Semiempirical energy calculations on model compounds of polypeptides. Crystal structures of DL‐acetylleucine N‐methylamide and DL‐acetyl‐amino‐n‐butyric acid N‐methylamide

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Abstract
The sum E of the packing and conformation energies of the crystals of DL‐acetylleucine N‐methylamide (ALNMA) and DL‐acetyl‐α‐amino‐n‐butyric acid N‐methylamide (ABAMA) is calculated as a function of the crystallographic parameters and the conformational angles. The intermolecular energy is assumed to be the pairwise sum of nonbonded and electrostatic atomic interactions, while both these terms and intrinsic terms describing barriers of internal rotation contribute to the intramolecular energy. For ALNMA E is minimized with respect to 18 parameters: the minimum found when starting from the experimental structure agrees with this within 0.07 Å and 3°, except for one angle which deviates by 6° the average deviations of the atomic coordinates are \documentclass{article}\pagestyle{empty}$ |\overline {\Delta x|} = 0.02,|\overline {\Delta y|} = 0.07,|\overline {\Delta z|} = 0.08 $ Å. Another minimum with about the same energy shows slightly worse agreement. A comparison between different sets of nonbonded functions is made. The prediction of conformation and intermolecular packing of ABAMA is attempted on the basis of the knowledge of the unit cell and the space group. In agreement with available experimental data it is found that only one‐di‐mensional arrays of molecules linked by pairs of hydrogen bonds are compatible with the unit cell. The more stable of two possible conformations of the main chain agrees approximately with the experimental conformation. The calculation is not conclusive with regard to the side‐chain conformation and the packing of non‐hydrogen‐bonded molecules.