Towards the charge-density study of proteins: a room-temperature scorpion-toxin structure at 0.96 angstrom resolution as a first test case

The number of protein structures refined at a resolution higher than 1.0 An gstrom is continuously increasing. Subatomic structures may deserve a more sophisticated model than the spherical atomic electron density. In very hig h resolution structural studies (d < 0.5 Angstrom) of small peptides, a mul tipolar atom model is used to describe the valence electron density. This a llows a much more accurate determination of the anisotropic thermal displac ement parameters and the estimate of atomic charges, This information is of paramount importance in the understanding of biological processes involvin g enzymes and metalloproteins. The structure of the scorpion Androctonus au stralis Hector toxin II has been refined at 0.96 Angstrom resolution using synchrotron diffraction data collected at room temperature. Refinement with a multipolar electron-density model in which the multipole populations are transferred from previous peptide studies led to the observation of valenc e electrons on covalent bonds of the most ordered residues. The refined net charges of the peptide-bond atoms were of the correct sign but were undere stimated. Such protein-structure refinements against higher resolution data collected at cryogenic temperature will enable the calculation of experime ntal atomic charges and properties such as electrostatic potentials.