ELECTRON DISTRIBUTIONS IN PEPTIDES AND RELATED MOLECULES - EXPERIMENTAL AND THEORETICAL-STUDY OF LEU-ENKEPHALIN TRIHYDRATE

The crystal structure, thermal vibrations, and electron density of leu -enkephalin trihydrate, C28H37N5O7.3H(2)O, have been analyzed using si ngle-crystal X-ray diffraction data measured at 100 K with Mo K alpha radiation to a resolution corresponding to [(sin theta)/lambda](max) = 1.15 Angstrom(-1). A multipolar atom density model was fitted against the 5444 unique data with I > 2 sigma(I) [R(F) = 0.038, R(w)(F) = 0.0 23, GOF = 0.73] in order to calculate the phases for the crystal struc ture factors and map the valence electron distribution. Ab initio SCF calculations have been carried out on molecular fragments with extende d basis sets including polarization functions. The experimental densit ies along the equivalent bonds of the four peptide groups are equal to within 0.05 e Angstrom(-3) and are in close agreement with those obta ined for smaller peptides. Fragment theoretical distributions derived either with or without the molecules of crystallization water are used to figure out the influence of hydrogen bonds on the deformation dens ity. Experimental and theoretical maps derived in the plane containing the intramolecular H bonds characteristic of the beta turns evidence the character of weak N-H...pi bonds of those interactions. Leu-enkeph alin trihydrate crystallizes in the orthorhombic space group P2(1)2(1) 2(1) With M(r) = 609.68, D-x= 1.34 g cm(-3), Z = 4, and cell dimension s (100 K) a 10.851(4) Angstrom, b = 13.095(3) Angstrom, c 21.192(4) An gstrom, and V = 3011(3) Angstrom(3).