Subsequent Slow Relaxation Research Articles

Collagen turnover in the adult femoral mid‐shaft: Modeled from anthropogenic radiocarbon tracer measurements

We have measured the (14)C content of human femoral mid-shaft collagen to determine the dynamics of adult collagen turnover, using the sudden doubling and subsequent slow relaxation of global atmospheric (14)C content due to nuclear bomb testing in the 1960s and 1970s as a tracer. (14)C measurements were made on bone collagen from 67 individuals of both sexes who died in Australia in 1990-1993, spanning a range of ages at death from 40 to 97, and these measurements were compared with values predicted by an age-dependent turnover model. We found that the dataset could constrain models of collagen turnover, with the following outcomes: 1) Collagen turnover rate of females decreases, on average, from 4%/yr to 3%/yr from 20 to 80 years. Male collagen turnover rates average 1.5-3%/yr over the same period. 2) For both sexes the collagen turnover rate during adolescent growth is much higher (5-15%/yr at age 10-15 years), with males having a significantly higher turnover rate than have females, by up to a factor of 2. 3) Much of the variation in residual bomb (14)C in a person's bone can be attributed to individual variation in turnover rate, but of no more than about 30% of the average values for adults. 4) Human femoral bone collagen isotopically reflects an individual's diet over a much longer period of time than 10 years, including a substantial portion of collagen synthesised during adolescence.

FERMION CONDENSATION IN A RELATIVISTIC DEGENERATE STAR: ARRESTED COLLAPSE AND MACROSCOPIC EQUILIBRIUM

Fermionic Cooper pairing leading to the BCS-type hadronic superfluidity is believed to account for periodic variations ("glitches") and subsequent slow relaxation in spin rates of neutron stars. Under appropriate conditions, however, fermions can also form a Bose–Einstein condensate of composite bosons. Both types of behavior have recently been observed in tabletop experiments with ultra-cold fermionic atomic gases. Since the behavior is universal (i.e., independent of atomic potential) when the modulus of the scattering length greatly exceeds the separation between particles, one can expect analogous processes to occur within the supradense matter of neutron stars. In this paper, I show how neutron condensation to a Bose–Einstein condensate, in conjunction with relativistically exact expressions for fermion energy and degeneracy pressure and the relations for thermodynamic equilibrium in a spherically symmetric space–time with Schwarzschild metric, leads to stable macroscopic equilibrium states of stars of finite density, irrespective of mass.

In situ study of stress relaxation mechanisms of pure Al thin films during isothermal annealing

Abstract It is known that the compressive stress relaxation of Al thin films occurs by various deformation mechanisms, such as hillock formation, microstructural changes, and creep. In order to clarify the characteristics and quantify the contributions of each deformation mechanism, the stress relaxations of Al thin film during isothermal annealing at 194 °C were investigated by wafer curvature measurement and in situ hillock observation. For the stress relaxation measured by the wafer curvature method, fast relaxation and subsequent slow relaxation processes were observed. In contrast, for the stress relaxation measured by hillock formation, only one relaxation process was observed, which had a similar time constant to that of the slow relaxation process observed in the wafer curvature measurement. These results indicate that hillock formation is the most plausible mechanism for slow stress relaxation.

Stress Relaxation during Isothermal Annealing at Elevated Temperatures in Electroplated Cu Films

ABSTRACTDeformation mechanisms of electroplated Cu thin films on TaN/SiO2/Si were investigated by performing isothermal annealing above 200 °C. Stress relaxation behavior during isothermal annealing was analyzed by curve fitting using exponential decay equations. During heating, fast relaxation and subsequent slow relaxation processes were observed. In contrast, during cooling, only slow relaxation process was observed. Among possible mechanisms for stress relaxation, diffusion creep was found to be the most plausible mechanism based on the obtained values of the activation energy. It was suggested that the slow relaxation process observed both in the heating and in the cooling processes was attributed to a grain-boundary diffusion creep. On the other hand, the fast relaxation process observed during heating was attributed to a surface-diffusion controlled mechanism. The surface diffusion mechanism was considered to be characteristic to Cu thin films that did not form stable surface oxide.

Protein kinase A increases the rate of relaxation but not the rate of tension development in skinned rat cardiac muscle.

To clarify the contribution of cross-bridge kinetics to the contraction profile of cardiac twitch during beta-adrenergic stimulation, we studied the rate of tension development and relaxation following laser flash photolysis of caged compounds in rat-skinned ventricular trabeculae before and after treatment with the catalytic subunit of protein kinase A (PKA, 0.5 U/microl, 40 min). Tension development following nitrophenyl (NP)-EGTA photolysis was fitted with a single exponential function. The rate constant increased with an increase in postphotolysis steady tension, and the relation between the rate constant and the tension was not influenced by PKA. The rate of relaxation following diazo-2 photolysis was fitted with a double exponential function. The rate of both initial rapid and subsequent slow relaxation was independent of the extent of relaxation. PKA increased the rate of initial rapid relaxation by about twofold, but showed no significant effect on the rate of subsequent slow relaxation. These results suggest that in beta-receptor stimulated rat cardiac muscle, the increased rate of tension development and the facilitated relaxation rate during twitch can be partly explained as being due to the combined effects of decreased Ca(2+) affinity of troponin C and increased cycling rate of cross-bridges (subtractive combination for tension development and additive combination for tension relaxation).

Binding of phenol and analogues to alanine complexes of tyrosine phenol-lyase from Citrobacter freundii: implications for the mechanisms of alpha,beta-elimination and alanine racemization.

We have examined the interaction of Citrobacter freundii tyrosine phenol-lyase with both L- and D-alanine. This enzyme catalyzes the racemization of alanine as a side reaction, in addition to the physiological beta-elimination of L-tyrosine to give phenol and ammonium pyruvate. The steady-state kinetic parameters for alanine racemization, kcat and Km, for D-alanine are 0.008 S-1 and 32 mM, respectively, while those for L-alanine are 0.03 S-1 and 11 mM. Incubation of tyrosine phenol-lyase with either L- or D-alanine forms a quinonoid complex that exhibits a strong peak at 500 nm. The presence of K+ increases the intensity of the 500-nm absorption with L-alanine, but decreases the intensity of the peak with D-alanine. Rate constants for the formation of these quinonoid intermediates and the effects of phenol and analogues on the reaction with either L- or D-alanine have been studied by rapid-scanning and single-wavelength stopped-flow spectrophotometry. Phenol binds to all the intermediates of tyrosine phenol-lyase with L- and D-alanine, but most strongly to the external aldimine complex, resulting in a decrease in the absorbance at 500 nm at equilibrium. Pyridine N-oxide binds selectively to the quinonoid complex of alanine, and thus causes an increase in the absorbance at 500 nm at equilibrium. 4-Hydroxypyridine causes a decrease in absorbance at 500 nm during the fast phase, but an increase in absorbance at 502 nm in a subsequent slow relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Reaction of indole and analogs with amino acid complexes of Escherichia coli tryptophan indole-lyase: detection of a new reaction intermediate by rapid-scanning stopped-flow spectrophotometry

The effects of indole and analogues on the reaction of Escherichia coli tryptophan indole-lyase (tryptophanase) with amino acid substrates and quasisubstrates have been studied by rapid-scanning and single-wavelength stopped-flow spectrophotometry. Indole binds rapidly (within the dead time of the stopped-flow instrument) to both the external aldimine and quinonoid complexes with L-alanine, and the absorbance of the quinonoid intermediate decreases in a subsequent slow relaxation. Indoline binds preferentially to the external aldimine complex with L-alanine, while benzimidazole binds selectively to the quinonoid complex of L-alanine. Indole and indoline do not significantly affect the spectrum of the quinonoid intermediates formed in the reaction of the enzyme with S-alkyl-L-cysteines, but benzimidazole causes a rapid decrease in the quinonoid peak at 512 nm and the appearance of a new peak at 345 nm. Benzimidazole also causes a rapid decrease in the quinonoid peak at 505 nm formed in the reaction with L-tryptophan and the appearance of a new absorbance peak at 345 nm. Furthermore, addition of benzimidazole to solutions of enzyme, potassium pyruvate, and ammonium chloride results in the formation of a similar absorption peak at 340 nm. This complex reacts rapidly with indole to form a quinonoid intermediate very similar to that formed from L-tryptophan. This new intermediate is formed faster than catalytic turnover (kcat = 6.8 s-1) and may be an alpha-aminoacrylate intermediate bound as a gem-diamine.

Dynamics of molecular organization in agarose sulphate

AbstractNongelling solutions of structurally regular chain segments of agarose sulphate show disorder–order and order–disorder transitions (as monitored by the temperature dependence of optical rotation) that are closely similar to the conformational changes that accompany the sol–gel and gel–sol transitions of the unsegmented polymer. The transition midpoint temperature (Tm) for formation of the ordered structure on cooling is ∼25 K lower than Tm for melting. Salt‐induced conformational ordering, monitored by polarimetric stopped‐flow, occurs on a millisecond time scale, and follows the dynamics expected for the process 2 coil ⇌ helix. The equilibrium constant for helix growth (s) was calculated as a function of temperature from the calorimetric enthalpy change for helix formation (ΔHcal = −3.0 ± 0.3 kJ per mole of disaccharide pairs in the ordered state), measured by differential scanning calorimetry. The temperature dependence of the nucleation rate constant (knuc), calculated from the observed second‐order rate constant (kobs) by the relationship kobs = knuc(1 − 1/s) gave the following activation parameters for nucleation of the ordered structure of agarose sulphate (1 mg mL−1; 0.5M Me4NCl or KCl): ΔH* = 112 ± 5 kJ mol−1; ΔS* = 262 ± 20 J mol−1 K−1; ΔG*298 = 34 ± 6 kJ mol−1; (knuc)298 = (7.5 ± 0.5) × 106 dm3 mol−1 s−1. The endpoint of the fast relaxation process corresponds to the metastable optical rotation values observed on cooling from the fully disordered form. Subsequent slow relaxation to the true equilibrium values (i.e., coincident with those observed on heating from the fully ordered state) was monitored by conventional optical rotation measurements over several weeks and follows second‐order kinetics, with rate constants of (2.25 ± 0.07) × 10−4 and (3.10 ± 0.10) × 10−4 dm3 mol−1 s−1 at 293.7 and 296.2 K, respectively. This relaxation is attributed to the sequential aggregation processes helix + helix → dimer, helix + dimer → trimer, etc., with depletion of isolated helix driving the much faster coil–helix equilibrium to completion. Light‐scattering measurements above and below the temperature range of the conformational transitions indicate an average aggregate size of 2–3 helices.