Precise measurement of deuteron tensor analyzing powers with BLAST

We report a precision measurement of the deuteron tensor analyzing powers T(20) and T(21) at the MIT-Bates Linear Accelerator Center. Data were collected simultaneously over a momentum transfer range Q=2.15-4.50 fm(-1) with the Bates Large Acceptance Spectrometer Toroid using a highly polarized deuterium internal gas target. The data are in excellent agreement with calculations in a framework of effective field theory. The deuteron charge monopole and quadrupole form factors G(C) and G(Q) were separated with improved precision, and the location of the first node of G(C) was confirmed at Q=4.19±0.05 fm(-1). The new data provide a strong constraint on theoretical models in a momentum transfer range covering the minimum of T(20) and the first node of G(C).     

Search for effects beyond the born approximation in polarization transfer observables in e(over→)p elastic scattering

Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, G(E)/G(M), obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H(e[over →],e(')p[over →]) reaction for three different beam energies at a Q(2)=2.5 GeV(2), spanning a wide range of the kinematic parameter ε. The ratio R, which equals μ(p)G(E)/G(M) in the Born approximation, is found to be independent of ε at the 1.5% level. The ε dependence of the longitudinal polarization transfer component P(?) shows an enhancement of (2.3±0.6)% relative to the Born approximation at large ε.     

Conditional glycosylation in eukaryotic cells using a biocompatible chemical inducer of dimerization

Chemical inducers of dimerization (CIDs) are cell-permeable small molecules capable of dimerizing two protein targets. The most widely used CID, the natural product rapamycin and its relatives, is immunosuppressive due to interactions with endogenous targets and thus has limited utility in vivo. Here we report a new biocompatible CID, Tmp-SLF, which dimerizes E. coli DHFR and FKBP and has no endogenous mammalian targets that would lead to unwanted in vivo side effects. We employed Tmp-SLF to modulate gene expression in a yeast three-hybrid assay. Finally, we engineered the Golgi-resident glycosyltransferase FucT7 for tunable control by Tmp-SLF in mammalian cells.     

Noncovalent complexes of APS reductase from <Emphasis Type="Italic">M. tuberculosis</Emphasis>: Delineating a mechanistic model using ESI-FTICR MS

ESI-FTICR MS was utilized to characterize a 4Fe-4S containing protein Mycobacterium tuberculosis APS reductase. This enzyme catalyzes the reduction of APS to sulfite and AMP with reducing equivalents from the protein cofactor, thioredoxin. Under nondenaturing conditions, a distribution of the apoprotein, a 2Fe-2S intermediate, and the 4Fe-4S holoprotein were observed. Accurate mass measurements indicated an oxidation state of +2 for the 4Fe-4S cluster, with no disulfide bond in the holoenzyme. Gas-phase stability of the 4Fe-4S cluster was investigated using both in-source and collision induced dissociation, which provided information regarding the relative gas-phase binding strength of iron towards protein ligands and inorganic sulfides. Noncovalent complexes of the holoprotein with several ligands, including APS, thioredoxin, and AMP, were also investigated. Calculated values of dissociation constants for the complexes indicate that AMP binds with a higher affinity to the enzyme intermediate than to the free enzyme. The implications of the binary and ternary complexes observed by gas-phase noncovalent interactions in the mechanism of APS reduction are discussed.     

Self-assembled cellular microarrays patterned using DNA barcodes

The successful integration of living cells into synthetic devices requires precise control over cell patterning. Here we describe a versatile platform that can accomplish this goal through DNA hybridization. Living cells functionalized with exogenous cell-surface DNA strands bind to cognate sequences of DNA printed on glass slides. Attachment via these "cell-adhesion barcodes" is rapid and specific, with close-packed arrays of cells forming within minutes. The biophysical properties of the system are characterized, and the technique is used to form complex cellular patterns with single-cell line widths and self-assembled cellular microarrays. Key advantages of DNA-directed cell binding include the ability to immobilize both adherent and non-adherent cells, to capture cells selectively from a mixed population, to tune the binding properties of the cells, and to reuse substrates prepared with widely available DNA printing technologies.     

Glycocalyx Engineering with a Recycling Glycopolymer that Increases Cell Survival In Vivo

Synthetic glycopolymers that emulate cell‐surface mucins have been used to elucidate the role of mucin overexpression in cancer. However, because they are internalized within hours, these glycopolymers could not be employed to probe processes that occur on longer time scales. In this work, we tested a panel of glycopolymers bearing a variety of lipids to identify those that persist on cell membranes. Strikingly, we found that cholesterylamine (CholA) anchored glycopolymers are internalized into vesicles that serve as depots for delivery back to the cell surface, allowing for the display of cell‐surface glycopolymers for at least ten days, even while the cells are dividing. As with native mucins, the cell‐surface display of CholA‐anchored glycopolymers influenced the focal adhesion distribution. Furthermore, we show that these mimetics enhance the survival of nonmalignant cells in a zebrafish model of metastasis. CholA‐anchored glycopolymers therefore expand the application of glycocalyx engineering in glycobiology.