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 angstrom-1. Measurements of 10 527 reflections gave 4913 unique data [R(int)(\F\2) = 0.019] of which 2641 had I > 3-sigma(I). A multipolar atomic density model was fitted [R(\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, M(r) = 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) angstrom, V = 1372 angstrom-3, D(x) = 1.26 mg mm-3; at 103 K, a = 8.209 (3), b = 11.016 (2), c = 14.760 (4) angstrom, V = 1335 angstrom-3, D(x) = 1.29 mg mm-3, mu = 0.083 mm-1 for lambda = 0.7107 angstrom.