NMR CHEMICAL SHIELDING SURFACE OF N-ACETYL-N'-METHYLALANINAMIDE - A DENSITY-FUNCTIONAL STUDY

The five energetically lowest minima on the potential energy surface o f N-acetyl-N'-methylalaninamide were optimized at the Becke3LYP/DZd le vel of theory to compare these density functional theory results with the literature findings at restricted Hartree-Fock/3-21G. While the re lative energies are very similar, the amide moiety is predicted to be much more flexible at Becke3LYP/DZd. As a consequence, the three minim a that favor a nonplanar amide group differ by up to 14 degrees in the ir phi and psi values between the two levels. To compare the change in the density functional NMR chemical shifts with respect to phi and ps i with experimental results, Becke3LYP/DZd was employed to optimize a structure for N-acetyl-N'-methylalaninamide at each 30 degrees interva l on the (phi,psi) surface in the regions that correspond to the alpha helix and the beta-pleated sheet and at each 60 degrees interval else where. The corresponding NMR chemical shielding surface was computed w ith the density functional program deMon. The resultant NMR chemical s hielding surfaces for N and C-beta are in good agreement with the expe riment, while the change in the NMR chemical shielding of C' and C-alp ha cannot be described only in terms of phi and psi. The chemical shif ts for those atoms also depend on the nonplanarity of the amide moiety . We evaluated this dependence for N-methylacetamide as a model system . Estimates of the parameters derived from N-methyl-acetamide allowed the NMR-shielding surfaces of C' and C-alpha to be corrected for the n onplanar nitrogen influence. Although the effect is less pronounced wi th lower level theoretical geometries, due to the smaller degree of py ramidalization of the amide nitrogen, the (phi, psi)NMR chemical shiel ding surfaces will need to be corrected. The agreement with the experi ment was much better for the corrected surface of C' when the nitrogen in the alpha helix had a nonplanar environment. (C) 1997 by John Wile y & Sons, Inc.