Dynamic rheology of agar gels: theory and experiments. Part II: gelation behavior of agar sols and fitting of a theoretical rheological model

A theoretical rheological model for agar gels, based on the bead spring model for linear flexible random coils and the model for crosslinked polymers, is successfully fitted to experimental gelation curves obtained over a wide range of cooling rates (0.5–20 °C min⁻¹) and agar concentration (1–3 wt%). The theoretical gelation temperature, T_gel^model increases with increasing agar concentration and decreasing cooling rate. The intrinsic net association rate increases significantly with increasing cooling rate. This increase is related to the higher probability of association of the non-associated agar molecules at higher cooling rates.     

Intraventricular insulin decreases kappa opioid-mediated sucrose intake in rats

The hormone insulin acts in the central nervous system (CNS) as a regulator of body adiposity and food intake. Recent work from our laboratory has provided evidence that one way by which insulin may decrease food intake is by decreasing the rewarding properties of food. Evidence from others suggests that endogenous opioids may mediate the palatable properties of foods, and insulin may decrease nonfood-related reward via interaction with some CNS kappa opioid systems. In the present study we examined the ability of insulin to interact with exogenous or endogenous kappa opioids to modulate feeding of palatable sucrose pellets by nondeprived rats. Insulin (5 mU intracerebroventricular (i.c.v.), t=−3 h) completely reversed the ability of the exogenous kappa agonist U50,488 (26 μg, i.c.v., t=−15 min) to stimulate 90-min sucrose feeding (211±32% reduced to 125±23% of 90-min baseline intake). Further, i.c.v. insulin (5 mU, t=−3 h) interacted with a subthreshold dose of the kappa receptor antagonist norbinaltorphimine (5 μg, i.c.v., t=−15 min) to decrease the 90-min sucrose intake baseline (77±11% versus 109±10% of 90 min baseline intake, insulin/norbinaltorphimine versus norbinaltorphimine). Together these studies provide new evidence that insulin in the CNS may decrease the action of CNS kappa opioid system(s) that mediate palatable feeding.     

Effect of urea on solution behavior and heat-induced gelationof chitosan-β-glycerophosphate

In this study, we have investigated the effect of urea on the physicochemical (pH and conductivity) and rheological properties of the chitosan-β-GP system in order to assess the main polymer–polymer interactions at low and high temperature. The pH of the solutions was slightly increased due to the consumption of H⁺ in solution through the hydrolysis of urea. Furthermore, the addition of urea considerably decreased the conductivity, and therefore the ionic strength of the solutions, and this effect was more important at high temperature. It indicated that urea strongly affects polymer–polymer interactions by weakening hydrogen bonds at low temperature, but in addition can hinder hydrophobic effect at high temperature since the reduction of ionic strength results in less screening of electrostatic repulsion between protonated glucosamine groups. At 15 °C, the addition of urea to chitosan-β-GP solutions decreased their elasticity, shortened relaxation times and simplified the relaxation process due to the disruption of hydrogen bonds. Heat-induced gelation of the chitosan system in the presence of urea showed higher gelation temperature (T_gel) in non-isothermal tests and longer gelation time (t_gel) in isothermal conditions. The activation energy for gelation also increased with increasing urea concentration. We concluded that the detrimental effect of urea on the gelation process was mainly related to a decrease in polymer-polymer hydrophobic effect, as shown by the decrease in conductivity.     

Two new hydrothermally synthesised hexamine bridged L–M–L type coordination polymers: Characterisation and magneto-structural correlation

Two new L–M–L type transition metal coordination polymers [M(C₆H₁₂N₄)(NCO)₂(H₂O)₂]_n, where M = Co(II) (1) and Ni(II) (2), have been synthesised under controlled hydrothermal condition and characterised spectroscopically and by thermal analyses. Here hmt or hexamethylenetetramine has behaved as a neutral organic bidentate spacer molecule. Both the complexes crystallise in the monoclinic system as confirmed from single crystal X-ray diffraction studies. Cryomagnetic susceptibility measurements over a wide range of temperature (2–300 K) under 0.5 T magnetic fields show a weak antiferromagnetic interaction of J = −0.65 cm⁻¹ for 1 and −1.6 cm⁻¹ for 2. The values have been given a favorable support by weak covalent and H-bonding interactions between octahedral M(II) metal centers as revealed from X-ray structure determination. The high dimensionality of the structures is probably a manifestation of extensively weak covalent interactions and H-bondings.     

DNA crosslinking and cell survival in human lymphoid cells treated with 8-methoxypsoralen and long wavelength ultraviolet radiation

8-Methoxypsoralen (8-MOP) when irradiated with long wavelength ultra-violet radiation (UV-A) inhibits DNA synthesis in lymphocytes in vitro and in vivo. 8-MOP binds reversibly to DNA in the dark; when exposed to UV-A, covalent monoadducts and cross-links are formed with the DNA. The present study correlates the cytotoxic effects of 8-MOP plus UV-A with DNA crosslinking. E-B virus transformed human lymphoblastoid cells were suspended in a colorless salt solution containing 8-MOP and exposed to UV-A from fluorescent lamps filtered to remove radiation below 320 nm (22.5 J/m²-sec). Cells were then returned to complete medium and assayed for survival (by daily counts of viable cells and by cloning in microtiter wells) and for DNA crosslinking by alkaline elution. 8-MOP alone or UV-A alone resulted in minimal to no alterations in survivial or in DNA crosslinking. DNA crosslinking was found to be linearly dependent on 8-MOP concentration (in the range of 0.01–1.0 μg/ml) for 3 different UV-A doses (3000–15000 J/m²). The surviving fraction declined exponentially as a function of the relative number of DNA crosslinks. These results suggest that the cytotoxic effects of photoactivated 8-MOP in human lymphoblastoid cells may depend on DNA interstrand crosslinks.     

Thermoregulatory use of heat increment of feeding in the tawny owl (Strix aluco)

The heat increment of feeding (HIF) was investigated in the tawny owl (Strix aluco) in central Norway (63°N, 10°E), close to the northern limit of its distribution. HIF was measured as the increase in heat production (measured as oxygen consumption) after force-feeding the owls with laboratory mice at thermoneutral conditions (20 °C) and during cold-exposure (5 °C and −5 °C). The basal metabolic rate of the owls (mean mass 419 g) was 4.39 kJ h⁻¹ and the lower critical temperature was approximately 16 °C. During cold conditions, HIF substituted for thermogenesis, and at an ambient temperature of −5 °C the substitution was complete. Calculations indicate that the substitution by HIF may save the owls as much as 60% of their daily thermoregulatory costs. This corresponds to about 10% of their total daily energy budget.     

Rheological properties of barley β-glucan

Health benefits of cereal β-glucan are linked to its high viscosity. Although viscosity of β-glucan gum solutions has been reported previously, there are conflicting reports about its behavior at elevated temperatures. Therefore, the viscosity behavior of barley β-glucan gum obtained in a pilot plant (PP) or in a laboratory (LAB) was determined at different shear rates (1.29–129 s⁻¹) and temperatures (0.1–75 °C) in this study. Viscosity decrease with temperature was demonstrated for both gums and activation energy E_a was calculated from the Arrhenius equation. None of the fresh gum solutions exhibited thixotropic behavior at ≤1% (w/w) concentration, but the measurement demonstrated that increased shear rate is not applicable to polymer solutions of low viscosity. Information about rheological properties of β-glucan will lead to better understanding of its behavior under physiological and processing conditions.     

The ability of isolated myocardial cells of the rat to contract at temperatures near freezing point

1. 1.|Single spontaneously-beating myocardial cells were prepared by enzymatic isolation from rat heart ventricles. Using a TV camera and videotape recorder, beating parameters were characterized with respect to temperature.

2. 2.|The cells obtained showed two types of contractions: (a) action-potential-coupled, fast contractions, which could be triggered electrically but sometimes also occurred spontaneously; and (b) a phasic type of contraction which occurs spontaneously as a slowly (25–200 μm s⁻¹) proceedings wave through the cell and is not coupled to the action potential.

3. 3.|Both the spontaneous phasic and electrically-triggered fast contractions continued down to about 0°C, where the cells went into an irreversible contracture, but recovered if the temperature was raised before the contracture occurred.

4. 4.|In physically-contracting cells, the beat rate and the velocity of the contraction wave were temperature dependent showing a linear relationship on an Arrhenius plot; apparent activation energies were 31 and 54 kJ mol⁻¹, respectively.

5. 5.|When fast contractions were triggered by electric field stimulation, the threshold of cells was reduced as temperature rose. The rheobase values showed that cell excitability decreased more steeply when the temperature fell below 20°C. This change could not be seen in the values of chronaxie, which showed a linear relationship on the Arrhenius plot within the whole temperature range used (6–35°C).

6. 6.|It is concluded that the lower temperature limit is not the same for all structural levels of the rat heart. Myocardial cells are able to contract still at the freezing point while mechanisms initiating the heart beat require higher temperatures to be functional.

Study of Vitamin C ester synthesis by immobilized lipase from Candida sp.

Oleoyl ester of -ascorbic acid was synthesized by using immobilized lipases from Candida sp. A series of solvents, such as ethanol, tetrahydrofuran, pyridine, butanol, tertiary amyl alcohol (t-amyl alcohol), hexanol, octanol and hexane (log P from −0.24 to 3.5) were investigated for the reaction, and t-amyl alcohol was found to be the most suitable from the standpoint of the substrate concentration and the enzyme activity. And the equilibrium of the reaction was affected by the addition of the molecular sieves and the temperature. Reaction carried out at 55 °C and with 50 g/l of 4 Å molecular sieves is good for the enzyme to keep its activity and for making the equilibrium go to the product. The kinetic model was studied and the result showed that the reaction can be described by Ping-Pong mechanism. Parameters value of V_m and K_m′ were obtained. Last, the pure products of the reaction were attained and determined by IR spectra, mass spectrometry and ¹H NMR spectra.     

Some physical and microstructural properties of genipin-crosslinked gelatin-maltodextrin hydrogels

The physical properties and microstructure of gelatin-maltodextrin hydrogels fixed with genipin (GP) were investigated as a function of pH (3-7), maltodextrin (MD) (0-9%, w/w) and GP (0-10 mM levels), at a constant gelatin (G) concentration (10%, w/w). Network strength (elastic modulus, E) and swelling behavior were characterized by large deformation testing and by swelling index (SI). In general, network strength increased and swelling decreased at higher pH, MD and GP levels, except at pH 3, where E was independent of the GP concentration until approximately 7.5 mM, above which it declined. Confocal scanning laser microscopy (CLSM) images showed phase separation to be suppressed at pH 3, whereas at pH 7, separation into a self-similar dispersed phase was apparent. Overall, the judicious use of GP to crosslink G was an appropriate means of kinetically trapping MD within the gelatin network.     

A feasible method to determine the critical DS for modified polysaccharide derivatives'' gelation by gel point, tg

The process of determining a critical DS to a polymer's gelation is a key link in identifying degradable polymers' cross-linking behaviors. A feasible method was developed from the rheological technique. In principle, it was a new application of gel point, t_g, in an overlapping field of polymers' modification and characterization, by which the gelling properties of carboxyl methylated guar gum (CMGG) and locust bean gum (CMLBG) were dynamically investigated by identifying their t_g. Differing from the routine research in rheology or synthesis chemistry, the present study focused on neither pure t_g nor pure DS but on their interrelation. The results demonstrated that CMGG/CMLBG's gel points could be observed only when their DS were 0.2. However, it was not guaranteed to yield the tested polymers' gel points unless the investigation was performed on a plateau region. Two preconditions, DS≤0.2 and ω=3–60 rad s⁻¹, must be satisfied simultaneously besides γ of 0.5% was maintained in order to obtain the expected gel point. Since DS was regarded as a contradictory focus related to the polymers' hydrophilicity and cross-linking behavior, the value DS=0.2 was exactly the upper limit of CMGG/CMLBG's gelation. The significance of the present study rests not only in the first successful attempt in determining galactomannan-based derivatives' DS_critical but also in its universal application for other polysaccharide-based derivatives because of a similarity in structure.     

Probing fluorescence induction in chloroplasts on a nanosecond time scale utilizing picosecond laser pulse pairs

The fluorescence induction and other fluorescence properties of spinach chloroplasts at room temperature were probed utilizing two 30-ps wide laser pulses (530 nm) spaced Δt (s) apart in time (Δt = 5–110 ns). The energy of the first pulse (P₁) was varied (10¹²–10¹⁶ photons · cm⁻²), while the energy of the second (probe) pulse (P₂) was held constant (5 · 10¹³ photons · cm⁻²). A gated (10 ns) optical multichannel analyzer-spectrograph system allowed for the detection of the fluorescence generated either by P₁ alone, or by P₂ alone (preceded by P₁). The dominant effect observed for the fluorescence yield generated by P₁ alone is the usual singlet-singlet exciton annihilation which gives rise to a decrease in the yield at high energies. However, when the fluorescence yield of dark-adapted chloroplasts is measured utilizing P₂ (preceded by pulse P₁) an increase in this yield is observed. The magnitude of this increase depends on Δt, and is characterized by a time constant of 28 ± 4 ns. This rise in the fluorescence yield is attributed to a reduction of the oxidized (by P₁) reaction center P-680⁺ by a primary donor. At high pulse energies (P₁ = 4 · 10¹⁴ photons · cm⁻²) the magnitude of this fluorescence induction is diminished by another quenching effect which is attributed to triplet excited states generated by intense P₁ pulses. Assuming that the P₁ pulse energy dependence of the fluorescence yield rise reflects the closing of the reaction centers, it is estimated that about 3–4 photon hits per reaction center are required to close completely the reaction centers, and that there are 185–210 chlorophyll molecules per Photosystem II reaction center.     

Pressure effects on the rates of Hg-assisted chloride release from some [CrCl(diamine)(triamine)] complexes

The rate of Hg²⁺-assisted chloride release from several mer-[CrCl(diamine)(triamine)]²⁺ complexes has been measured as a function of pressure, Hg²⁺ concentration and temperature. The calculated activation volumes are independent of [Hg²⁺] and temperature and kinetic parametes 10⁴ k_Hg (25 °c) (M⁻¹ s⁻¹), ΔH^‡ (kJ mol⁻¹), ΔS^‡ (J K⁻¹ mol⁻¹), ΔV^‡ (cc mol⁻¹) are: (en)(dpt): 6.44. 75.5, −52, −5.0; (ibn)(dpt): 5.81, 89.5, −6, −0.03; (Me₂tn)(dpt): 22.2, 84.9, −11, −0.5; (tn)(dpt): 29.1, 87, −1, +0.3; (en)(2,3-tri): 1.94, 87.0, −24, −5.7; (en)(Medpt): 0.417, 94.6, −11, −0.8; (tn)(Medpt): 9.14, 98.3, +26, +1.8.     

Halophila ovalis (R. Br.) Hook. f. from a submarine hot spring in southern Japan

Colonies of the seagrass Halophila ovalis are found growing adjacent to coral Acropora sp. and Seriatopora hystrix in a submarine hot spring (at 15.7 m depth, 28.6°C) at the north coast of Taketomi Island, near the southern tip of Japan. Halophila plants grow in sea water containing sulphide 930 μg S ml⁻¹ and on the substratum with fine precipitates of the submarine hot spring which have sulphide content up to 5400 μg S g⁻¹ DW. The accumulated sulphide concentration reaches as high as 8400 μg S g⁻¹ DW in under ground tissues and 5700 μg S g⁻¹ DW in above-ground tissues, respectively. It is suggested that, not the sulphide concentration but light and possibly water temperature are the limiting factors for the Halophila colonization in the submarine hot spring.     

A performance comparison of choline biosensors: anodic or cathodic detections of H2O2 generated by enzyme immobilized on a conducting polymer

Amperometric choline biosensors were fabricated by the covalent immobilization of an enzyme of choline oxidase (ChO) and a bi-enzyme of ChO/horseradish peroxidase (ChO/HRP) onto poly-5,2′:5′,2″-terthiophene-3′-carboxylic acid (poly-TTCA) modified electrodes (CPMEs). A sensor modified with ChO utilized the oxidation process of enzymatically generated H₂O₂ in a choline solution at +0.6 V. The other one modified with ChO/HRP utilized the reduction process of H₂O₂ in a choline solution at −0.2 V. Experimental parameters affecting the sensitivity of sensors, such as pH, applied potential, and temperature were optimized. A performance comparison of two sensors showed that one based on ChO/HRP/CPME had a linear range from 1.0×10⁻⁶ to 8.0×10⁻⁵ M and the other based on ChO/CPME from 1.0×10⁻⁶ to 5.0×10⁻⁵ M. The detection limits for choline employing ChO/HRP/CPME and ChO/CPME were determined to be about 1.0×10⁻⁷ and 4.0×10⁻⁷ M, respectively. The response time of sensors was less than 5 s. Sensors showed good selectivity to interfering species. The long-term storage stability of the sensor based on ChO/HRP/CPME was longer than that based on ChO/CPME.     

Finger temperatures during military field training at 0 to −29 °C

1. Skin and rectal temperatures were recorded continuously in 70 measurements during typical tasks of infantry and artillery training at 0 to −29 °C. The duration of the measurements varied from 55 min to 9.5 h.

2. The distribution of finger skin temperatures was quite similar at ambient temperature ranges 0 to −10 °C and −10 to −20 °C, while at −20 to −30 °C the finger temperatures were clearly lower.

3. At different ambient temperature ranges, 20–69% of finger temperatures were low enough to cause cold thermal sensations.

4. Sensation of cold was experienced at a finger temperature of 11.6±3.7 °C (mean±SD).     

Thermal denaturation and degradation of schizophyllan

Size exclusion chromatography and low-angle laser light scattering have been used for studying the evolution of schizophyllan polysaccharide during a thermal treatment (t > 100°C) in aerated solution. Thermal denaturation of the native triple helices into single chains is initiated above 135°C and is complete in 10 min at 160°C. Both conformations can coexist in the 130–140°C temperature range. In the presence of oxygen, both forms of the biopolymer undergo severe thermal degradation. The rate of degradation was found to be independent of chain length and conformation. An activation energy of 104 kJ mol⁻¹ was determined. The reaction was base-catalyzed. Analysis of chromatographic patterns indicate that the degradation probably occurs through an ‘all-or-none’ process.     

Dielectric relaxation in hydroxyethyl cellulose

This paper presents the results of the dielectric analysis of hydroxyethyl cellulose (HEC) in the temperature range 100–350 K and in the frequency range 20 Hz–1 MHz. The results show a distinct broad relaxation process in the temperature range 150–250 K with activation energy of about 33.4 MJ/kmol. The strength of the relaxation, ε_s−ε∞, increases only slightly with temperature and the apparent increase of the height of the relaxation maximum is attributed to an increase of the co-operativity parameter . This increase of is interpreted as decrease of the co-operativity of dipole motions with increasing temperature. The origin of the relaxation is ascribed to the reorientation of double ethylene oxide groups or part of these.     

Thermodynamics of the disproportionation of adenosine 5'-diphosphate to adenosine 5'-triphosphate and adenosine 5'-monophosphate, II. Experimental data

High-pressure liquid-chromatography and microcalorimetry have been used to determine equilibrium constants and enthalpies of reaction for the disproportionation reaction of adenosine 5′-diphosphate (ADP) to adenosine 5′-triphosphate (ATP) andadenosine 5′-monophosphate (AMP). Adenylate kinase was used to catalyze this reaction. The measurements were carried out over the temperature range 286 to 311 K, at ionic strengths varying from 0.06 to 0.33 mol kg⁻¹, over the pH range 6.04 to 8.87, and over the pMg range 2.22 to 7.16, where pMg = -log a(Mg²⁺). The equilibrium model developed by Goldberg and Tewari (see the previous paper in this issue) was used for the analysis of the measurements. Thus, for the reference reaction: 2 ADp³⁻ (ao) AMp²⁻ (ao)+ ATp⁻ (ao), K° = 0.225 ± 0.010, ΔG° = 3.70 +- 0.11 kJ mol ⁻¹, ΔH° = −1.5 ± 1. 5 kJ mol ⁻¹, °S ° = −17 ± 5 J mol⁻¹ K⁻¹, and ACP_p°≈ = −46 J mo1l⁻¹ K⁻¹ at 298.15 K and 0.1 MPa. These results and the thermodynamic parameters for the auxiliary equilibria in solution have been used to model the thermodynamics of the disproportionation reaction over a wide range of temperature, pH, ionic strength, and magnesium ion morality. Under approximately physiological conditions (311.15 K, pH 6.94, [Mg²⁺] = 1.35 × 10⁻³ mol kg⁻¹, and I = 0.23 mol kg⁻¹) the apparent equilibrium constant (K_A′ = m(ΣAMP)m(ΣATP)/[ m(ΣADP)]²) for the overall disproportionation reaction is equal to 0.93 ± 0.02. Thermodynamic data on the disproportionation reaction and literature values for this apparent equilibrium constant in human red blood cells are used to calculate a morality of 1.94 × 10⁻⁴ mol kg⁻¹ for free magnesium ion in human red blood cells. The results are also discussed in relation to thermochemical cycles and compared with data on the hydrolysis of the guanosine phosphates.     

Bench-scale fermentation of Trichoderma viride on wastewater sludge: Rheology, lytic enzymes and biocontrol activity

Conidiation and lytic enzyme production by Trichoderma viride at different solids concentration of pre-treated municipal wastewater sludge was examined in a 15-L fermenter. The maximum conidia concentration (5.94 × 10⁷ CFU mL⁻¹ at 96 h) was obtained at 30 g L⁻¹ suspended solids. The maximum lytic enzyme activities were achieved around 12–30 h of fermentation. Bioassay against a fungal phytopathogen, Fusarium sp. showed maximum activity in the sample drawn around 96 h of fermentation at 30 g L⁻¹ suspended solids concentration. Entomotoxicity against spruce budworm larvae showed maximum value ≈17290 SBU μL⁻¹ at 30 g L⁻¹ suspended solids concentration at the end of fermentation (96 h). Plant bioassay showed dual action of T. viride, i.e., disease prevention and growth promotion. The rheological analyses of fermentation sludges showed the pseudoplastic behaviour. In order to maintain required dissolved oxygen concentration ≥30%, the agitation and aeration requirements significantly increased at 35 g L⁻¹ compared to 30 and 25 g L⁻¹. The oxygen uptake rate and volumetric oxygen mass transfer coefficient, k_La at 35 g L⁻¹ did not increase in comparison to 30 g L⁻¹ due to rheological complexity of the broth during fermentation. Thus, the successful fermentation operation of the biocontrol fungus T. viride is a rational indication of its potential for mass-scale production for agriculture and forest sector as a biocontrol agent.