Myoglobin models and steric origins of the discrimination between O 2 and CO

 Synthetic models of the myoglobin active site have provided much insight into factors that affect CO and O 2 binding in the proteins. "Capped" and "pocket" metal porphyrin systems have been developed to probe how steric factors affect ligand binding and ultimately to elucidate important aspects of the mechanism of CO discrimination in the proteins. These model porphyrins are among the most thoroughly characterized systems to date. From the twenty-one known crystal structures, analysis of the types of distortion that occur upon ligand binding under the cap, including porphyrin doming and ruffling, lateral and horizontal movement of the cap, and bending and tilting of the Fe–C–O bond, provides an indication of how steric interactions will affect structure in Hb and Mb. The model porphyrin systems discussed range from those that discriminate against O 2 binding compared to biological systems to those with similar CO and O 2 binding strength to myoglobin, and also to those that bind both O 2 and CO very weakly or not at all. The primary type of distortion observed upon CO binding is vertical or lateral movement of the cap and some ruffling of the porphyrin plane. Minimal bending or tilting of the M–C–O bond is observed, suggesting that the Fe–C–O bending that has been found from crystal structures of the hemoproteins is unlikely.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42324/1/775-2-4-521_70020521.pd

The crystal structure of uytenbogaardtite, Ag3AuS2, and its relationships with gold and silver sulfides-selenides

The file attached is the Accepted/final draft post-refereeing version of the article

X-ray scattering intensities of water at extreme pressure and temperature

We have calculated the coherent x-ray scattering intensity of several phases of water at 1500 and 2000 K under high pressure, using ab initio Density Functional Theory (DFT). Our calculations span the molecular liquid, ice VII, and superionic solid phases, including the recently predicted symmetrically hydrogen bonded region of the superionic phase. We show that wide angle x-ray scattering intensity could be used to determine phase boundaries between these high pressure phases, and we compare the results for ice VII and superionic water. We compute simulated spectra and provide new atomic scattering form factors for water at extreme conditions, which take into account frequently neglected changes in ionic charge and electron delocalization. We show that our modifed atomic form factors allow for a nearly exact comaprison to the total x-ray scattering intensities calculated from DFT. Finally, we analyze the effect our new form factors have on determination of the oxygen-oxygen radial distribution function

INFRARED EVIDENCE FOR A LARGE ISOTOPE EFFECT ON THE POTENTIAL ENERGY CURVE OF THE HYDROGEN BOND IN HCrO2HCrO_{2}

$^{1}$R. M. Douglass, Acta Cryst, 10, 423 (1957).Author Institution: Shell Development CompanyFrom x-ray diffraction studies $HCrO_{2}$ is $reported^{1}$ to belong to the trigonal system and to have one molecule per unit cell with a hydrogen bond of length 2.54 \AA. Its infrared spectrum is consistent with the proposed structure and with the short hydrogen bond However, the spectrum of $DCrO_{2}$ dues not correspond to an $HCrO_{2}$ spectrum which has been modified simply by a mass change. We find: (1) the ratio of the stretching frequencies $\nu OH/$OD is near unity (theoretical 1.38);(2) the ratio of the bending frequencies $\nu \angle_{OHO}/\nu \angle_{ODO}$ exceeds considerably the theoretical value (observed 1.47: theoretical 1.39); (3) the O-H stretching band is a singlet but the O-D stretching hand is a doublet; (4) in the lattice mode region (700-$300 cm^{-1}$) there are two bands for $HCrO_{2}$ but there are five for $DCrO_{2}$). The above findings become explicable if we assume that the potential energy curve for the hydrogen bond is altered in going from $HCrO_{2}$ to $DCrO_{2}$. Specifically we require for $HCrO_{2}$ that the potential barrier separating the two minima be lower than the ground vibrational level of the proton; for $DCtO_{2}$ we require that the Lanier be considerably higher than the ground level for the deuterium Such a change in the height of the hairier is reasonable if the oxygens of the hydrogen bond are separated more in $DCrO_{2}$ than in $HCrO_{2}$. We have measured the lattice constants of $HCrO_{2}$ and $DCrO_{2}$ and find significant differences which if interpreted as arising solely from a change in the O-O distance, indicate a 0.04 expansion in that distance in going from $HCrO_{2}$ to $DCrO_{2}$, This expansion appears sufficient to lead to a significantly weaker hydrogen bond in $DCrO_{2}$