Crystal Structure and Dielectric Property of Endohedral Lithium Fullerene

Endohedral lithium fullerene Li+@C60 can have a dielectric polarization by the off-centered location of the Li+ cation inside the C60 cage. The x-ray structure analysis of the PF6- salt [Li@C60](PF6) revealed that the Li+ cation occupies two off-centered equivalent positions at 20 K and hence the crystal is non-polar [1]. The disordered structure at low temperature is explained by a static orientation disorder of polar Li+@C60 cations and/or a dynamic tunneling of the Li+ cation inside the C60 cage. The Li+ tunneling would be suppressed by an intermolecular interaction at lower temperature and a dielectric phase transition might be induced. We reveal the dielectric property and crystal structure of [Li@C60](PF6) below 20 K in this study. The temperature dependence of the dielectric permittivity was measured for the single crystal down to 9 K. The dielectric permittivity increases with decreasing temperature according to the Curie-Weiss law. Such a behavior was also observed in H2O@C60 crystal but not in empty C60 crystal [2]. No dielectric phase transition was observed in H2O@C60 down to 8 K. In contrast, a dielectric anomaly suggesting a phase transition was observed in [Li@C60](PF6) around 18 K. The single-crystal x-ray diffraction experiment below 20 K was also performed at SPring-8 BL02B1. The crystal has a cubic structure at 20 K [1]. The temperature dependence of the cubic lattice constant shows no anomaly around 18 K. However, diffraction peaks that are forbidden for the given structure appear below 18 K. Thus the crystal symmetry is lowered by the dielectric phase transition. We present the result of the crystal structure analysis of the newly discovered low-temperature phase.

Crystal Structure analyses of dipeptidyl aminopeptidase BII from P. mexicana WO24.

The peptidase family S46 that contains the dipeptidyl aminopeptidase BII (DAP BII) from Pseudoxanthomonas mexicana WO24 is the only exopeptidase family in clan PA peptidases. Our present phylogenetic and experimental studies indicated that the catalytic triad of DAP BII is composed of His 86, Asp 224 and Ser 657 and implied that unknown large helical domains involved in exopeptidase activity[1]. However, three-dimensional structure of a family S46 peptidase has not yet been reported. Thus, the crystal structure of DAP BII is essential not only to understand the catalytic mechanism of family S46 peptidases but also to clarify the structural origin of the exo-type peptidase activities of these enzymes. Recently, we have successfully crystallized the DAP BII and collected X-ray diffraction data to 2.3 Å resolution from the crystal. This crystal belonging to space group P2₁2₁2₁, with unit-cell parameters a = 76.55 Å, b = 130.86 Å, c = 170.87 Å[2]. Structural analysis by the multi-wavelength anomalous diffraction method is underway[3]. Here, we report the first crystallization and structural analysis of the DAP BII from P. mexicana WO24 as family S46 peptidase. Other enzymes that belong to this family are DPP7 and DPP11 from Porphyromonas gingivalis, DPP11 from Porphyromonas endodontalis (periodontal pathogen) and DPP11 from Shewanella putrefaciens (multidrug resistance associated opportunistic pathogen). These gram-negative bacterial pathogens are known to asaccharolytic. Especially, Porphyromonas gingivalis is known to utilize dipeptides, instead of free amino acids, as energy source and cellular material. Since S46 peptidases are not found in mammals, we expect our study will be useful for the discovery of specific inhibitors to S46 peptidases from these pathogens.