Powder-bed additive manufacturing for aerospace application: Techniques, metallic and metal/ceramic composite materials and trends

The current paper is devoted to classification of powder-bed additive manufacturing (PB-AM) techniques and description of specific features, advantages and limitation of different PB-AM techniques in aerospace applications. The common principle of “powder-bed” means that the used feedstock material is a powder, which forms “bed-like” platform of homogeneous layer that is fused according to cross-section of the manufactured object. After that, a new powder layer is distributed with the same thickness and the “printing” process continues. This approach is used in selective laser sintering/melting process, electron beam melting, and binder jetting printing. Additionally, relevant issues related to powder raw materials (metals, ceramics, multi-material composites, etc.) and their impact on the properties of as-manufactured components are discussed. Special attention is paid to discussion on additive manufacturing (AM) of aerospace critical parts made of Titanium alloys, Nickel-based superalloys, metal matrix composites (MMCs), ceramic matrix composites (CMCs) and high entropy alloys. Additional discussion is related to the quality control of the PB-AM materials, and to the prospects of new approaches in material development for PB-AM aiming at aerospace applications

Novel complex MAD phasing and RNase H structural insights using selenium oligonucleotides

The crystal structures of protein–nucleic acid complexes are commonly determined using selenium-derivatized proteins via MAD or SAD phasing. Here, the first protein–nucleic acid complex structure determined using selenium-derivatized nucleic acids is reported. The RNase H–RNA/DNA complex is used as an example to demonstrate the proof of principle. The high-resolution crystal structure indicates that this selenium replacement results in a local subtle unwinding of the RNA/DNA substrate duplex, thereby shifting the RNA scissile phosphate closer to the transition state of the enzyme-catalyzed reaction. It was also observed that the scissile phosphate forms a hydrogen bond to the water nucleophile and helps to position the water molecule in the structure. Consistently, it was discovered that the substitution of a single O atom by a Se atom in a guide DNA sequence can largely accelerate RNase H catalysis. These structural and catalytic studies shed new light on the guide-dependent RNA cleavage

Limitations in current acetylcholinesterase structure–based design of oxime antidotes for organophosphate poisoning

Acetylcholinesterase (AChE; EC 3.1.1.7), an essential enzyme of cholinergic neurotransmission in vertebrates, is a primary target in acute nerve agent and organophosphate (OP) pesticide intoxication. Catalytically inactive OP-AChE conjugates formed between the active-center serine and phosphorus of OPs can, in principle, be reactivated by nucleophilic oxime antidotes. Antidote efficacy is limited by the structural diversity of OP-AChE conjugates resulting from differences in the structure of the conjugated OP, the different active-center volumes they occupy when conjugated to the active-center serine of AChE, and the distinct chemical characteristics of both OPs and oximes documented in numerous X-ray structures of OP-conjugated AChEs. Efforts to improve oxime reactivation efficacy by AChE structure-based enhancement of oxime structure have yielded only limited success. We outline here the potential limitations of available AChE X-ray structures that preclude an accurate prediction of oxime structures, which are necessary for association in the OP-AChE gorge and nucleophilic attack of the OP-conjugated phosphorus