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There is an alarming rate of human African trypanosomiasis recrudescence in many parts of sub-Saharan Africa. Yet, the disease has no successful chemotherapy. Trypanosoma lacks the enzymatic machinery for the de novo synthesis of purine nucleotides, and is critically dependent on salvage mechanisms. Inosine 5′-monophosphate dehydrogenase (IMPDH) is responsible for the rate-limiting step in guanine nucleotide metabolism. Here, we characterize recombinant Trypanosoma brucei IMPDH (TbIMPDH) to investigate the enzymatic differences between TbIMPDH and host IMPDH. Size-exclusion chromatography and analytical ultracentrifugation sedimentation velocity experiments reveal that TbIMPDH forms a heptamer, different from type 1 and 2 mammalian tetrameric IMPDHs. Kinetic analysis reveals calculated Km values of 30 and 1300 μm for IMP and NAD, respectively. The obtained Km value of TbIMPDH for NAD is approximately 20–200-fold higher than that of mammalian enzymes and indicative of a different NAD binding mode between trypanosomal and mammalian IMPDHs. Inhibition studies show Ki values of 3·2 μm, 21 nM and 3·3 nM for ribavirin 5′-monophosphate, mycophenolic acid and mizoribine 5′-monophosphate, respectively. Our results show that TbIMPDH is different from its mammalian counterpart and thus may be a good target for further studies on anti-trypanosomal drugs.
Photonic crystals, in which the refractive index changes periodically, provide an exciting new tool for the manipulation of photons and have received keen interest from a variety of fields. This article reviews recent progress in the manipulation of photons by photonic crystals. First, the article covers spontaneous emission, a fundamental phenomenon associated with all photonic devices that emit light, which now can be successfully controlled. Light emission is suppressed in areas where the photonic crystal is complete, while strong emission occurs in the areas where there are artificial defects. Next, it is shown that a very strong confinement of photons in a small volume on the scale of cubic wavelengths becomes possible by using photonic crystals, where nanocavities with ultrahigh-Q values of more than 2 million have been successfully demonstrated. Finally, photonic crystals promise to realize unprecedented types of lasers, which can produce tailored beams on demand, while keeping stable single longitudinal and lateral modes.
We introduce the lasing principle and important characteristics of photonic-crystal surface-emitting laser (PC-SELs). Specifically, we demonstrate two-dimensional coherent lasing oscillation with GaN PC-SELs, using a unique crystal growth technique called “air hole retained overgrowth” (AROG). Above the threshold, we obtained a two-dimensionally distributed near-field pattern, and a distinctive far-field pattern with a divergence angle less than 1°. We also investigate a suitable sample structure for the reduction of the threshold current, where the PC structure is moved from an n-cladding layer to a p-cladding one. This is an important step towards the realization of novel light sources that can be integrated two dimensionally for a variety of new scientific and engineering applications in the blue to ultraviolet wavelengths.
A channel add-filtering function of a device based on defects in a two dimensional photonic crystal slab is investigated in addition to a channel drop-filtering function reported previously. It is considered that photons irradiated to a point defect from free space are trapped by the point defect, are transferred to a line defect waveguide nearby, and are emitted from the waveguide edge. It is shown that the spectrum of the add-filtering function is almost same as that of the drop-filtering function, which demonstrates that photons can be not only dropped-from but also added-to a line-defect waveguide through the resonant trapping by point defects.
There has been increasing interest in photonic crystals in which the refractive index changes periodically. A photonic bandgap can be formed in the crystals, and the propagation of electromagnetic waves is prohibited for all wave vectors in this bandgap. Various important scientific and engineering applications, such as control of spontaneous emission, sharp bending of light, trapping of photons, and so on, may be realized by creating photonicbandgap crystals and engineering the defects and light-emitters. In the field of two-dimensional (2D) photonic crystals, some important contributions aiming at device applications have included Scherer et al.'s demonstration that a single defect can be utilized as a very tiny cavity for light emission, and Joannopoulos et al.'s work on 2D photonic-crystal circuits. Here, the present status of our work in III–V semiconductor-based 2D and 3D photonic crystals is briefly reviewed.
Based on a set of requirements identified for photonic crystals intended for use in optoelectronic devices, we have developed a method of fabricating three-dimensional photonic crystals that involves stacking air/semiconductor gratings by wafer fusion approach. Precise alignment of the stacked layers is achieved through the use of a laser beam assisted very precise alignment system, and three-dimensional photonic crystal has been successfully fabricated for the infrared and optical communication wavelength regions. We have also developed a photonic crystal waveguide providing sharp 90° bend.
Substratum heterogeneity on rocky shores can affect the distribution pattern of sessile epibenthic organisms. The rocky shore at Usujiri, southern Hokkaido, Japan, is composed of well-developed, columnar jointed dolerites. In this area, landscape is characterized by a mosaic of barnacle and mussels, where species composition in patches differs among typical topographic-classes, i.e. barnacles dominate on horizontal planes and vertical planes while mussels only inhabit in grooves. To determine the factors maintaining those distribution patterns, the recruitment of dominant sessile species in the typical topographic-classes with and without adults of barnacles and mussels were examined by field experiment. The results showed that the distribution pattern was determined by recruitment, and the recruitment pattern was strongly affected by rock-surface topography. Moreover, the presence of conspecific adults contributed to the maintenance of this topography-related mosaic by facilitating recruitment. These results show: (1) the presence of resource division for rocky topography among barnacles and mussels; and (2) that intraspecific positive interaction rather than interspecific competition may play important role in maintaining the topography-related mosaic of barnacle and mussels at Usujiri.
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