The speed limit of optoelectronics.

Light-field driven charge motion links semiconductor technology to electric fields with attosecond temporal control. Motivated by ultimate-speed electron-based signal processing, strong-field excitation has been identified viable for the ultrafast manipulation of a solid's electronic properties but found to evoke perplexing post-excitation dynamics. Here, we report on single-photon-populating the conduction band of a wide-gap dielectric within approximately one femtosecond. We control the subsequent Bloch wavepacket motion with the electric field of visible light. The resulting current allows sampling optical fields and tracking charge motion driven by optical signals. Our approach utilizes a large fraction of the conduction-band bandwidth to maximize operating speed. We identify population transfer to adjacent bands and the associated group velocity inversion as the mechanism ultimately limiting how fast electric currents can be controlled in solids. Our results imply a fundamental limit for classical signal processing and suggest the feasibility of solid-state optoelectronics up to 1 PHz frequency.

Next Generation Driver for Attosecond and Laser-plasma Physics.

The observation and manipulation of electron dynamics in matter call for attosecond light pulses, routinely available from high-order harmonic generation driven by few-femtosecond lasers. However, the energy limitation of these lasers supports only weak sources and correspondingly linear attosecond studies. Here we report on an optical parametric synthesizer designed for nonlinear attosecond optics and relativistic laser-plasma physics. This synthesizer uniquely combines ultra-relativistic focused intensities of about 1020 W/cm2 with a pulse duration of sub-two carrier-wave cycles. The coherent combination of two sequentially amplified and complementary spectral ranges yields sub-5-fs pulses with multi-TW peak power. The application of this source allows the generation of a broad spectral continuum at 100-eV photon energy in gases as well as high-order harmonics in relativistic plasmas. Unprecedented spatio-temporal confinement of light now permits the investigation of electric-field-driven electron phenomena in the relativistic regime and ultimately the rise of next-generation intense isolated attosecond sources.

Partitioning of polar fatty acids into lymph and portal vein after intestinal absorption in the rat.

We tested the hypothesis that fatty acids destined for the portal vein after intestinal absorption would be diverted into lymph when infused along with a saturated long-chain fatty acid. Thoracic fistula rats were infused intraduodenally with either linolenic (18:3), lauric (12:0), or decanoic (10:0) acid, or with each fatty acid in combination with 5 mM palmitic acid (16:0), in micellar solutions of taurocholate (10 mM) and 2-mono-oleoylglycerol. Lymphatic transport of linolenic acid was enhanced by co-absorption with palmitic acid: when 0.1 mM linolenic acid was infused alone, 32 +/- 8% of that absorbed and transported beyond the mesentery was carried in lymph. The addition of palmitic acid to the infusate increased the percentage transported in lymph to 56 +/- 10% (P less than 0.005). The increment was due to enhanced intracellular re-esterification of linolenate into triacylglycerol. When 5 mM linolenic acid was infused, the comparable figures for lymphatic transport were 55 +/- 2% for linolenate infused alone and 66 +/- 6% for linolenate infused with palmitate (P less than 0.005). In contrast, the predominantly portal venous transport of lauric and decanoic acids was unaffected by co-absorption with palmitate. We conclude that the partitioning of long-chain fatty acids between portal blood and lymph is dependent on the luminal milieu, in addition to polarity of the fatty acid and the rate of absorption. Unsaturated long-chain fatty acids have a substantial portal transport under conditions which simulate normal food ingestion.

SV40 VP1 assembles into disulfide-linked postpentameric complexes in cell-free lysates.

The simian virus 40 (SV40) capsid is composed of pentameric capsomeres of the major structural protein, Vp1. The chemical nature of Vp1-Vp1 interactions, as well as the role of the minor structural proteins, Vp2 and Vp3, in SV40 assembly is not clear. We show here that Vp1 molecules synthesized in rabbit reticulocyte lysates self-assembled into postpentameric 12S complexes in the absence of other viral structural proteins and in a time and concentration dependent manner. The 12S complexes were resistant to perturbants of noncovalent interactions but were sensitive to reduction by dithiothretiol. Nonreducing SDS-PAGE analysis revealed disulfide-linked VP1 complexes of > 400 kDa. Our results are consistent with crystallography studies of SV40 which suggest involvement of disulfide bonds at a postcapsomeric stage of viral assembly.

Portal venous transport of long-chain fatty acids absorbed from rat intestine.

We studied the route of transport of long-chain fatty acids (LCFA) from the rat intestine. Lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18:0), linoleic (18:2), and linolenic (18:3) acids were infused intraduodenally for 4 h as micellar solutions into unanesthetized thoracic duct-fistula rats. Proportionally more of each LCFA was transported by the portal vein at infusion rates of 0.3 mumol/h than at 15 mumol/h. Of the LCFA absorbed at low rates of infusion, 72% of lauric, 58% of myristic, 41% of palmitic, 28% of stearic, 58% of linoleic, and 68% of linolenic acid bypassed the lymphatic pathway. To test the inference that 58% of absorbed linoleate was transported by the portal vein, [14C]linoleic acid was infused at 0.3 or 15 mumol/h into unanesthetized rats equipped with both thoracic duct and portal venous fistulas. Portal venous blood (0.1 ml) was withdrawn every 30 min for 4 h. Proportionally more [14C]linoleate was recovered in portal blood at the low infusion rate. To examine the morphology of fat absorption, segments of rat jejunum in anesthetized rats were infused with micellar linoleic acid at 3 or 150 mumol/h. At 3-mumol/h infusion rate, an appearance identical to the fasting state was seen by electron microscopy. At 150 mumol/h, many larger chylomicron-sized particles appeared in absorptive cells, intercellular spaces, and lymphatics. We conclude that a substantial proportion of unsaturated LCFA is transported from rat intestine in portal venous blood.

Poliovirus 3C protease-mediated degradation of transcriptional activator p53 requires a cellular activity.

Infection of HeLa cells with poliovirus leads to rapid shut-off of host cell transcription by RNA polymerase II. Previous results have suggested that both the basal transcription factor TBP (TATA-binding protein) and transcription activator proteins such as CREB (cyclic AMP-responsive element-binding protein) and Oct-1 (the octamer-binding factor) are cleaved by the viral-encoded protease, 3C(Pro). Here we demonstrate that the transcriptional activator (and tumor suppressor) p53 is degraded by the viral protease 3C both in vivo and in vitro. Unlike other transcription factors that are directly cleaved by 3C(pro), degradation of p53 requires a HeLa cell activity in addition to 3C(Pro). The degradation of p53 by 3C(Pro) does not appear to involve the ubiquitin pathway of protein degradation. Vaccinia virus infection of HeLa cells leads to inactivation of the cellular activity required for 3C(Pro)-mediated degradation of p53. The vaccinia-encoded protein (CrmA) is known to inhibit caspase I (ICE protease) that converts inactive IL-1beta to an active secreted form. Incubation of HeLa cells with caspase I inhibitor Z-VAD-fmk does not interfere with 3C(Pro)-mediated degradation of p53. The cellular activity present in extracts of HeLa cells can be fractionated through phosphocellulose. A partially purified fraction that elutes at 0.6 M KCl from phosphocellulose contains the activity that degrades p53 in a 3C(Pro)-dependent manner. These results suggest that both poliovirus-encoded protease 3C(Pro) and a cellular activity are required for the degradation of p53 observed in cells infected with poliovirus.