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A persistent spin helix Just as a body moving in a vacuum tends to stay in motion, the axis of a spinning electron tends to remain fixed in direction. Both phenomena are conservation laws that ultimately derive from the uniformity of empty space. By contrast, an electron moving in a semiconductor sees a lattice of charged atoms flying past at nearly 1% of light speed, causing its spin direction to fluctuate wildly. Now Koralek et al . demonstrate that the application of an external electric field to a semiconductor can precisely balance the spin-destabilizing effect of the charged lattice. The collective spin of the entire gas of electrons, rather than that of each individual particle, then emerges as a new conserved quantity — a property well suited for 'spintronics' applications. The axis of a spinning electron tends to remain fixed in direction: in contrast, an electron moving in a semiconductor sees a lattice of charged atoms flying past, causing its spin direction to fluctuate. Koralek and colleagues demonstrate that an electric field applied to the semiconductor can balance this spin-destabilizing effect; the collective spin of the entire gas of electrons is conserved, a property well-suited for 'spintronics' applications. According to Noether’s theorem 1 , for every symmetry in nature there is a corresponding conservation law. For example, invariance with respect to spatial translation corresponds to conservation of momentum. In another well-known example, invariance with respect to rotation of the electron’s spin, or SU(2) symmetry, leads to conservation of spin polarization. For electrons in a solid, this symmetry is ordinarily broken by spin–orbit coupling, allowing spin angular momentum to flow to orbital angular momentum. However, it has recently been predicted that SU(2) can be achieved in a two-dimensional electron gas, despite the presence of spin–orbit coupling 2 . The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the ‘persistent spin helix’ 2 . SU(2) is realized, in principle, when the strengths of two dominant spin–orbit interactions, the Rashba 3 (strength parameterized by α ) and linear Dresselhaus 4 ( β 1 ) interactions, are equal. This symmetry is predicted to be robust against all forms of spin-independent scattering, including electron–electron interactions, but is broken by the cubic Dresselhaus term ( β 3 ) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as α approaches β 1 . Here we report experimental observation of the emergence of the persistent spin helix in GaAs quantum wells by independently tuning α and β 1 . Using transient spin-grating spectroscopy 5 , we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point. Excellent quantitative agreement with theory across a wide range of sample parameters allows us to obtain an absolute measure of all relevant spin–orbit terms, identifying β 3 as the main SU(2)-violating term in our samples. The tunable suppression of spin relaxation demonstrated in this work is well suited for application to spintronics 6 , 7 .

An electron propagating through a solid carries spin angular momentum in addition to its mass and charge. Of late there has been considerable interest in developing electronic devices based on the transport of spin that offer potential advantages in dissipation, size and speed over charge-based devices 1 . However, these advantages bring with them additional complexity. Because each electron carries a single, fixed value (- e ) of charge, the electrical current carried by a gas of electrons is simply proportional to its total momentum. A fundamental consequence is that the charge current is not affected by interactions that conserve total momentum, notably collisions among the electrons themselves 2 . In contrast, the electron's spin along a given spatial direction can take on two values, ± ℏ /2 (conventionally ↑,↓), so that the spin current and momentum need not be proportional. Although the transport of spin polarization is not protected by momentum conservation, it has been widely assumed that, like the charge current, spin current is unaffected by electron–electron ( e – e ) interactions. Here we demonstrate experimentally not only that this assumption is invalid, but also that over a broad range of temperature and electron density, the flow of spin polarization in a two-dimensional gas of electrons is controlled by the rate of e – e collisions.

Various classes of nucleotidyl polymerases with different transcriptional roles contain a conserved core structure. Less is known, however, about the distinguishing features of these enzymes, particularly those of the RNA-dependent RNA polymerase class. The 1.9 Å resolution crystal structure of hepatitis C virus (HCV) nonstructural protein 5B (NS5B) presented here provides the first complete and detailed view of an RNA-dependent RNA polymerase. While canonical polymerase features exist in the structure, NS5B adopts a unique shape due to extensive interactions between the fingers and thumb polymerase subdomains that serve to encircle the enzyme active site. Several insertions in the fingers subdomain account for intersubdomain linkages that include two extended loops and a pair of antiparallel α-helices. The HCV NS5B apoenzyme structure reported here can accommodate a template:primer duplex without global conformational changes, supporting the hypothesis that this structure is essentially preserved during the reaction pathway. This NS5B template:primer model also allows identification of a new structural motif involved in stabilizing the nascent base pair.

A detailed technical overview summarizes and discusses available evidence concerning the effects of fish oil n-3 polyunsaturated fatty acids (PUFA's) on reducing cardiovascular disease risks. Attention is given to: the characteristics and metabolism of n-6 and n-3 PUFA's and the effects of n-3 PUFA's on plasma lipid profiles; the role of n-3 PUFA's in reducing atherosclerosis risk; possible protection of n-3 PUFA's during myocardial infarction; the relative importance of eicosapentaenoic vs. docosahexaneoic acids; and appropriate intakes of n-3 PUFA's. The potential adverse health effects from ingesting fish oils also are discussed.(wz)

The high affinity of the noncovalent interaction between biotin and streptavidin forms the basis for many diagnostic assays that require the formation of an irreversible and specific linkage between biological macromolecules. Comparison of the refined crystal structures of apo and a streptavidin:biotin complex shows that the high affinity results from several factors. These factors include the formation of multiple hydrogen bonds and van der Waals interactions between biotin and the protein, together with the ordering of surface polypeptide loops that bury the biotin in the protein interior. Structural alterations at the biotin binding site produce quaternary changes in the streptavidin tetramer. These changes apparently propagate through cooperative deformations in the twisted $\\beta $ sheets that link tetramer subunits.

Dioxygenases catalyse the cleavage of molecular oxygen with subsequent incorporation of both oxygen atoms into organic substrates. Some of the best-studied dioxygenases have been isolated from bacteria where they catalyse the critical ring-opening step in the biodegradation of aromatic compounds. These bacterial enzymes generally contain nonheme ferric iron as the sole cofactor. Protocatechuate 3,4-dioxygenase (3,4-PCD) was one of the first such enzymes recognized and catalyses the intradiol cleavage of protocatechuic acid by oxygen to produce beta-carboxy-cis,cis-muconic acid. Previous studies have shown that the 3,4-PCD found in Pseudomonas aeruginosa is an oligomer with a relative molecular mass (Mr) of 587,000 (587K) containing 12 copies each of alpha (22.3K) and beta (26.6K) subunits. The X-ray structure determination of 3,4-PCD reveals the catalytic iron environment required for oxygenolytic cleavage of aromatic rings and also provides a novel holoenzyme assembly with cubic 23(T) symmetry and first examples of mixed beta-barrel domains.

Increased consumption of n-3 fatty acids reduces mortality from sudden cardiac death, indicating that such acids have anti-arrhythmic effects. We did electrophysiological testing in ten patients with implanted cardioverter defibrillators who were at high risk of sudden cardiac death. To assess their immediate effects on the induction of sustained ventricular tachycardia, n-3 fatty acids were infused. Such tachycardia was not induced in five of seven patients. Our findings show that infusion of n-3 polyunsaturated fatty acids does not induce arrhythmia, but did result in a reduction of sustained ventricular tachycardia in some patients.

We examined whether the synthesis of interleukin-1 or tumor necrosis factor, two cytokines with potent inflammatory activities, is influenced by dietary supplementation with n—3 fatty acids. Nine healthy volunteers added 18 g of fish-oil concentrate per day to their normal Western diet for six weeks. We used a radioimmunoassay to measure interleukin-1 (IL-1β and IL-1α) and tumor necrosis factor produced in vitro by stimulated peripheral-blood mononuclear cells. With endotoxin as a stimulus, the synthesis of IL-1β was suppressed from 7.4±0.9 ng per milliliter at base line to 4.2±0.5 ng per milliliter after six weeks of supplementation (43 percent decrease; P = 0.048). Ten weeks after the end of n-3 supplementation, we observed a further decrease to 2.9±0.5 ng per milliliter (61 percent decrease; P = 0.005). The production of IL-1α and tumor necrosis factor responded in a similar manner. Twenty weeks after the end of supplementation, the production of IL-1βIL-1α, and tumor necrosis factor had returned to the presupplement level. The decreased production of interleukin-1 and tumor necrosis factor was accompanied by a decreased ratio of arachidonic acid to eicosapentaenoic acid in the membrane phospholipids of mononuclear cells. We conclude that the synthesis of IL-1β, IL-1α, and tumor necrosis factor can be suppressed by dietary supplementation with long-chain n—3 fatty acids. The reported antiinflammatory effect of these n—3 fatty acids may be mediated in part by their inhibitory effect on the production of interleukin-1 and tumor necrosis factor. (N Engl J Med 1989;320:265–71.) INTERLEUKIN-1 and tumor necrosis factor are polypeptide cytokines synthesized by monocytes and other cells in response to injury, as well as to infectious, inflammatory, or immunologic challenges. 1 2 3 These cytokines induce local inflammatory changes and mediate several systemic acute-phase responses. 4 , 5 Moreover, interleukin-1 and tumor necrosis factor often act synergistically — e.g., on vascular endothelium, 6 insulin production, 7 and the synthesis of metabolites of arachidonic acid. 8 The n—3 (also called omega—3) fatty acids are polyunsaturated fatty acids in which the last double bond is located between the third and fourth carbon atom from the methyl end of the fatty acid chain. These fatty . . .

New treatments for HCV (Hepatitis C virus) infections are likely to arise from inhibition of the essential, virally-encoded enzymes. These targets include the serine protease required for processing of the HCV polyprotein. The protease constitutes one functional domain of the bifunctional HCV NS3 (non-structural protein 3). Here, insights regarding the NS3 structure and recently synthesized NS3 inhibitors are reviewed. Interestingly, many NS3 protease inhibitors have taken advantage of an unusual product inhibition by Nterminal products of cleavage at the polyprotein processing sites.

Helicases are nucleotide triphosphate (NTP)-dependent enzymes responsible for unwinding duplex DNA and RNA during genomic replication. The 2.1 Å resolution structure of the HCV helicase from the positive-stranded RNA hepatitis C virus reveals a molecule with distinct NTPase and RNA binding domains. The structure supports a mechanism of helicase activity involving initial recognition of the requisite 3′ single-stranded region on the nucleic acid substrate by a conserved arginine-rich sequence on the RNA binding domain. Comparison of crystallographically independent molecules shows that rotation of the RNA binding domain involves conformational changes within a conserved TATPP sequence and untwisting of an extended antiparallel β-sheet. Location of the TATPP sequence at the end of an NTPase domain β-strand structurally homologous to the ‘switch region’ of many NTP-dependent enzymes offers the possibility that domain rotation is coupled to NTP hydrolysis in the helicase catalytic cycle.