Funders, publishers, and research institutions must act to ensure that research computer code is made widely available.

In 1953, Pauling and Corey predicted that enantiomeric β-sheet peptides would coassemble into so-called “rippled” β-sheets, in which the β-sheets would consist of alternating l- and d-peptides. To date, this phenomenon has been investigated primarily with amphipathic peptide sequences composed of alternating hydrophilic and hydrophobic amino acid residues. Here, we show that enantiomers of a fragment of the amyloid-β (Aβ) peptide that does not follow this sequence pattern, amyloid-β (16–22), readily coassembles into rippled β-sheets. Equimolar mixtures of enantiomeric amyloid-β (16–22) peptides assemble into supramolecular structures that exhibit distinct morphologies from those observed by self-assembly of the single enantiomer pleated β-sheet fibrils. Formation of rippled β-sheets composed of alternating l- and d-amyloid-β (16–22) is confirmed by isotope-edited infrared spectroscopy and solid-state NMR spectroscopy. Sedimentation analysis reveals that rippled β-sheet formation by l- and d-amyloid-β (16–22) is energetically favorable relative to self-assembly into corresponding pleated β-sheets. This work illustrates that coassembly of enantiomeric β-sheet peptides into rippled β-sheets is not limited to peptides with alternating hydrophobic/hydrophilic sequence patterns, but that a broader range of sequence space is available for the design and preparation of rippled β-sheet materials.

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*Abstract:*It has been nearly twenty years since section 512 of the Digital Millennium Copyright Act established the so-called notice and takedown process. Despite its importance to copyright holders, online service providers, and Internet speakers, very little empirical research has been done on how effective section 512 is for addressing copyright infringement, spurring online service provider development, or providing due process for notice targets.This report includes three studies that draw back the curtain on notice and takedown:1. using detailed surveys and interviews with more than three dozen respondents, the first study gathers information on how online service providers and rightsholders experience and practice notice and takedown on a day-to-day basis;2. the second study examines a random sample from over 100 million notices generated during a six-month period to see who is sending notices, why, and whether they are valid takedown requests; and3. the third study looks specifically at a subset of those notices that were sent to Google Image Search.The findings suggest that whether notice and takedown “works” is highly dependent on who is using it and how it is practiced, though all respondents agreed that the Section 512 safe harbors remain fundamental to the online ecosystem. Perhaps surprisingly in light of large-scale online infringement, a large portion of OSPs still receive relatively few notices and process them by hand. For some major players, however, the scale of online infringement has led to automated, “bot”-based systems that leave little room for human review or discretion, and in a few cases notice and takedown has been abandoned in favor of techniques such as content filtering. The second and third studies revealed surprisingly high percentages of notices of questionable validity, with mistakes made by both “bots” and humans.The findings strongly suggest that the notice and takedown system is important, under strain, and that there is no “one size fits all” approach to improving it. Based on the findings, we suggest a variety of reforms to law and practice.Note: This is an updated version of the original paper. It includes two new appendices and some minor updates and corrections.Also available at SSRN: http://ssrn.com/abstract=2755628

Amphipathic peptides with alternating polar and nonpolar amino acid sequences efficiently selfassemble into functional b-sheet fibrils as long as the nonpolar residues have sufficient hydrophobicity. For example, the Ac-(FKFE) 2 -NH 2 peptide rapidly self-assembles into b-sheet bilayer nanoribbons, while Ac-(AKAE) 2 -NH 2 fails to self-assemble under similar conditions due to the significantly reduced hydrophobicity and b-sheet propensity of Ala relative to Phe. Herein, we systematically explore the effect of substituting only two of the four Ala residues at various positions in the Ac-(AKAE) 2 -NH 2 peptide with amino acids of increasing hydrophobicity, b-sheet potential, and surface area (including Phe, 1-naphthylalanine (1-Nal), 2-naphthylalanine (2-Nal), cyclohexylalanine (Cha), and pentafluorophenylalanine (F 5 -Phe)) on the self-assembly propensity of the resulting sequences. It was found that double Phe variants, regardless of the position of substitution, failed to self-assemble under the conditions used in this study. In contrast, all double 1-Nal and 2-Nal variants readily self-assembled, albeit at differing rates depending on the substitution patterns. To determine whether this was due to hydrophobicity or side chain surface area, we also prepared double Cha and F 5 -Phe variant peptides (both side chain groups are more hydrophobic than Phe). Each of these variants also underwent effective self-assembly, with the aromatic F 5 -Phe peptides doing so with greater efficiency. These findings provide insight into the role of amino acid hydrophobicity and sequence pattern on self-assembly proclivity of amphipathic peptides and on how targeted substitutions of nonpolar residues in these sequences can be exploited to tune the characteristics of the resulting self-assembled materials.hydrophobicity, peptide, self-assembly | I N TR ODU C TI ONThe development of biomaterials derived from self-assembled peptides is of great current interest due to biomedical applications including controlled release drug delivery, vaccine development, tissue engineering, and wound healing. Amphipathic peptides that consist of alternating hydrophobic and hydrophilic amino acids are a privileged class of peptide that readily self-assembles into b-sheet bilayer nanoribbons that share the basic characteristics of cross-b amyloid assemblies (Figure 1). [23][24][25][26][27][28][29][30] These bilayer nanoribbons effectively sequester all hydrophobic functionality to the bilayer interior, leaving hydrophilic groups exposed to solvent on the exterior face. [24,[31][32][33][34] As a result, these nanoribbons maintain high solubility in aqueous solvents, making them ideal materials for biological applications. Because of the high utility of this class of amphipathic peptide, they have been widely adopted as models to study the fundamental physicochemical basis of peptide self-assembly processes in order to facilitate the rational design of next-generation materials derived from self-assembled b-sheet peptides.[ [35][36][37][38][39]...