The effect of surface roughness was studied on the formation of femtosecond laser-induced ripple structures for 4H-SiC. Of the two types of ripple structures, fine and coarse, the threshold fluence for the formation of the fine ripple was remarkably reduced on the surface roughened by intentional rubbing, while the threshold for the formation of the coarse ripple was not affected. The result suggests that the initial surface roughness plays a significant role only for the formation of fine ripples. A brief explanation will be given on the qualitative basis for the specific effect of surface morphology to the fine ripple formation.

We found that irradiation by a single femtosecond laser pulse produced a surface wave structure on a (100) silicon surface. The surface wave structure bearing a concentric shape was produced only in the edge regions in the direction of the electric field from the center. The increase in the number of irradiating pulses allowed the concentric structure gradually to align perpendicularly to the laser beam’s electric field. The period of the surface wave structure was nearly equal to the wavelength of the irradiating laser pulse. In this accord, we propose that this structure is a precursor of ripple formation.

Upon femtosecond pulsed-laser irradiation, periodic structures referred to as “ripples” were fabricated on the surface of a 4H–SiC single crystal. The periodic structures consisted of two concentric regions in the irradiated spots which were clearly distinguished by the period. Surface morphologies were characterized as a function of energy, accumulation number, and interval of pulses. The difference in the threshold of fine and coarse ripples was identified for the first time. The possible formation mechanisms of these structures were discussed.

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Oligopeptidase B (OPB) is a member of the prolyl oligopeptidase (POP) family of serine proteases. OPB in trypanosomes is an important virulence factor and potential pharmaceutical target. Characteristic structural features of POP family members include lack of a propeptide and presence of a β-propeller domain (PD), although the role of the β-PD has yet to be fully understood. In this work, residues Glu(172), Glu(490), Glu(524) and Arg(689) in Trypanosoma brucei OPB (Tb OPB), which are predicted to form inter-domain salt bridges, were substituted for Gln and Ala, respectively. These mutants were evaluated in terms of catalytic properties and stability. A negative effect on kcat/Km was obtained following mutation of Glu(172) or Arg(689). In contrast, the E490Q mutant exhibited markedly decreased thermal stability, although this mutation had less effect on catalytic properties compared to the E172Q and R689A mutants. Trypsin digestion showed that the boundary regions between the β-PD and catalytic domains (CDs) of the E490Q mutant are unfolded with heat treatment. These results indicated that Glu(490) in the CD plays a role in stabilization of Tb OPB, whereas Glu(172) in the β-PD is critical for the catalytic activity of Tb OPB.