ELECTRON CRYSTALLOGRAPHY

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(Supported by NIH GMS 46733 to D. Dorsett). During the last few years it has become obvious that electron crystallography is a practical method for determining molecular structure at the atomic level. Further, theoretical advances have made it possible to solve such structures using the traditional methods of crystallography with and without the use of images to make initial phase guesses. Dr. Dorset is a world leader in the development of these methodologies in general and in the use of direct methods of structure determination in particular. A co-worker of Dr. Dorset's, Dr. Herbert Hauptman, received the Noble Prize for development of these methods as applied to x-ray crystallography. One of the major problems limiting the advancement of electron crystallography is the treatment of secondary and dynamic scattering. Both of these phenomena are reduced if not eliminated by collecting the diffraction data using high-voltage, i.e. megavolt, electron beams. Previous progress reports have described the successful determination of the structures of copper perchloro- and perbromo-phthalocyanine. These model compounds illustrated both the possibility of determining atomic-level structures using electron crystallography and the necessity of high-voltage for the reduction of dynamical scattering. In contrast to the phthalocyanines, anthracene requires that a three-dimensional (3-D) data set be collected, which makes it a logical next step in the development of our capability. Anthracene also requires the use of our cryostage which allows us to gain experience in cryoelectron diffraction using relatively inexpensive and easily prepared specimens. A 3-D data set from anthracene is being collected to test our ability to extend our methods from two-dimensional projections to true 3-D structure determination. The anthracene data are being analyzed to determine both the intensities and the zone axes. Many patterns have been collected and analyze d, and this is ongoing. We are still working on determining how many different zone axes are represented, and will subsequently use the data set to identify problems with the merging of the data into a 3-D set. After an adequate data set has been assembled a full 3-D structure will be determined. A through-focus series of germanium film was caried out for determining the spherical aberration coefficient of the HVEM, needed for this work.