Electron distributions in peptides and related molecules. 1. An experimental and theoretical study of N-acetyl-L-tryptophan methylamide.

The thermal vibrations and electron density of N-Ac-L-Trp-NHMe have been analyzed using single-crystal X-ray diffraction data measured at 103 K with Mo K alpha radiation to a resolution corresponding to (sin theta max)/ lambda = 1.17 A-1. Measurements of 10,527 reflections gave 4913 unique data [R(int)(magnitude of F2) = 0.019] of which 2641 had I greater than 3 sigma (I). A multipolar atomic density model was fitted [R(magnitude of F) = 0.028] in order to calculate phases for the crystal structure factors and map the valence-electron distribution. The phase problem for determining deformation densities by Fourier synthesis for noncentrosymmetric crystals is discussed. The experimental density agrees well with the theoretical density from an ab initio SCF molecular wave function calculated at the crystallographic molecular geometry with a split-valence basis set. Both the experimental and theoretical analyses confirm that the electron distribution is the same in the two different peptide groups in the molecule. Crystal data: C14H17N3O2, Mr = 259.31, orthorhombic, P2(1)2(1)2(1), Z = 4, F(000) = 522 e from 295 to 103 K; at 295 K, a = 8.152(2), b = 11.170 (2), c = 15.068 (3) A, V = 1372 A3, Dx = 1.26 mg mm-3; at 103 K, a = 8.209 (3), b = 11.016 (2), c = 14.760 (4) A, V = 1135 A3, Dx = 1.29 mg mm-3, mu = 0.083 mm-1 for lambda = 0.7107 A.