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

The number of protein structures refined at a resolution higher than 1.0 Angstrom is continuously increasing. Subatomic structures may deserve a more sophisticated model than the spherical atomic electron density. In very high resolution structural studies (d < 0.5 Angstrom) of small peptides, a multipolar atom model is used to describe the valence electron density. This allows a much more accurate determination of the anisotropic thermal displacement parameters and the estimate of atomic charges, This information is of paramount importance in the understanding of biological processes involving enzymes and metalloproteins. The structure of the scorpion Androctonus australis 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 valence 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 underestimated. Such protein-structure refinements against higher resolution data collected at cryogenic temperature will enable the calculation of experimental atomic charges and properties such as electrostatic potentials.