TRANSFERABILITY OF ELECTRON DENSITY: APPLICATION TO MACROMOLECULAR SYSTEMS. V. Pichon-Pesme, H. Lachekar, C. Lecomte, LCM3B, URA CNRS ndeg. 809, Université Henri Poincaré, Nancy 1, BP 239, 54506 Vandoeuvre-lés-Nancy Cédex, France

Because electron density is a local property, e^- density studies of the peptide molecules show that the nonspherical part of the deformation density (i.e. the P_lm parameters of Hansen-Coppens model) and the P_v valence population parameters remain essentially the same for a given atom in the same environment. We have determined for each chemical type of a given atom a small set of pseudoatom multipole parameters and we have used them to describe aspherical form factors.

The refinement of the structure of a peptide for which only low resolution data are available was improved by the use of these aspherical factors^[1]. The statistical indices decreased by about 30 %, the molecule pass the rigid bond test very well. On leu-enkephalin, we will show a comparison between the multipole refinement^[2] and a structure refinement with aspherical form factors and net atomic charges derived from our previous studies on other peptides. The agreement factors, the experimental deformation density map, the result of rigid bond test are comparable in both cases. These results lead us to consider this method for bigger molecules like small protein. Then, we are creating a data bank containing all the multipole parameters for each type of atom for the 20 amino acid residues. The preliminary results applied to high resolution data of a scorpio toxin will be shown.

[1] V. Pichon-Pesme, C. Lecomte, H. Lachekar, J. Phys. Chem., 1995, 99, 6242-6290

[2] R. Wiest, V. Pichon-Pesme, M. Bénard and C. Lecomte, J. Phys. Chem., 1994, 98, 1351-1362