Accurate protein crystallography at ultra-high resolution: Valence electron distribution in crambin

The charge density distribution of a protein has been refined experimentall y. Diffraction data for a crambin crystal were measured to ultra-high resol ution (0.54 Angstrom) at tow temperature by using short-wavelength synchrot ron radiation. The crystal structure was refined with a model for charged, nonspherical, multipolar atoms to accurately describe the molecular electro n density distribution. The refined parameters agree within 25% with our tr ansferable electron density library derived from accurate single crystal di ffraction analyses of several amino acids and small peptides, The resulting electron density maps of redistributed valence electrons (deformation maps ) compare quantitatively well with a high-level quantum mechanical calculat ion performed on a monopeptide. This study provides validation for experime ntally derived parameters and a window into charge density analysis of biol ogical macromolecules.