Science Concentrates MATERIALS
Pulling on polyladderenes Insulating polyladderene unzips to form semiconducting polymer Using mechanical force, rather than light or heat, to transform molecules from one form to another has gained traction in the past decade, creating the field of polymer mechanochemistry. Stanford University chemists have made a polymer that transforms from a nonconducting form to a semiconducting one in response to n
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onds. With longer sonication, the material darkens and transforms into an insoluble mesh of semiconducting nanowires (Science 2017, DOI: 10.1126/science.aan2797). The polymer could be used, for example, to report on physical stresses in a material. “It’s a creative work of mechanochemical beauty,” comments Jeffrey S. Moore, a mechanochemistry pioneer at the University of Illinois, Urbana-Champaign. “I wish we’d have thought of this ourselves.” Next, the chemists hope to create sim-
showed him the ladderane lipids his group had been synthesizing. The two realized these ladder-like fused cyclobutanes would be a great motif for the polymer Xia envisioned. When subjected to mechanical force, they imagined that the σ bonds in the cyclobutanes would unzip to form continuous π bonds as in polyacetylene
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mechanical force. One day, such a material could find use in mimicking the sense of touch or hearing, in which a mechanical force is converted into an electrical signal. Yan Xia’s group spent two years trying to design the polymer without success. One day Xia was chatting with his Stanford colleague Noah Z. Burns, when Burns
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(shown). Their colleague Todd J. Martinez joined the collaboration to study the mechanism of this transformation via computer modeling. When sonicated in solution—a common technique for applying mechanical force— the polyladderene structure they made goes from colorless to blue in a matter of sec-
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pler structures that accomplish the same task. The polyladderene synthesis currently yields decent amounts, Burns says. “But if we ever wanted to do commercial applications, our synthesis, as it stands, would not be viable,” he says. “So we are actively making simpler monomers that take less steps to get to.”—BETHANY HALFORD
BIOCHEMISTRY
Janitorial system in red blood cells discovered
C R E D I T: S H UT T E RSTO CK
Enzyme helps trash most of the proteins inside cells that are transforming into red blood cells Red blood cells are a stripped-down version of most other cells: The oxygen-carrying workhorses are packed with hemoglobin and little else. As immature red blood cells called reticulocytes transform into dedicated oxygen carriers, they trash their nucleus, organelles, and most proteins. Two groups of scientists are now reporting that this massive and selective clearance event occurs thanks to a protein degradation pathway involving an enzyme called UBE2O. A team led by Daniel Finley and Mark D. Fleming at Harvard
Medical School report that in the last stages of red blood cell differentiation, UBE2O remodels the proteome inside reticulocytes by tagging the small protein ubiquitin onto protein “trash” to signal for its breakdown, leaving the cell with approximately 98% αand β-globin, the subunits that join to form hemoglobin (Science 2017, DOI: 10.1126/ science.aan0218). Meanwhile, Ramanujan S. Hegde and his team at the Medical Research Council Laboratory of Molecular Biology in Cambridge, England, discovered that in reticulocytes, UBE2O also degrades rogue
α-globin proteins that have not formed functional hemoglobin. In other types of cells, Hegde’s team similarly found that UBE2O helps clear out orphan proteins that have also failed to join multiprotein complexes. The enzyme recognizes the proteins and tags them at many sites with individual ubiquitin molecules to signal for degradation. This is unlike other ubiquitin-based degradation pathways, which tag proteins for destruction with long chains of ubiquitin molecules, explain Randolph Y. Hampton at the University of California, San Diego, and Catherine Dargemont at Paris Diderot University in an associated commentary (Science 2017, DOI: 10.1126/science. aao1896). “Further understanding of UBE2O and other quality-control pathways might open new therapeutic avenues” they add.—SARAH EVERTS AUGUST 7, 2017 | CEN.ACS.ORG | C&EN
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