The proposed research aims to develop new biocrystallographic methodologies to extract maximal information from atomic-resolution diffraction data from protein crystals. Such data are now becoming available at a rapidly increasing rate, and from such data, crystallography can reveal, in quantitative detail, not only the three-dimensional geometric atomic structure of protein crystals, but also their three dimensional electrostatic atomic structure, their acid-base hydrogen structure, and the dynamical structure of their modes of anisotropic mean-square atomic displacement due to disorder and/or thermal vibration. The proposed work will employ and develop ultrahigh-resolution, ultralow-cryotemperature crystallographic techniques in order to map and analyze at atomic, or even sub-atomic, resolution the aspherical atomic valence electron density distributions, the molecular electrostatic potential distributions, and the structural dynamical modes in protein crystals. The crystallographic charge density and electrostatic potential analyses will employ an established experimental databank of transferable, amino acid and oligopeptide, electron density parameters; and a complementary theoretical databank based on high-level quantum chemical calculations will be built, tested, and compared with the experimental databank. Experimental charge density distributions will be used to derive experimental net atomic charges, electrical dipole and higher multiple moments, and electrostatic interaction energies, for atoms and functional groups of atoms and protein molecules interacting with one another and with the molecules in their solvation shells in crystallography. In addition, crystallographic analyses of mean-square atomic displacements will fit structural dynamical models of flexibly-joined rigid-body segments or domains to model the external and internal modes of protein molecular motions in crystallography. Test cases for the development of these new biocrystallographic methodologies will be analyses of the roles of structured electrostatics and structured dynamical modes in the phenomena of insulin hexamer allosterism and ribonuclease. Both problems will be analyzed by both X-ray and neutron diffraction.
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