Inverted Temperature Dependence Research Articles

Kinetic Isotope Effect of Prostaglandin H Synthase Exhibits Inverted Temperature Dependence

Conversion of arachidonic acid to prostaglandin G2/H2 catalyzed by prostaglandin H synthase (PGHS) is proposed to involve initial transfer of the C13 pro-(S) hydrogen atom from arachidonate to the Tyr385 radical in PGHS, followed by insertion of two oxygen molecules and several chemical bond rearrangements. The initial hydrogen-transfer was recently concluded to be a rate-limiting step in cyclooxygenase catalysis based on the observed intrinsic deuterium kinetic isotope effect values (Dkcat). In the present study, we have found that Dkcat values of both PGHS-1 and -2 show an unusual increase with temperatures in the range of 288–310 K, exhibiting an inverted temperature dependence. The value of lnDkcat, however, decreased linearly with 1/T, consistent with a typical Arrhenius relationship.

Modeling and characterization of thermally induced skew on clock distribution networks of nanometric ICs

Temperature has traditionally been a key parameter to take into account during the many stages of IC design flows, and in particular, during the sign-off phases of critical circuit components like the Clock Distribution Networks (CDNs). While for old technologies this task was accomplished by means of worst case corner-based static analysis, the advent of nanometric CMOS technologies made this approach intrinsically inadequate.This paper provides a detailed analysis of clock skew variations induced by non-uniform thermal profiles on tree-like CDNs. Using a dedicated simulation framework, we characterized the complex thermal effects that metal interconnects and buffers under inverted temperature dependence (ITD) may induce on the clock tree.Experiments conducted on a synthetic, thermal-programmable benchmark underline the presence of unexpected behaviors that standard tools are not able to catch.

Dual-[formula omitted] assignment policies in ITD-aware synthesis

Traditionally, the effects of temperature on delay of CMOS devices have been evaluated using the highest operating temperature as a worst-case corner. This conservative approach was based on the fact that, in older technologies, CMOS devices systematically degraded their performance as temperature increases.With the progressive scaling of technology, however, there has been a continuous reduction of the gap between supply and threshold voltages of devices, mostly due to low-power constraints. The latter have accelerated this trend by using libraries containing multiple instances of a cell with different ranges of threshold voltages; in particular, the use of high-Vt cells to control sub-threshold leakage currents has made this gap smaller and smaller.The consequence of this trend is the occurrence of the so-called inverted temperature dependence (ITD), under which cells get faster as temperature increases. This new thermal dependence has made the old worst-case design approach obsolete, posing new EDA challenges. Beside complicating timing analysis, in particular, ITD has important and unforeseeable consequences for power-aware design, especially in dual-Vt logic synthesis. Due to a contrasting temperature dependence between low-Vt cells (which enjoy the classical, direct temperature dependence) and high-Vt cells (for which an inverted temperature dependence holds), a single-temperature worst-case design approach fails to generate netlists that are compliant with timing constraints for the entire temperature range.In this work, we first validate the relevance of ITD on an industrial 65nm CMOS multi-Vt library. Then, we describe an ITD-aware, dual-Vt assignment algorithm that guarantees temperature-insensitive operation of the circuits, together with a significant reduction of both leakage and total power consumption. The algorithm has been tested over standard benchmarks using three different replacement policies.Experimental results show an average leakage power savings of 50% w.r.t. circuits synthesized with a standard, commercial flow that does not take ITD into account and thus, to ensure that no temperature-induced timing faults occur, needs to resort to over-design (i.e., over-constraining the timing bound so as to make sure that temperature fluctuations never make the circuits violating the specified required time for all paths).

Handling inverted temperature dependence in static timing analysis

In digital circuit design, it is typically assumed that cell delay increases with decreasing voltage and increasing temperature. This assumption is the basis of the cornering approach with cell libraries in static timing analysis (STA). However, this assumption breaks down at low supply voltages because cell delay can decrease with increasing temperature. This phenomenon is caused by a competition between mobility and threshold voltage to dominate cell delay. We refer to this phenomenon as the inverted temperature dependence (ITD). Due to ITD, it becomes very difficult to analytically determine the temperatures that maximize or minimize the delay of a cell or a path. As such, ITD has profound consequences for STA: (1) ITD essentially invalidates the approach of defining corners by independently varying voltage and temperature; (2) ITD makes it more difficult to find short paths, leading to difficulties in detecting hold time violations; and (3) the effect of ITD will worsen as supply voltages decrease and threshold voltage variations increase. This article analyzes the consequences of ITD in STA and proposes a proper handling of ITD in an industrial sign-off STA tool. To the best of our knowledge, this article is the first such work.

Local structure ofLiNiO2studied by neutron diffraction

The nature of the magnetic state of $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$ has been controversial. In this compound Ni spins $(S=1∕2)$ form a triangular lattice with the possibility of magnetic frustration, but the exact state of spin correlation has not yet been known in spite of the extensive research work. A factor that complicates understanding of the magnetic state is the orbital state of ${\mathrm{Ni}}^{3+}$ which is a Jahn-Teller (JT) ion. While there is no signature of long-range Jahn-Teller distortion, local JT distortion has been suspected. We have performed neutron diffraction and atomic pair-density function analyses up to unprecedented large distances to discover a number of unusual features, such as anomalous peak broadening, local JT distortion, sharp oxygen-oxygen distance correlations, and inverted temperature dependence of medium range correlation. These observations are best explained by local orbital ordering of ${\mathrm{Ni}}^{3+}$ ions into three sublattices. This orbital ordering, however, cannot develop into long-range order because of the strain field it generates, and domains of about $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in size are formed. Domains are susceptible to random pinning by impurities (site disorder) resulting in the loss of structural long-range order. We suggest that this local orbital ordering is the basis for the complex magnetic properties observed in this compound.

Superconducting high-temperature pairing–theoretical considerations on an unusual phenomenon

In the theoretical framework of a recently developed first-order mechanism for the superconducting pairing an inverted temperature dependence for the transition between the unpaired and a paired state has been observed. We suggest the existence of a paired state at high temperatures T emerging from an unpaired low-temperature state. The entropy driven stabilization of paired states is the outcome of a T-dependent competition between the electronic energy and the electronic configuration entropy. With increasing T the entropy admixtures to the free energy of a given quantum ensemble can exceed energetic effects. The unusual T-dependence of superconductivity has been derived by cluster calculations in the two-parameter Hubbard approximation. The clusters chosen are of the four-electron-four-center type.

Temperature Dependence of the Thermodynamics and Kinetics of Micellar Solutions

We predict theoretically the thermodynamics and relaxation kinetics of solutions of cylindrical branched micelles. Using a recently developed theory in combination with the experimental data, we explain the unusual, inverted temperature dependence of the phase separation observed in wormlike micelles and dilute microemulsions. We extend the model to treat the temperature dependence of the relaxation kinetics and explain the observations.

Temperature dependence of solute vibrational relaxation in supercritical fluids: experiment and theory

Vibrational lifetime ( T 1) data for the asymmetric CO stretching mode of W(CO) 6 in supercritical ethane and carbon dioxide as a function of temperature at various fixed densities are compared to a recent extended hydrodynamic theory [1]. In ethane at the critical density, as the temperature is raised, T 1 initially becomes longer, reaching a maximum ∼70 K above the critical temperature. T 1 decreases with further increase in temperature. The theory is able to reproduce this behavior nearly quantitatively without free parameters. At high density in ethane and in CO 2, the inverted temperature dependence is not observed, in agreement with theoretical calculations.

3D structure of microperoxidase-11 by NMR and molecular dynamic studies

Microperoxidase-11 (MP11) is an undecapeptide, obtained from enzymatic cleavage of cytochrome c, which still exhibits peroxidase activity. It retains residues 11–21 of the protein, the heme-c group and His18 as fifth ligand at the iron atom. We studied MP11 in the paramagnetic, Fe(III) low-spin state by NMR and constrained molecular dynamics (MD). Well-resolved 1H and 13C spectra were obtained in water–trifluoroethanol and water–methanol mixtures with different axial ligands to the iron, i.e. imidazole, NH3, OH, CN, at different temperatures from −15 to +25 °C. All the protons of the heme and the peptide moiety were assigned by using TOCSY and NOESY experiments. Some stereospecific assignments were also performed and eight coupling constants between the amide and the α-protons were measured. Seventy-one inter-residue and heme peptide and 40 intra-residue NOE interactions were translated into inter-proton distances. Ten structures satisfying these distance constraints to within ±0.4 Å were obtained and showed that almost the whole peptide (segment 12–19) can form a right-handed helix. The envelope structure of the peptide around the heme is well defined based on a number of NOE interactions between heme and peptide protons. Variable-temperature experiments were performed in order to obtain information on the protons affected by the paramagnetic shift. An inverted temperature dependence (Curie law) was observed for the heme and meso protons, except for 3-Me, when the sixth axial ligand is NH3 or imidazole. The pattern of 13C hyperfine shifts of heme methyl groups, similar in MP11 and cytochrome c, suggests that the orientation of His ring along the α–γ meso axis is conserved in MP11, which is consistent with MD calculations. Copyright © 2000 John Wiley & Sons, Ltd.

Inverted Temperature Dependence of the Decomposition of Carbon Dioxide on Oxide-Supported Polycrystalline Copper

The paper reports an investigation of the decomposition of carbon dioxide on an industrial (I.C.I.) methanol synthesis catalyst, which in effect is oxide-supported polycrystalline copper. In the temperature range 173 to 333 K using the technique of reactive frontal chromatography it is shown that (i) the rate of decomposition of carbon dioxide increases on decreasing the dosing temperature, and (ii) the extent of oxidation of copper by the decomposition of carbon dioxide increases on decreasing the dosing temperature. The former result is in agreement with an earlier conclusion that carbon dioxide decomposition on unsupported polycrystalline copper was precursor-state moderated. The "negative" activation energy obtained is in fact the difference in energy between the heat of adsorption of the precursor state and the surface decomposition activation energy of the adsorbed precursor state, which is the same argument as that used by G. Ertl [ in "Catalytic Ammonia Synthesis" (J. R. Jennings, Ed.), p. 123, Plenum, New York, 1991] to explain the negative activation energy for the nitrogen sticking probability on potassium-promoted single crystal iron. The inverted temperature dependence of the extent of the oxidation of copper by the decomposition of carbon dioxide is rationalised on the basis that the oxided copper surface reconstructs in an activated way to an unreactive state. The energy barrier to that reconstruction is too high for it to occur at 173 K (the lowest temperature studied) where the extent of surface oxidation (25% of a monolayer) is therefore solely a function of oxygen poisoning of the reactive surface. The CO/CO 2 reactive frontal lineshape at 173 K is exactly the same as that of the N 2/N 2O lineshape at 333 K; the fewer oxygen coverage at cessation of reaction by oxygen poisoning for the CO/CO 2 reaction shows the reaction to be structure sensitive. Infrared spectroscopic data show the reconstruction to involve the loss of the (110) face and the growth of the (211) face while CO desorption data confirm the activated nature of the reconstruction.

Dimer-dimer interactions in octameric mitochondrial creatine kinase

Mitochondrial creatine kinase (Mi-CK) forms octamers and dimers, which are readily interconvertible in vitro. The kinetic and thermodynamic octamer stability of wild-type and two mutant, octamer-destabilized forms of chicken sarcomeric Mi-CK was investigated at varying temperatures, pHs, and salt and substrate concentrations, in order to identify parameters which might regulate the octamer/dimer ratio in vivo and to assess the nature of octamer-stabilizing interactions. For wild-type Mi-CK, the rate of the transition state analogue complex (TSAC)-induced octamer decay increased with increasing temperature up to 28 degrees C; increasing pH markedly accelerated the decay in a biphasic manner. The substrate-dependent decay data suggest that also the productive enzymatic transition state of Mi-CK induces an octamer-destabilizing conformation. Thermodynamically, the octamers are stabilized by a combination of hydrophobic and polar contributions. Van't Hoff analysis showed that hydrophobic interactions dominate both in the absence of substrates and in the TSAC conformation, since the equilibrium octamer fractions increased with increasing temperatures, in spite of the accelerated decay kinetics. For the Mi-CK mutant E4Q, a similar temperature dependence was found; in contrast, mutant W264C exhibited an inverted temperature dependence, suggesting that hydrophobic interactions might be largely abolished in this mutant. Both the kinetic and the thermodynamic data seem to suggest that the octamer-dimer transitions of Mi-CK might not play a major role in a fast regulation of mitochondrial energy metabolism, but could rather be involved in slow long-term modulations.

Dynamic displacement anisotropy of atoms, streamer discharges and phonon focusing in BeO and CdSe crystals and I-VII compounds

Streamer discharges at 77 K and crytallographically oriented surface spark breakdown at 295 K have been obtained in BeO crystals. An inverted temperature dependence has been found for the breakdown anisotropy compared with that in CdSe crystals. It is shown that the temperature dependences of the excitation thresholds of the incomplete electric breakdown at different crystallographic orientations in dielectrics as well as those of the streamer discharges in hexagonal semiconductors are controlled by the anisotroply of ionic dynamic displacements. The directions of maximum phonon focusing have been calculated in the localoisation planes of the incomplete breakdown in NaCl, KBr and LiF crystals at room temperature in accordance with well-known breakdown model along phonon streams. It is shown that the alkali halide breakdown data do not agree with this model. An anisotropy is predicted of the dynamic displacement of atoms in alkali halide crystals as well as in the basis plane and mirrorsymmetric directions of planes containing the crystallographic c-axis in hexagonal II–VI compounds, which allows us to explain the observable particularities of the discharge anisotropy in both cubic and hexagonal crystals.

Photoluminescence properties of the Al0.48In0.52As/InP interface and the diffusion of carriers thereto

A study of the photoluminescence (PL) and photoluminescence excitation (PLE) properties of the interface luminescence of the staggered-aligned Al0.48In0.52As/InP is presented, together with a study of the diffusion of carriers to this interface. Two PL peaks originating from the interface were measured at energies higher than commonly reported. This is the result of growth of the Al0.48In0.52As layer directly on the semi-insulating InP substrate, which results in a much sharper triangular well in the conduction band than when grown on an n-type InP buffer layer. Data from PL in a magnetic field and from PLE both showed that the PL transitions are excitonic in character. Furthermore, an inverted S-shape temperature dependence was found for the PL energy, which is characteristic of carrier localization. PLE measurements showed that at 4.2 K both electrons and holes participating in the interface PL are provided by exciton diffusion from the InP, while at 70 K they are provided by exciton diffusion from the Al0.48In0.52As. The difference arises from carrier localization in the Al0.48In0.52As top layer below 50 K. Unexpectedly, for the InP at 4.2 K the PL intensities of both excitonic and donor-to-acceptor transitions were independent of the absorption of laser light in the Al0.48In0.52As top layer.

The effect of the heat of association on homogeneous nucleation rates in methanol vapor

Homogeneous nucleation rates in methanol vapor were measured using the expansion pulse technique. Our results show an apparently inverted temperature dependence. The obtained data are consistent with earlier expansion chamber investigations, reported in the literature, however, strong deviations from corresponding diffusion chamber data are observed. In the present paper we show that these deviations can be explained by considering vapor association during adiabatic expansion. The homogeneous nucleation rates are found to be drastically reduced due to the latent heat released during the formation of subcritical clusters. Thereby the severe discrepancies between expansion chamber experiments for methanol and the corresponding diffusion chamber data are now essentially resolved. Furthermore, an apparently anomalous behavior of ethanol vapor, observed in expansion chambers at higher temperatures, is now qualitatively understood.