On the single-pixel approximation in maximum-entropy analysis

Charge density studies utilizing powder diffraction and MEM. Exploring of high Tc superconductors, C60 superconductors and manganites

The recent progress of the accurate charge density studies by the Maximum Entropy Method(MEM) utilizing X-ray powder diffraction is reviewed with some examples. Results for PrBCO (PrBa2Cu3O7-δ), YBCO (YBa2Cu3O7-δ), C60 superconductors (Rb2CsC60, K2RbC60, Na2RbC60) and the layered manganite, NdSr2Mn2O7, which is well known as colossal magnetoresistive (CMR)-related material, are given. For non-super conductor, PrBCO, it is found in the MEM charge density that there exist directional robes of the charge density from Pr atom toward the O atoms in the CuO2 conduction planes. On the other hand, for a very well known high Tc super conductor, YBCO, appreciable charge densities in the interatomic region around the Y atom is not recognized in the MEM charge density. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr(4f)–O(2pπ) orbitals which supports the idea that the hole trapping by the hybridizes states suppresses the superconductivity in PrBCO. The MEM charge densities of the fullerene superconductors, Rb2CsC60, K2RbC60 and Na2RbC60, show distinct structural differences from that of non-superconductors, C60 and Li2CsC60, reflecting the superconducting properties. And the charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have strong correlation to the superconducting transition temperature, Tc. As the bigger the charge charge trnsfer, the higher the Tc. In the accurate MEM charge density of antiferromagnetic manganite, NdSr2Mn2O7,the direct imaging of spontaneous ordering of the dx 2-y 2 orbital indicating anisotropic exchange couplings between the local-spins on the Mn sites, which causes an unique layered-type spin order. The theoretical background of the MEM is also mentioned in some detail.

Peptide binding in OppA, the crystal structures of the periplasmic oligopeptide binding protein in the unliganded form and in complex with lysyllysine

The periplasmic oligopeptide binding protein, OppA, acts as the initial receptor for the uptake of peptides by the oligopeptide permease (Opp) in Gram-negative bacteria. Opp will handle peptides between two and five amino acid residues regardless of their sequence. The crystal structures of a series of OppA-peptide complexes have revealed an enclosed but versatile peptide binding pocket and have illustrated how tri- and tetrapeptide ligands are accommodated. Here, the crystal structures of (i) OppA complexed with a dipeptide (lysyllysine) and (ii) unliganded OppA have been solved using X-ray data extending to 1.8 and 2.4 A spacing, respectively. In the dipeptide complex, the alpha-amino group of the ligand is anchored through an ion pair interaction with Asp419, as observed in complexes with longer peptides. However, its alpha-carboxylate group forms water-mediated interactions with the guanidinium groups of Arg404 and Arg413 rather than the direct salt bridges to Arg413 and His371 observed in the tripeptide and tetrapeptide complexes, respectively. Isothermal titration calorimetric measurements of the binding of lysine-containing peptides of different lengths to OppA show that the dipeptide, KK, is bound with approximately 60-fold lower affinity than related tri- and tetrapeptides (KKK and KKKA, respectively). These data are discussed with reference to the calculated enthalpic and entropic contributions to ligand binding and the structures of the OppA peptide complexes. In the unliganded molecule, domain III has rotated as a rigid body through 26 degrees away from domains I and II, exposing the ligand binding site. The water structure in the binding cleft shows similarities to that in the various OppA-peptide complexes.