DELTAsigma(E(x), V(xx), V(yy), V(zz)) f(tau), are obtained, and the m ean values, DELTAsigmaBAR, correlate well with the actual shift patter n and overall shift range observed experimentally (Luck, L. A.; Falke, J. J. Biochemistry 1991, 30, 4248-4256). The computed F-19 NMR shield ing of the pentapeptide Gly-Gly-[5-F]Trp-Gly-Gly in H2O is close to, b ut somewhat more shielded than, that of the denatured protein. Almost all computed F-19 chemical shifts are upfield of the field-free value, in accord with the results of ab initio calculations. The chemical sh ifts calculated are sensitive to the charge chosen for F in the LRF-MD trajectories, and best agreement with experiment is obtained with q(F ) = -0.25. Calculations on the Salmonella typhimurium galactose bindin g protein yield extremely similar chemical shift spectra, consistent w ith the approximately 7% difference in amino acid composition. The uni form field components make the largest contributions to the shielding patterns observed, presumably because the gradient terms fall off more rapidly with distance. The exposed residue, Trp 284, has the largest amplitude of fluctuation associated with its shielding trajectory, pos sibly due to the rapid and random movement of neighboring water molecu les. Trp 284 is highly shielded due to interaction with water, althoug h other buried residues (e.g. Trp 133) may also be highly shielded, du e to electric field effects within the protein. Our results imply that van der Waals interactions do not play a major role for fluorine shie lding nonequivalence in proteins, since the experimental results can b e reproduced by using solely the computed field and field gradient ter ms. The ability to predict protein NMR shielding patterns and ranges o ffers promise for structural analysis and also provides a way of valid ating different methods of computing protein electrostatics. In this r espect, it is instructive to note that the charge fields from ionized surface groups are found to be largely shielded by the dielectric resp onse of the solvent and the protein.