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PubMed2001

Ligand-modulation of the stability of the glucose transporter GLUT 1.

The glucose transporter GLUT 1 was isolated from human erythrocytes and reconstituted into endogenous membrane lipids. Results from thermal denaturation studies, using differential scanning calorimetry, indicate that the thermal denaturation temperature of GLUT 1 is significantly lower in the presence of ATP. The lowering of this transition temperature is very dependent on pH. At more acidic pH, ATP has a greater effect of lowering the thermal denaturation temperature of the protein. For example, with 4.8 mM ATP, the denaturation endotherm is lowered by over 10 degrees at pH 4.3, whereas at pH 7.4, ATP does not alter this transition temperature. However, a change in pH alone, in the absence of ATP, has very little effect on the denaturation temperature. Both glucose and salt partially reverse the lowering of the temperature of thermal denaturation caused by ATP. Studies of acrylamide quenching of the Trp residues of GLUT 1 indicate that at neutral pH, ATP increases the Stern-Volmer quenching constant, while glucose lowers it. The results indicate that ATP binds to GLUT 1 and destabilizes the native structure, leading to a lowering of the thermal denaturation temperature and an increase in acrylamide quenching. The effects of ATP are reversed in part by glucose and are also partly electrostatic in nature.

Acrylamide↗

PubMed1981

Erythrocyte membrane in myotonic dystrophy. A study with acetylcholinesterase.

Erythrocyte membrane in myotonic dystrophy (MyD) was studied with respect to acetylcholinesterase (AchE), an enzyme localized on the external surface of the membrane. The activity was determined over the temperature range of 7-41 degrees C (in hemolysates) and 25-41 degrees C (in ghosts). The activity, the transition temperature in the Arrhenius plots, and the activation energy either above or below the transition temperature did not differ between MyD and controls. The Hill coefficient for the inhibition by fluoride was approximately 1 in MyD and in controls at 13, 25 and 37 degrees C. The dose of fluoride for 50% inhibition of the enzyme activity differed between different temperatures but not between MyD and controls. There seems to be no gross abnormality in AchE or its environment in erythrocyte membrane in MyD.

Acetylcholinesterase↗

PubMed1984

Differential scanning calorimetry of Cd(II) alkaline phosphatases.

Differential scanning calorimetry has been employed to monitor structural alterations induced in the dimeric enzyme alkaline phosphatase on binding of Cd(II) (to the metal-free apoenzyme) and phosphate (Pi) (to the Cd(II) enzyme). Cd(II) addition to the apoenzyme at pH 6.5 results in an increased transition temperature, suggesting a stabilizing effect of the bound metal ion. Two distinct structural forms of the protein are detected as discrete calorimetric transitions (Tm = 69-84 degrees C; 87-94 degrees C, respectively). Distribution of the enzyme between these forms is found to depend on the exogenous Cd(II) concentration and the protocol of Cd(II) addition. These results indicate that conversion between the conformational forms is a slow process which appears to require specific levels of metal ion site occupancy. These studies, in which the exogenous Cd(II) concentration was varied from 10(-5) M to 10(-3) M suggest a structural basis for previously observed hysteretic phenomena observed on Cd(II) binding to the enzyme. Even at a minimum stoichiometry of Cd(II) (2 eq/mol of dimer) a single equivalent of Pi is sufficient to accelerate assumption of a stabilized form of the protein (Tm = 90 degrees C). This is followed by a slow structural change paralleling the time course of formation of the functional 2 Cd(II) phosphoryl enzyme which displays two calorimetric transitions (Tm = 65 degrees C, 88 degrees C). The low temperature transition does not appear if Pi is initially present at millimolar concentrations and is abolished on addition of Pi at concentrations in excess of 0.1 mM. These observations suggest the presence of a second, distinct Pi binding site on the 2 Cd(II) phosphoryl enzyme. This is supported by the changes observed in the 31P NMR chemical shift of Pi added to comparable enzyme samples. These data, including assessment of the effect of the presence of Mg(II), are discussed in terms of the mechanism of metal ion association to the enzyme and rearrangement of bound metal ions induced by Pi binding.

Alkaline Phosphatase↗

PubMed1978

The effect of free fatty acids on the thermotropic phase transition of dimyristoyl glycerophosphocholine.

The effect of free fatty acids on the phase transition characteristics and fluidity of bilayers of dimyristoyl glycerophosphocholine were studied by pyrene eximer fluorescence and differential scanning calorimetry. High melting saturated fatty acids with chain lenghts of 12--18 carbon atoms raise the phase transition temperature and enhance the ability of pyrene to form clusters in the gel state while not affecting the fluidity of the membrane in the liquid crystal state. Low melting unsaturated fatty acids lower the phase transition temperature and decrease the ability of pyrene to form clusters in the gel state while not affecting the fluidity of the membrane in the liquid crystal state. The effects of the very long chain fatty acids, arachidic (C 20) and behenic (C 22) appears to be similar to those of cholesterol in that they cause a broadening of the phase transition with a lowering of the transition enthalpy but have little effect on the temperature at which the phase transition occurs.

Calorimetry↗

PubMed2002

The activation energy at Tg and the fragility index of indomethacin, predicted from the influence of the heating rate on the temperature position and on the intensity of thermally stimulated depolarization current peak.

PURPOSE: The purpose of this study was to estimate the activation energy at the glass transition temperature (and the fragility index) of amorphous indomethacin from the influence of heating rate on the features of the relaxation peaks obtained by thermally stimulated depolarization currents (TSDC) and to compare the obtained results with those obtained by other procedures based on TSDC data. METHODS: The glass transition temperature region of amorphous indomethacin was characterized at different heating rates by TSDC in a way similar to that used to determine the kinetics of the glass transition relaxation by differential scanning calorimetry. The features of a thermal sampled TSDC peak, namely the temperature location and the intensity, depend on the heating rate. RESULTS: The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating rate dependence of the TSDC peak location, T(m), and of the maximum intensity of the TSDC peak, I(T(m)). CONCLUSIONS: The methods for determining the activation energy for structural relaxation and fragility of indomethacin from TSDC data obtained with different heating rates were compared with other procedures previously proposed. TSDC, which is not a very familiar technique in the community of pharmaceutical scientists, proved to be a very convenient technique to study molecular mobility and to determine the fragility index in glass-forming systems. The value of approximately 60 appears as a reasonable value of the fragility index of indomethacin.

Drug Stability↗

PubMed2006

Functional reconstitution of the integral membrane enzyme, isoprenylcysteine carboxyl methyltransferase, in synthetic bolalipid membrane vesicles.

Three bipolar archaeal-type diglycerophosphocholine tetraether lipids (also known as bolalipids) have been prepared to determine (1) the influence of molecular structure on the physical properties of bolalipid membranes and (2) their impact on the functional reconstitution of Ste14p, a membrane-associated isoprenylcysteine carboxyl methyltransferase from Saccharomyces cerevisiae. Three bolalipids were synthesized: C20BAS, C32BAS, and C32phytBAS. These bolalipid structures differ in that the C20BAS derivative has a short sn-1 glyceryl diether C20H40 transmembrane alkyl chain and two ether-linked sn-2 n-decyl chains, whereas the C32BAS and C32phytBAS derivatives have a longer sn-1 diether C32H64 membrane-spanning chain and two ether-linked sn-2 n-hexadecyl or phytanyl chains, respectively. Differential scanning calorimetry and temperature-dependent 31P NMR was used to determine the gel-to-liquid crystalline phase transition temperatures of the bolalipids (C32BAS Tm > 85 degrees C; C32phytBAS Tm = 14 degrees C; and C20BAS Tm = 17 degrees C). The bolalipid lateral diffusion coefficients, determined by fluorescence recovery after photobleaching at 25 degrees C, were 1.5 x 10(-8) and 1.8 x 10(-9) cm2/s for C20BAS and C32phytBAS, respectively. The mobility of C32BAS could not be measured at this temperature. Ste14p activity was monitored by an in vitro methyltransferase assay in reconstituted vesicle dispersions composed of DMPC, C20BAS/E. coli polar lipid, C20BAS/POPC, C32phytBAS/E. coli polar lipid, and C32phytBAS/POPC. Ste14p activity was lost in vesicles composed of 75-100 mol % C20BAS and 0-100 mol % C32BAS but retained in vesicles with 0-50 mol % C20BAS and 0-100 mol % C32phytBAS. Confocal immunofluorescence microscopy confirmed the presence of Ste14p in 100 mol % C20BAS and 100 mol % C32phytBAS vesicle dispersions, even though the lamellar liquid crystalline phase thickness of C20BAS is only 32 A. Because Ste14p activity was not affected by either the gel-to-liquid-crystal phase transition temperature of the lipid or the temperature of the assay, the low activity observed in 75-100 mol % C20BAS membranes can be attributed to hydrophobic mismatch between this bolalipid and the hydrophobic surface of Ste14p.

Cell Membrane↗

PubMed1994

Ethanol unfolds firefly luciferase while competitive inhibitors antagonize unfolding: DSC and FTIR analyses.

Firefly luciferase has gained popularity as a protein model in elucidating anaesthesia mechanism because the bioluminescence of the purified enzyme system is extremely sensitive to volatile anaesthetics. This study analysed the thermal unfolding of firefly luciferase by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). DSC showed that the transition of firefly luciferase from the folded (N) to unfolded (D) state occurred at 41.7 degrees C with the excess heat flow of 1.6 cal g-1 protein. Ethanol decreased the transition temperature dose dependently. In contrast, luciferin competitors, anilinonaphthalenesulphonate (ANS), toluidinonaphthalenesulphonate (TNS), and myristic acid increased the transition temperature. The competitive inhibitors antagonized unfolding and stabilized the N-state. Ethanol promoted unfolding and stabilized the D-state. Temperature scan by FTIR agreed with the DSC data. The intensities of amide-I' and amide-II' bands started to increase at 20-25 degrees C. This temperature coincides with the temperature where the bioluminescence of firefly luciferase is maximal. The unfolding effect of ethanol was evident even at 5 degrees C. ANS, TNS, and myristic acid completely protected the enzyme from the thermal unfolding. This is the first demonstration that the noncompetitive inhibitors induce the isothermal first-order phase transition in a functional protein, whereas competitive inhibitors protect the enzyme from thermal unfolding. The action mode of competitive inhibitors on firefly luciferase is completely different from that of noncompetitive inhibitors.

Calorimetry, Differential Scanning↗

PubMed1975

Glucose transport in Acholeplasma laidlawii B: dependence on the fluidity and physical state of membrane lipids.

The uptake of D-glucose by Acholeplasma laidlawii B occurs via a mediated transport process, as shown by the following observations: (i) glucose permeates A. laidlawii B cells at a rate at least 100 times greater than would be expected if its entry occurred only by simple passive diffusion; (ii) the apparent activation energy for glucose uptake in A. laidlawii is significantly lower than that expected and observed for the passive permeation of this sugar; (iii) glucose uptake appears to be a saturable process; (iv) glucose uptake can be completely inhibited by low concentrations of phloretin and phlorizin; and (v) glucose uptake is markedly inhibited at temperatures above 45 C, whereas the passive entry of erythritol continues to increase logarithmically until at least 60 C. The metabolism of D-glucose by this organism is rapid and, at low glucose concentrations, the intracellular radioactivity derived from D-[14-C]glucose is at any given time a reflection of the net effect of glucose transport, glucose metabolism, and loss from the cell of radioactive metabolic products. Care must thus be taken when attempting to determine the rate of glucose transport by measuring the accumulation by the cells of the total radioactivity derived from D-[14-C]glucose. The rate of uptake of D-glucose by A. laidlawii B cells is markedly dependent on the fatty acid composition and cholesterol content of the plasma membrane and exhibits a direct dependence on the fluidity of the membrane lipids as measured by their reversible, thermotropic gel to liquie-crystalline phase transition temperatures. In contrast to the transport rates, the apparent activation energy for glucose uptake above the phase transition temperature is not dependent on membrane lipid composition. At the temperature range within the membrane lipid phase transition region, the apparent activation energy of glucose uptake is different from the activation energy observed at temperatures above the phase transition. This may reflect the superimposed operation within the phase transition region of more than one temperature-dependent process.

Acholeplasma laidlawii↗

PubMed2000

Assessment of polymer-polymer interactions in blends of HPMC and film forming polymers by modulated temperature differential scanning calorimetry.

PURPOSE: To assess the miscibility and phase behavior of binary blends of hydroxypropylmethyl cellulose (HPMC) with hydroxypropyl cellulose (HPC), methylcellulose (MC), and polyvinylpyrrolidone (PVP). METHODS: Polymer-polymer miscibility was assessed by measurement of the glass transition temperature (Tg) and the width of the glass transition temperature (W-Tg), using modulated temperature differential scanning calorimetry (MTDSC). RESULTS: HPMC K4M/PVP and HPMC E5/MC blends were miscible as evidenced by a single, composition dependent, Tg throughout the entire composition range. HPMC/HPC blends were immiscible at all compositions. For the miscible blends, the variation in Tg with blend composition was compared to the values predicted by the Fox and Couchman-Karasz equations. At intermediate blend compositions, HPMC K4M/PVP blends exhibited negative deviations from ideal behavior. The Tg of the HPMC E5/MC blends was found to follow the Fox equation. The W-Tg measurements of the miscible blends gave evidence of phase separation at certain compositions. CONCLUSIONS: MTDSC was shown to be a useful technique in characterizing the interactions between some commonly used pharmaceutical polymers.

Calorimetry, Differential Scanning↗

PubMed2000

Strongly temperature dependent sliding friction for a superconducting interface

A sudden drop in mechanical friction between an adsorbed nitrogen monolayer and a lead substrate occurs when the lead passes through the superconducting transition temperature. We attribute this effect to a sudden drop at the superconducting transition temperature of the substrate Ohmic heating. The Ohmic heating is due to the electronic screening current that results from the sliding adsorbed film.

Journal Article↗

PubMed1999

Structural and thermodynamic studies of KM+, a d-mannose binding lectin from Artocarpus integrifolia seeds.

The KM+ lectin exhibits a novel and unusual circular dichroism (CD) spectrum that could be explained by a high proline content that would be inducing deformation of the beta-structure and/or unusual turns. KM+ was shown to be a very rigid lectin, which was very stable under a broad variety of conditions (urea, guanidine, hydrolysis, pH, etc.). Only incubation for 60 min at 333-338 K and extreme basic pH were able to induce conformational changes which could be observed by CD and fluorescence measurements. Data from CD are typical for protein denaturing associated with changes in the overall secondary structure. Data from high-performance size exclusion chromatography (SEC) showed that the denatured forms produced at pH 12.0 are eluted in clusters that co-elute with the native forms. A significant contribution from the tyrosines to the fluorescence emission upon denaturation was observed above 328 K. In fact at 328 K some broadening of the emission spectrum takes place followed by the appearance of a shoulder (approx. 305 nm) at 333 K and above. The sensitivity of tryptophan fluorescence to the addition of sugar suggests a close proximity of the tryptophan residues to the sugar binding site, K(a)=(2.9+/-0.6)x10(3) M(-1). The fraction of chromophore accessible to the quencher obtained is f(a)=0.43+/-0.08, suggesting that approximately 50% of the tryptophan residues are not accessible to quenching by d-mannose. KM+ thermal denaturation was found to be irreversible and was analyzed using a two-state model (N-->D). The results obtained for the activation energy and transition temperature from the equilibrium CD studies were: activation energy, E(a)=134+/-11 kJ/mol and transition temperature, T(m)=339+/-1 K, and from the fluorescence data: E(a)=179+/-18 kJ/mol and T(m)=337+/-1 K. Kinetic studies gave the following values: E(a)=108+/-18 kJ/mol and E(a)=167+/-12 kJ/mol for CD and fluorescence data, respectively.

Journal Article↗

PubMed2004

Binding analyses between Human PPARgamma-LBD and ligands.

The binding characteristics of a series of PPARgamma ligands (GW9662, GI 262570, cis-parinaric acid, 15-deoxy-Delta(12,14)-prostaglandin J(2), LY171883, indomethacin, linoleic acid, palmitic acid and troglitazone) to human PPARgamma ligand binding domain have been investigated for the first time by using surface plasmon resonance biosensor technology, CD spectroscopy and molecular docking simulation. The surface plasmon resonance biosensor determined equilibrium dissociation constants (KD values) are in agreement with the results reported in the literature measured by other methods, indicating that the surface plasmon resonance biosensor can assume a direct assay method in screening new PPARgamma agonists or antagonists. Conformational changes of PPARgamma caused by the ligand binding were detected by CD determination. It is interesting that the thermal stability of the receptor, reflected by the increase of the transition temperature (T(m)), was enhanced by the binding of the ligands. The increment of the transition temperature (DeltaT(m)) of PPARgamma owing to ligand binding correlated well with the binding affinity. This finding implies that CD could possibly be a complementary technology with which to determine the binding affinities of ligands to PPARgamma. Molecular docking simulation provided reasonable and reliable binding models of the ligands to PPARgamma at the atomic level, which gave a good explanation of the structure-binding affinity relationship for the ligands interacting with PPARgamma. Moreover, the predicted binding free energies for the ligands correlated well with the binding constants measured by the surface plasmon resonance biosensor, indicating that the docking paradigm used in this study could possibly be employed in virtual screening to discover new PPARgamma ligands, although the docking program cannot accurately predict the absolute ligand-PPARgamma binding affinity.

Apoproteins↗

PubMed2002

An autosampling differential scanning calorimeter instrument for studying molecular interactions.

A new ultrasensitive differential scanning calorimeter (DSC) instrument is described, which utilizes autosampling for continuous operation. High scanning rates to 250 deg/h with rapid cooling and equilibration between scans facilitates higher sample throughput up to 50 samples during each 24 h of unattended operation. The instrument is suited for those pharmaceutical applications where higher throughput is important, such as screening drug candidates for binding constant or screening solution conditions for stability of liquid protein formulations. Results are presented on the binding of five different anionic inhibitors to ribonuclease A, which included cytidine 2'-monophosphate (2'CMP), 3'CMP, uridine 3'-monophosphate, pyrophosphate, and phosphate. Binding constants K(B) (or dissociation constants K(d)) are obtained from the shift in the transition temperature T(M) for ribonuclease thermal unfolding in the presence of ligand relative to the transition temperature in the absence of ligand. Measured binding constants ranged from 155 M(-1) (K(d) = 6.45 mM) for the weak-binding phosphate anion to 13100 M(-1) (K(d) = 76.3 microM) for the strongest binding ligand, 2'CMP. The DSC method for measuring binding constants can also be extended to ultratight interactions involving either ligand-protein or protein-protein binding.

Algorithms↗

PubMed1981

Maintenance of Membrane Fluidity during Development of Freezing Tolerance of Winter Wheat Seedlings.

Fluidity of membrane lipids of shoot and root tissue and of chloroplasts from young wheat seedlings of contrasting freezing tolerance was investigated by measuring the motion and order parameters after spin labeling. A striking similarity was observed in membrane lipid fluidity of the five cultivars grown at 22 C. After cold hardening by growth at 2 C, a small change in membrane lipid fluidity was observed, but this was not correlated with the development of freezing tolerance, and there was no alteration in the transition temperature of membrane lipids. The results show that neither changes in membrane lipid fluidity nor transition temperature are a necessary feature of cold acclimation in wheat.

Journal Article↗

PubMed1976

Phase transitions in the membrane of a marine bacterium, Pseudomonas BAL-31.

An unsaturated fatty acid auxotroph, strain UFA, isolated from the marine pseudomonad Pseudomonas BAL-31, host cell of the lipid-containing bacteriophage PM2, was grown in media supplemented with different unsaturated fatty acids. Under these conditions the fatty acid composition of the cell could be altered drastically. The phase transition in the native membrane and in the extracted lipids was analyzed by electron spin resonance using a nitroxide spin probe. Membranes prepared from strain UFA grown in cis16:1 or trans16:1 showed one transition at 9.4 degrees C and 12.4 degrees C respectively. Extracted lipids in both cases had almost the same transition temperature as that of the intact membrane. Membranes prepared from Pseudomonas BAL-31 had one transition at approximately 12 degrees C, on the other hand there was no clear cut phase transition using extracted lipids. Replication of bacteriophage PM2 took place below the transition temperature of the membrane lipids in the case where strain UFA was grown in tran16:1. Other cases were not studied.

Bacteriophages↗

PubMed2001

The effect of osmotic pressure on the membrane fluidity of Saccharomyces cerevisiae at different physiological temperatures.

Membrane fluidity in whole cells of Saccharomyces cerevisiae W303-1A was estimated from fluorescence polarization measurements using the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, over a wide range of temperatures (6-35 degrees C) and at seven levels of osmotic pressure between 1.38 MPa and 133.1 MPa. An increase in phase transition temperatures was observed with increasing osmotic pressure. At 1.38 MPa, a phase transition temperature of 12 +/- 2 degrees C was observed, which increased to 17 +/- 4 degrees C at 43.7 MPa, 21+/- 7 degrees C at 61.8 MPa, and 24 +/- 9 degrees C at an osmotic pressure of 133.1 MPa. From these results we infer that, with increases in osmotic pressure, the change in phospholipid conformation occurs over a larger temperature range. These results allow the representation of membrane fluidity as a function of temperature and osmotic pressure. Osmotic shocks were applied at two levels of osmotic pressure and at nine temperatures, in order to relate membrane conformation to cell viability.

Membrane Fluidity↗

PubMed1991

Polymorphism of the bilayer membranes in the ordered phase and the molecular origin of the lipid pretransition and rippled lamellae.

The lamellar-to-undulated-lamellar phase transition (L beta'----P beta', pretransition) in lipid bilayers is shown to be a phenomenon which exists only if lipid polar headgroups are sufficiently hydrated and if the interchain packing is sufficiently weak. The minimal lipid hydrophilicity and the critical amount of the lipid-bound water can be related to the lipid chain-melting transition temperature; the latter must not exceed some maximal, chainlength-dependent value if the pretransition is to exist. The minimally required amount of the lipid-bound water itself is essentially chainlength independent, however, and unaffected by the method of hydration variation: physical dehydration, hydrational competition between the lipid molecules and the substances dissolved in the aqueous subphase, or decreasing lipid headgroup polarity all affect the pretransition temperature similarly on the appropriate scale. Simple, phenomenological expressions for the evaluation of the bilayer subtransition, pretransition and chain-melting phase transition temperature as a function of the lipid chainlength are presented. They show that, even in excess water, bilayers will tend to undulate only as long as each of the two identical lipid chains will contain between 12 +/- 1 and 22 +/- 1 carbon atoms, the P beta-phase region for the less polar lipids being as a rule narrower. To get a theoretical means for quantitatively studying the effects of the lipid hydration on the bilayer pretransition, an interaction-balance method is proposed for describing undulated membranes at the molecular level. This is based on comparing the free-energy gain from the increased headgroup hydration with the free-energy loss caused by the reduced chain-chain attraction upon ripple formation. A rationale is thus found for scaling the pretransition temperature in terms of the hydration-induced chain-melting phase transition shift or of the lipid surface hydrophilicity. Within the framework of such a model the recently reported (de)hydration dependence of the bilayer-undulation period is reproduced with reasonable accuracy. Furthermore, it is estimated that at least 12 +/- 2 water molecules must be associated with each lipid head for the bilayer undulation to be feasible. The closer the system is to this boundary condition the longer is the repeat-distance for the surface undulations and the less stable is the undulated bilayer phase.

Gels↗

PubMed1997

Cooperativity in the Iron(II) Spin-Crossover Compound [Fe(ptz)(6)](PF(6))(2) under the Influence of External Pressure (ptz = 1-n-Propyltetrazole).

The iron(II) spin-crossover compound [Fe(ptz)(6)](PF(6))(2) (ptz = 1-propyltetrazole) crystallizes in the triclinic space group P&onemacr;, with a = 10.6439(4) Å, b = 10.8685(4) Å, c = 11.7014(4) Å, alpha = 75.644(1) degrees, beta = 71.671(1) degrees, gamma = 60.815(1) degrees, and Z = 1. In [Fe(ptz)(6)](PF(6))(2), the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin state, DeltaV degrees (HL), is 24(2) Å(3)/molecule. The interaction constant Gamma, as a measure of cooperativity, is within experimental error independent of external pressure and has a value of 101(5) cm(-)(1). In contrast to the case of the related compound [Fe(ptz)(6)](BF(4))(2) (Decurtins et al. Inorg. Chem.1985, 24, 2174), there is no hysteresis due to a first-order crystallographic phase transition, nor is there a hysteresis induced by external pressure as in the mixed crystal [Zn(1)(-)(x)()Fe(x)()(ptz)(6)](BF(4))(2), x = 0.1 (Jeftic et al. J. Phys.Chem. Solids 1996, 57, 1743). However, in [Fe(ptz)(6)](PF(6))(2), the interaction constant Gamma is found to be very close to the critical value above which a hysteresis solely due to the cooperative effects is expected. In addition, high-spin --> low-spin relaxation measurements were performed under external pressures of up to 1 kbar in the temperature interval between 50 and 60 K. An external pressure of 1 kbar accelerates the high-spin --> low-spin relaxation by 1 order of magnitude.

Journal Article↗