Calorimetric Signals Research Articles

Adsorption-induced conformational changes of proteins onto ceramic particles: Differential scanning calorimetry and FTIR analysis

Three model proteins, bovine serum albumin, hen's egg lysozyme and bovine serum fibrinogen, were adsorbed from aqueous solution onto finely dispersed ceramic particles, namely different kinds of alumina and hydroxyapatite particles. The influence of adsorption on protein secondary structure was investigated. The FTIR spectroscopic findings were compared with the results of DSC measurements. In almost all cases it was found that adsorption results in destabilisation and structural loss of the bound protein. A decrease in transition enthalpy is correlated with a loss in α-helical structure, which seems to be the most sensitive structure on adsorption-induced rearrangements. A total collapse of structure in the adsorbed proteins was not determined on any ceramic surface. Some residual structure is always retained. Structural changes in the D- or E-domains of fibrinogen could be independently observed by two different calorimetric signals. The two techniques applied in the present study—micro-DSC and FTIR spectroscopy—can be concluded to provide complementary information on adsorption-induced structural changes on both the molecular (thermal stability, overall structure) and the sub-molecular level (secondary structure).

Si-N membrane-based microcalorimetry: Heat capacity and thermal conductivity of thin films

We have used silicon micromachining techniques to fabricate devices for measuring specific heat or other calorimetric signals from microgram-quantity samples over a temperature range from 1.7 to at least 525 K in magnetic fields to date up to 8 T. The devices are based on a robust silicon-nitride membrane with thin film heaters and thermometers. Different types of thermometers are used for different purposes and in different temperature ranges. These devices are particularly useful for thin film samples (typically 100–400 nm thick at present) deposited directly onto the membrane through a Si micromachined evaporation mask. They have also been used for small bulk samples attached by conducting grease, Ga or In, and for powder samples dissolved in a solvent and dropped onto devices. The measurement technique used (relaxation method) is particularly suited to high field measurements because the thermal conductance can be measured once in zero field and is field independent, while the time constant of the relaxation does not depend on thermometer calibration.

Contradicting perturbations of the segmental and secondary relaxation dynamics of polymer strands constrained in nanopores

The glass transition and the sub-glassy relaxations in 1 mm bulk-like poly(methyl methacrylate) (PMMA) films and PMMA confined in ∼5 nm silica pores were studied using several variants of thermally stimulated currents (TSCs) spectroscopy applied in the temperature range from −263 to +160 °C. The hindering of the segmental dynamics, demonstrated by the strong high-temperature shift of the α mode, is accompanied by a substantial broadening of both the dielectric and calorimetric signals compared to the bulk phase. Contrastingly, the TSC signatures of local transitions ascribed to –COOCH 3 (β mode) and α-CH 3 (γ mode) pendant groups shift to lower temperatures. These perturbations appear irrespective of the initiator (benzoyl peroxide or azo-bis-isobutyro-nitrile) used for the in situ free-radical polymerisation process. The data are interpreted in the framework of the balancing of enthalpic and entropic effects of the nanoconstraint, which generate a motional gradient with hindered relaxing units near the attractive interface and sterically or kinetically facilitated rotational motions in the core.

Simultaneous heat-flow differential calorimetry and thermogravimetry for fast determination of sorption isotherms and heat of sorption in environmental or food engineering

A new experimental technique is described for the determination of desorption characteristics for engineering purposes. A TGA-DSC set-up is used in isothermal mode to achieve the thermal desorption of deformable standard materials like microcrystalline cellulose and kaolin in dry air. Assumptions on heat and mass transfer are made and discussed in order to derive desorption isotherms and heat of sorptions from the calorimetric and gravimetric signals. The method is rapid and accurate for high-temperature desorption processes T⩾40 ∘C. It is particularly reliable for small water activity values 0⩽ a w ⩽0.4 where a good agreement with standard methods is observed in spite of small apparent diffusion coefficients D≅10 −10 m 2 s −1 for the two products tested. The method is suitable for heat sensitive biological products because of the small residence time in the furnace (1 or 2 h ).

Isothermal microcalorimetry in applied biology

Techniques of isothermal microcalorimetry have been much improved during the past two decades. In addition to their use in fundamental research, applications of practical importance have been established in some areas. However, no significant use of isothermal microcalorimetry has yet been seen in practical applications of biology, despite many methodological studies reported from that area. The main problem appears to be that the sample throughput of isothermal microcalorimeters is low compared to other techniques used in that field. Further, the non-specificity of calorimetric signals is in some cases a serious limitation. Significant progress has recently been made in the design of multi-channel isothermal microcalorimeters and in techniques where specific analytical methods have been combined with isothermal microcalorimeters. Some conclusions will be drawn with respect to the use of these techniques in applied work on living materials.

Icosahedral phase in rapidly solidified Al–Fe–Ce alloy

To study the mechanisms for the high strength of Al–TM–Re alloys, we synthesized metallic glasses Al 90Cu 5Ce 5 and Al 90Fe 5Ce 5 by melt spinning. Icosahedral particles surrounded by f.c.c.-Al were found to appear as a primary precipitation phase in the amorphous Al 90Fe 5Ce 5 alloy. The mechanical properties of the amorphous Al 90Fe 5Ce 5 alloy can be enhanced by using a mixed structure of icosahedral nanoparticles, Al nanoparticles and amorphous matrix. Microstructure analysis suggests that icosahedral structural units are homogeneously distributed in the matrix of the amorphous Al 90Fe 5Ce 5 alloy. These structural units can be identified by the characteristic isothermal calorimetric signals associated with their transformation to fine quasicrystalline grains. The unusual glass formability of the Al–Fe–Ce alloy is discussed in this report. A distinct pre-peak is observed in the X-ray diffraction patterns of the amorphous Al 90Cu 5Ce 5 and the Al 90Fe 5Ce 5 alloys. The relaxation and glass transition phenomena are clearly observed for the Al 90Cu 5Ce 5 alloy. Based on the structure difference between Al 90Fe 5Ce 5 and Al 90Cu 5Ce 5, the origin of the pre-peak can be attributed to the inter-metallic compounds pre-existent in the amorphous matrix.

The effect of pH and concentration upon aggregation transitions in aqueous solutions of poloxamine T701

Thermally induced aggregation transitions have been investigated for aqueous solutions of the poloxamine block copolymer T701—(OE 4OP 13) 2NCH 2CH 2N(OP 13OE 4) 2—using differential scanning calorimetry. The calorimetric signals obtained were fitted to a mass action model description of aggregation using a previously reported analytical procedure (Patterson et al., Langmuir 13 (1997) 2219). The presence of a central ethylene diamine moiety in the molecular structure renders the T701 molecule basic; this was confirmed and measured by acid/base titration. Basicity is shown to have an important impact upon aggregation. At low pH (2.5), the poloxamine exists in its protonated form and the bulk solution proton concentration is sufficient to suppress de-protonation, aggregation—as a consequence—is shifted to a higher temperature range. Any increase in pH reduces the temperature range over which aggregation occurs. The derived experimental calorimetric parameters, obtained from model fitting procedures, can be used to compute the fraction of poloxamine existing in an aggregated form, at any particular temperature. The data sets obtained were interpolated to show that at human body temperature (310.6 K) the fraction of poloxamine found in its aggregated form is zero at a pH of 2.5. However at a pH of 6.8, the percentage aggregation increases to about 85%. These aggregation characteristics of T701 have important implications for the design of drug delivery systems, which incorporate poloxamines.

Thermogravimetry in the analysis of results of a calorimetric experiment

A method based on a refined measurement equation is proposed for enhancing the accuracy of measurements of the specific heat of solids. The equation uses calorimetric and gravimetric output signals along with analytical representation of the output and baseline signals.

Microbial Calorimetry of Supported Cultures and Its Application to the Study of Antimicrobial Action.

Microbial calorimetry was applied to urethane foam-supported cultures of Klebsiella pneumoniae and Aspergillus oryzae to quantitatively evaluate the growth activity of microbes. The amplitude of calorimetric signals observed for the growth of microbes in a medium with porous urethane foam support was found to be markedly greater than that in stationary cultures in ordinary liquid media. For quantitative comparison, the maximal amplitude of the calorimetric signal, the growth rate constant and the doubling time were evaluated from the growth thermograms observed. The result obtained was that the growth activity of K. pneumoniae cultured on urethane foam increased about 10-fold that of cultures in liquid medium, as judged by the thermopile output signal at the peak time and about 1.7-fold in terms of the growth rate constant. The values observed for the supported culture of A. oryzae were about 1.5-fold and about 3-fold higher, respectively. The method was also applied to study the effect of sodium benzoate on the urethane foam-supported culture of A. oryzae, and its inhibitory action was quantitatively discussed.

Heat- and cold-setting gels of β-lactoglobulin solutions. A DSC and TEM study

We extracted data from the differential scanning calorimetric signals obtained with two different solutions of β-lactoglobulin at pH 3.5 and 7 and at a concentration which favours protein-protein interactions. The transition peaks were partially reversible only with solutions at pH 3.5. The temperatures corresponding to the first increase in heat flow, Θ onset, and to the amplitude of maximum deviation, Θ max, and the calorimetric enthalpy changes of heat reaction, Δ r H(cal) were higher at pH 3.5 than at pH 7. These differences indicated higher conformational stability at pH 3.5 than at pH 7: the heat transitions from the initial to the final conformational states could process through the reversible ‘two-state model’ at pH 3.5, while stable intermediate species might be created at pH 7. Isothermal treatments at Θ onset< Θ< Θ max+5 K, applied to the study of the sol-gel transition showed that solutions which denatured reversibly formed cold-setting gels, while solutions which denatured irreversibly formed heat-setting gels. Furthermore, the curve (Θ-t) which demarcated the diagrams of the sol-gel states intersected at Θ approx. equal to Θ max and the gel structures observed by transmission electronic microscope showed that cold-setting gels were constituted by linear aggregates (‘string of beads’) while heat-setting gels were constituted by very large clumped aggregates. All these results are discussed in terms of protein-protein interaction forces.

Deconvolution of Scanning Calorimetric Signals Obtained for Aqueous Mixtures of Poly(Oxypropylene) Oligomers

High-sensitivity differential scanning calorimetric (HSDSC) data obtained for mixtures of poly(oxypropylene) (POP) oligomers, of different molecular masses, in aqueous solution have been deconvoluted using a previously reported mass action thermodynamic model of aggregation (Armstrong, J.; et al. J. Phys. Chem. 1995, 99, 4590) to investigate polymer aggregation/phase separation in these mixed systems. The data shows that POP oligomer solutions will phase separate at well defined temperatures as the POP solution is warmed. The data obtained shows no scan rate dependence which prompts the hypothesis that phase separation is adequately described by a nucleation and growth mechanism. It is concluded that the nucleation step is observable by HSDSC while the growth phase is calorimetrically silent. The model derived parameters indicate that the phase transitions occurring at the lower temperature are only slightly modified compared to those values obtained for the respective single-polymer solutions, whereas the higher temperature transitions are markedly altered. A descriptive model is proposed which is used to explain the obtained results. Essentially the model suggests that the higher molecular mass component, which phase separate at lower temperatures, aggregates and phase separates independently of the lower molecular mass component. However, depending upon the molar ratio of the two components, the lower molecular mass component will, at the appropriate temperature, either join the already existing high molecular mass component aggregates or will form separate aggregate nuclei.

Time-domain optimal filtering

A method based on the Wiener approach has been presented for filtering calorimetric signals with the technique of windowing the power spectrum in the frequency domain instead of an abrupt rectangular cut-off of the noise frequencies. This paper presents a time-domain approach which, for the ideal case of additive noise-corrupted signals with the noise normal, does not require any user input except for considerations of implementation such as filter length. The key point lies in the norming of the final filter coefficients vector for various targets; here the steady state unity gain is selected.

Optimal filtering of calorimeter signals

A technique based on the Wiener filter is presented for filtering calorimetric signals. The filter was applied to a simulated calorimetric signal with three different signal-to-noise levels. Smoothing takes place in the frequency domain using windows that allow the power spectrum of the signal noise to be estimated a priori. In addition, the filter removes the parasitic oscillations that the fast Fourier transform introduces during deconvolution.