Low Activation Energy Process Research Articles

Complex Plane and 7Li NMR Studies of Highly Conductive Sulfide‐Based Lithium Glasses

The activation energy for lithium diffusion in highly conductive glasses with varying lithium concentrations is measured by 7Li NMR linewidth techniques, and is compared to activation energies for ionic conductivity obtained by complex plane techniques. The 7Li NMR spectra for the glasses have been examined as a function of temperature; the HB equation for motional narrowing was used to calculate the activation energies for lithium movement in the various glasses. From the NMR data, two activation energies were observed over the temperature range of −100°C to room temperature. The low temperature, high activation energy values correlate well with the activation energies obtained from ionic conductivity measurements, while the high temperature, low activation energy process is apparently due to some sort of short‐range motion detected only by the 7Li NMR measurements.

Reliability of InGaAs Photodiodes for SL Applications

A major objective of the TAT-8 submarine cable development program is the reliability assurance of its critical components. A relatively new device, the InGaAs photodiode, is used as the detection element in optical receivers and as the monitor of laser output in optical transmitters. In this paper, we describe a comprehensive reliability program aimed at assessing and assuring the reliability of InGaAs photodiodes. A major portion of this work has involved device operation at overstress conditions. Results to date indicate that for receiver photodiodes a device design exists which is predicted to meet a 1-FIT reliability objective. Tests of monitors of similar design are in progress. Critical to the success of the reliability assurance program is the ability to identify weak devices which are likely to fail early in the system life cycle. A conventional high-temperature burn-in is shown to be impractical, and it would not necessarily remove devices which fail by low activation energy processes. Overvoltage provides a means of accelerating such failure mechanisms. A “purge” using a combination of accelerants (high temperature and overvoltage) is shown to have considerable promise in failing weak devices, while inducing significant changes in those devices which subsequently fail early in a life test (elevated temperature at normal bias conditions). Devices which are unaffected by the purge are shown by extended life tests to be robust devices which have a high probability of meeting TAT-8 requirements.

Kinetics of thermal denaturation of antithrombin III

Purified antithrombin III (AT III), a single-chain human plasma glycoprotein, molecular weight 58,000 daltons, and one of the major serine protease inhibitors, was heated in the 60-70 degrees C range for inactivating possible contaminations by hepatitis B virus (HBV). Loss of inhibitory activity, unfolding of tertiary structure, and the rate of aggregate formation of AT III were monitored experimentally during heatig. Sucrose and sodium citrate were demonstrated to stabilize the protein. From the rate data the calculated activation energies (E) showed E(tert. struct.) < E(biol. act.) < E(aggreg.) indicating the order (lower activation energy process first) in which heat causes these changes in the protein molecule. The activation energy corresponding to denaturation of HBV was estimated to be at least fourfold lower than that associated with the unfolding of the tertiary structure of the protein. Purified AT III, thus stabilized and pasteurized, should be therapeutically effective, and the risk for transmission of hepatitis B should be decreased significantly.

Hydrogen isotope exchange kinetics of single protons in bovine pancreatic trypsin inhibitor

The exchange kinetics of the slowest exchanging BPTI beta-sheet protons are complex compared to model peptides; the activation energy, E alpha, and the pH dependence are temperature dependent. We have measured the exchange kinetics in the range pH 1--11, 33--71 degrees C, particularly the temperature dependence. The data are fit to a model in which exchange of each proton is determined by two discrete dynamical processes, one with E alpha approximately 65 kcal/mol and less than first order dependence on catalyst ion, and one with E alpha 20--30 kcal/mol and approaching first order in catalyst ion. The low activation energy process is the mechanism of interest in the native conformation of globular proteins and involves low energy, small amplitude fluctuations; the high activation energy process involves major unfolding. The model is simple, has a precedent in the hydrogen exchange literature, and explains quantitatively the complex feature of the exchange kinetics of single protons in BPTI, including the following. For the slowest exchanging protons, in the range 36 degrees--68 degrees C, E alpha is approximately 65 kcal/mol at pH approximately 4, 20--30 kcal/mol at pH greater than 10, and rises to approximately 65 kcal/mol with increasing temperature at pH 6--10; the Arrhenius plots converge around 70 degrees C; the pH of minimum rate, pHmin, is greater than 1 pH unit higher at 68 degrees C than for model compounds; and at high pH, the pH-rate profiles shift to steeper slope; the exchange rates around pHmin are correlated to the thermal unfolding temperature in BPTI derivatives (Wagner and Wüthrich, 1979, J. Mol. Biol. 130:31). For the more rapidly exchanging protons in BPTI the model accounts for the observation of normal pHmin and E alpha of 20--30 kcal/mol at all pH's. The important results of our analysis are (a) rates for exchange from the folded state of proteins are not correlated to thermal lability, as proposed by Wuthrich et al. (1979, J. Mol. Biol. 134:75); (b) the unfolding rate for the BPTI cooperative thermal transition is equal to the observed exchange rates of the slowest exchanging protons between pH 8.4--9.6, 51 degrees C; (c) the rates for exchange of single protons from folded BPTI are consistent with our previous hydrogen-tritium exchange results and with a penetration model of the dynamic processes limiting hydrogen exchange.

Search for low activation energy abstraction of H from C–H bonds by D atoms in hydrocarbons at 77 K

Experiments have been done to determine whether thermal D atoms produced at 77 K in 3-methylpentane-d14 containing ∼1% C–H bonds as an isotopic impurity are able to abstract H by a low activation energy process, as CH3 radicals are known to do. The D atoms have been produced by 254 nm photolysis of DI. The trends in HD/D2 ratio with changing ratio of C–H bonds to [DI] are consistent with the conclusion that the D+C6H13H→ HD+C6H13 abstraction occurs at 77 K with low activation energy requiring either a tunneling mechanism or unusual molecular distortion in the glass.

Nuclear resonance study of lithium motion in β-eucryptite (LiAlSiO 4)

Several diffusion processes have been observed in ceramic β-eucryptite using NMR methods. One low activation energy (0.25 eV) process has been identified with the high temperature disordered phase of the lattice. NMR deduced jump rates agree well with Raman scattering experiments.

Further Studies on Factors Affecting the Efflux of Betacyanin from Beet Root: A Note on Thermal Effects

AbstractAs judged by betacyanin efflux, beet root tissue differs in stability toward O2 at low and high temperatures (45–60°C and 60–100°C respectively). The effect of temperature itself can he divided into a high activation energy (93 Kcal mol−1) process in the lower temperature range and a low activation energy process (19 Kcal mor−1) in the higher range (&gt; 60°C). From these data it is suggested that initially, elevating temperatures bring about reversible conformational changes in the membrane. With continuing increase in temperature in the presence of O2, membrane chemical groups susceptible to oxidation are exposed and, upon oxidation, render conformational changes irreversible.