1.
Influence of surface cracks on laser-induced damage resistance of brittle KH₂PO₄ crystal.
Cheng, J, Chen, M, Liao, W, Wang, H, Wang, J, Xiao, Y, Li, M
Optics express. 2014;(23):28740-55
Abstract
Single point diamond turning (SPDT) currently is the leading finishing method for achieving ultra-smooth surface on brittle KH(2)PO(4) crystal. In this work, the light intensification modulated by surface cracks introduced by SPDT cutting is numerically simulated using finite-difference time-domain algorithm. The results indicate that the light intensification caused by surface cracks is wavelength, crack geometry and position dependent. Under the irradiation of 355 nm laser, lateral cracks on front surfaces and conical cracks on both front and rear surfaces can produce light intensification as high as hundreds of times, which is sufficient to trigger avalanche ionization and finally lower the laser damage resistance of crystal components. Furthermore, we experimentally tested the laser-induced damage thresholds (LIDTs) on both crack-free and flawed crystal surfaces. The results imply that brittle fracture with a series of surface cracks is the dominant source of laser damage initiation in crystal components. Due to the negative effect of surface cracks, the LIDT on KDP crystal surface could be sharply reduced from 7.85J/cm(2) to 2.33J/cm(2) (355 nm, 6.4 ns). In addition, the experiment of laser-induced damage growth is performed and the damage growth behavior agrees well with the simulation results of light intensification caused by surface cracks with increasing crack depths.
2.
Effect of inorganic phosphate on FMN binding and loop flexibility in Desulfovibrio desulfuricans apo-flavodoxin.
Muralidhara, BK, Chen, M, Ma, J, Wittung-Stafshede, P
Journal of molecular biology. 2005;(1):87-97
Abstract
The complex between flavin mononucleotide (FMN) and apo-flavodoxin is dominated by isoalloxazine-stacking interactions and 5'-phosphate hydrogen bonds. We show here that FMN binding to Desulfovibrio desulfuricans apo-flavodoxin is faster and the affinity is higher in the presence of inorganic phosphate as compared to in its absence (I=110 mM, pH 7, 20 degrees C). The transition-state of complex formation was investigated by phi-value analysis using Trp60Ala and Tyr98Ala apo-flavodoxin variants. We find that Tyr98 is highly involved in the FMN/protein transition state independent of inorganic phosphate, whereas the participation of Trp60 is modulated by inorganic phosphate. The phi-value for Trp60 is higher without phosphate, implying that at this condition stronger binding of Trp60 is required in the transition state to assure successful complex formation. Consistent with the experimental data, all-atom molecular dynamic simulations reveal that the presence of an anion in the phosphate subsite restricts the mobility of the Trp60-containing loop in terms of both backbone and side-chain movements, but has no effect on the Tyr98-containing loop. The overall thermodynamic stability of apo-flavodoxin is higher in the presence of inorganic phosphate as compared to in its absence (I=110 mM, pH 7, 20 degrees C). Kinetic experiments reveal that the additional stability originates in slower unfolding. The combined experimental and computational observations demonstrate that phosphate has an ordering effect on the Trp60-containing loop, which positions Trp60 favorably for FMN binding and increases the barrier for protein unfolding.