Neutron diffraction analysis of myoglobin: structure of the carbon monoxide derivative.

The locations of hydrogen and deuterium atoms and water molecules have been investigated in carbon monoxide myoglobin using neutron diffraction, and the results are compared with earlier work on metmyoglobin. Parallel real space refinements on the two molecules show relatively few changes, but do show the carbon monoxide molecule with the iron atom moving into the heme plane.

Changing postdoctoral career patterns for biomedical scientists.

Between 1973 and 1977 the total number of Ph.D.'s holding postdoctoral appointments in the biomedical sciences increased at a rate of more than 550 individuals (12.5 percent) per year. During this same period the total number of doctorates awarded each year in these disciplines showed very little change. The postdoctoral growth can be attributed to substantial increases in both the numbers of recent graduates taking postdoctorals and the length of stay on these appointments. The lack of alternative employment opportunities has contributed heavily to the postdoctoral buildup. Continued growth is likely to have important consequences for biomedical research and research training.

The structure of carbon monoxide myoglobin: real-space refinement.

The refinement results in optimum positioning of atoms in the density in most cases, and the juxtaposition of positive and negative density leads to high contrast in the maps and rapid convergence of the refinement. The real-space technique is particularly useful in handling the problem of peak overlap, which is difficult in the neutron case. All refinements to date have been done on the original map, which was phased without hydrogen and deuterium atoms, but the next series of refinements will contain H and D atoms, hydrogen exchange information, and structured water molecules. Comparison of the refinements of CO, Mb and met Mb have shown the iron atom to move nearly into the heme plane. No deuterium atom exists to bond the distal histidine to the CO ligand. Atomic coordinates of all atoms and water molecules will be published on completion of the refinements.

Use of the tangent formula for the refinement of neutron protein data.

The tangent refinement technique has been shown to work marginally for protein neutron data, at least to the extent that the images of all atoms except hydrogen are sharpened. However, the process is very sensitive to over-refinement, and all the maps were sharpened excessively. Refinements in the squared structure were more stable, and the maps were somewhat better than those from the original structure refinement. This technique might be helpful in certain instances, but the poor quality of the maps prevents it from being generally useful. Perhaps the tangent refinement technique will be more fruitful in application to neutron protein data when experimental phase information is available.

The analyzer in neutron protein crystallography.

The use of a pyrolytic graphite analyzer is shown to contribute to the cleanliness of a neutron protein crystallographic study. If neutrons of approximately 1.5-A wavelength are used, higher orders are reduced by nearly an order of magnitude, and background (arising largely from incoherent inelastic neutron-proton scattering) is reduced by nearly a factor of five. These advantages are gained at the expense of approximately 50% of measured integrated intensity and a distortion of integrated intensity with scattering angle. Because background scatter is generally large compared with peak reflectivity of a protein, the large background reduction by the analyzer more than compensates for reduced peak intensity to improve the statistics of most peak reflectivity measurements. The luminance function distortion of intensity data is not large, produces a slight smearing of atomic scattering density, and can be calculated and adjusted for in the data.