*authors contributed equally to this work 2 DNA origami is a robust molecular assembly technique by which a single-stranded DNA template is folded by annealing it with hundreds of short 'staple' strands. 1-4 The guiding design principle of nanofabrication by DNA self-assembly is that the target structure is the single most stable configuration; 5 however, the pathway and kinetics of origami assembly are poorly understood. The folding transition is cooperative 4,6,7 , and there is a strong analogy with protein folding: both are governed by information encoded in polymer sequence. [8][9][10][11] Misfolded structures are kinetic traps. The yield of well-folded DNA origami can be low: 2 yield is improved by titration of cations 2,12 or by following empirical design rules, 12,13 but it is frequently necessary to separate wellfolded origami from misfolded objects. 2, 3, 14-16 Here, we present an origami structure that is designed to reveal the assembly process. Our system has the unusual property of having a small set of distinguishable, well-folded shapes that represent discrete and approximately degenerate energy minima in a vast folding landscape. We obtain a high yield of well-folded origami structures, demonstrating the existence of efficient folding pathways. The distribution of shapes provides information about individual trajectories through the folding landscape. We show that the assembly pathway can be steered by rational design and identify similarities to protein folding: assembly is highly cooperative; reversible bond-formation is important in recovering from transient misfoldings; and the early formation of long-range connections can be very effective in forcing particular folds. Expanding the rational design process to include the assembly pathway is the key to reproducible synthesis, which is essential if nucleic acid selfassembly is to continue its rapid development 1-3,17-19 and become a reliable manufacturing technology. 20 3This study is based on a simplified version of the archetypal origami tile 1 and, in particular, on the distribution of observed folds of a 'dimer' variant which contains two copies of the template sequence in head-to-tail repeat. The 'monomer' tile ( Fig. . 1) (Fig. 1c); approximately 80% of tiles appear to be well folded.The 'dimer' template is also circular. It contains two identical copies of the monomer joined head-to-tail and can therefore bind two copies of each staple (Fig. 2). Each pair of body and seam staples can bind in one of two configurations (Fig. 2a) to form either an internal link within each copy of the monomer sequence or a pair of cross-links between the two copies.The total number of possible domain pairings is 2 76 ≈ 10 23 . Although many of these configurations are sterically inaccessible it is clear that the result of reducing the specificity of staple binding is that, as in the case of protein folding, the number of possible states of the system is overwhelmingly greater than the number of well-folded structures. However, in contrast to proteins (and t...
In the nacre or aragonite layer of the mollusk shell there exist proteomes which regulate both the early stages of nucleation and nano-to-mesoscale assembly of nacre tablets from mineral nanoparticle precursors. Several approaches have been developed to understand protein-associated mechanisms of nacre formation, yet we still lack insight into how protein ensembles or proteomes manage nucleation and crystal growth. To provide additional insights we have created a proportionally-defined combinatorial model consisting of two nacre-associated proteins, C-RING AP7 (shell nacre, H. rufescens) and pseudo-EF hand PFMG1 (oyster pearl nacre, P. fucata) whose individual in vitro mineralization functionalities are well-documented and distinct from one another. Using SEM, flow cell STEM, AFM, Ca(II) potentiometric titrations and QCM-D quantitative analyses, we find that both nacre proteins are functionally active within the same mineralization environments, and at 1:1 mole ratios, synergistically create calcium carbonate mesoscale structures with ordered intracrystalline nanoporosities, extensively prolong nucleation times and introduce an additional nucleation event. Further, these two proteins jointly create nanoscale protein aggregates or phases that under mineralization conditions further assemble into protein-mineral PILP-like phases with enhanced ACC stabilization capabilities, and there is evidence for intermolecular interactions between AP7 and PFMG1 under these conditions. Thus, a combinatorial model system consisting of more than one defined biomineralization protein dramatically changes the outcome of the in vitro biomineralization process.
The SARS-CoV-2 virus is primarily transmitted through virus-laden fluid particles ejected from the mouth of infected people. Face covers can mitigate the risk of virus transmission but their outward effectiveness is not fully ascertained. Objective: by using a background oriented schlieren technique, we aim to investigate the air flow ejected by a person while quietly and heavily breathing, while coughing, and with different face covers. Results: we found that all face covers without an outlet valve reduce the front flow through by at least 63% and perhaps as high as 86% if the unfiltered cough jet distance was resolved to the anticipated maximum distance of 2-3 m. However, surgical and handmade masks, and face shields, generate significant leakage jets that may present major hazards. Conclusions: the effectiveness of the masks should mostly be considered based on the generation of secondary jets rather than on the ability to mitigate the front throughflow. INDEX TERMS COVID-19 pandemic, face coverings, face masks, aerosol dispersal, aerosol generating procedures. IMPACT STATEMENT These results show the effectiveness of face coverings in mitigating aerosol dispersion and can aid policy makers to make informed decisions and PPE developers to improve their product effectiveness.
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