Melting properties of peptides and their solubility in water. Part 1: dipeptides based on glycine or alanine

Melting properties (melting temperature, melting enthalpy and heat capacity difference between liquid and solid phase) of biomolecules are indispensable for natural and engineering sciences. The direct determination of these melting properties by using conventional calorimeters for biological compounds is often not possible due to decomposition during slow heating. In the current study this drawback is overcome by using fast scanning calorimetry (FSC) to directly measure the melting properties of five dipeptides (glycyl-glycine, glycyl-l-alanine, l-alanyl-glycine, l-alanyl-l-alanine and cyclo(l-alanyl-glycine)). The experimental melting properties were used as inputs into a thermodynamic solid–liquid equilibrium relation to predict solubility of the dipeptides in water. The required activity coefficients were predicted with PC-SAFT using solubility-independent model parameters. PC-SAFT predicted different solubility profiles (solubility vs. temperature) of isomers. The predictions were validated by new experimental solubility data, and the crystal structure of the dipeptides in saturated solution was verified by X-ray diffraction. The different water solubility profiles of isomers (glycyl-l-alanine and l-alanyl-glycine) were found to be caused by the big difference in the melting enthalpy of the two dipeptides. To conclude, combining the PC-SAFT and FSC methods allows for accurate prediction of dipeptide solubility in water in a wide temperature range without the need to fit any model parameters to experimental solubility data.

Combination of the PC-SAFT and FSC methods allows for accurate prediction of dipeptide solubility in water in a wide temperature range without the need to fit any model parameters to experimental solubility data.

Nanometer scale thermal response of polymers to fast thermal perturbations

Nanometer scale thermal response of polymers to fast thermal perturbations is described by linear integro-differential equations with dynamic heat capacity. The exact analytical solution for the non-equilibrium thermal response of polymers in plane and spherical geometry is obtained in the absence of numerical (finite element) calculations. The solution is different from the iterative method presented in a previous publication. The solution provides analytical relationships for fast thermal response of polymers even at the limit t → 0, when the application of the iterative process is very problematic. However, both methods give the same result. It was found that even fast (ca. 1 ns) components of dynamic heat capacity greatly enhance the thermal response to local thermal perturbations. Non-equilibrium and non-linear thermal response of typical polymers under pulse heating with relaxation parameters corresponding to polystyrene and poly(methyl methacrylate) is determined. The obtained results can be used to analyze the heat transfer process at the early stages of crystallization with fast formation of nanometer scale crystals.

Influence of the Cross-Link Density on the Rate of Crystallization of Poly(ε-Caprolactone)

Cross-linked poly(ε-caprolactone) (PCL) is a smart biocompatible polymer exhibiting two-way shape memory effect. PCL samples with different cross-link density were synthesized by heating the polymer with various amounts of radical initiator benzoyl peroxide (BPO). Non-isothermal crystallization kinetics was characterized by means of conventional differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC). The latter technique was used to obtain the dependence of the degree of crystallinity on the preceding cooling rate by following the enthalpies of melting for each sample. It is shown that the cooling rate required to keep the cooled sample amorphous decreases with increasing cross-link density, i.e., crystallization process slows down monotonically. Covalent bonds between polymer chains impede the crystallization process. Consequently, FSC can be used as a rather quick and low sample consuming method to estimate the degree of cross-linking of PCL samples.

Fast scanning calorimetry of lysozyme unfolding at scanning rates from 5 K/min to 500,000 K/min

BACKGROUND

Protein denaturation is often studied using differential scanning calorimetry (DSC). However, conventional instruments are limited in the temperature scanning rate available. Fast scanning calorimetry (FSC) provides an ability to study processes at much higher rates while using extremely small sample masses [ng]. This makes it a very interesting technique for protein investigation.

METHODS

A combination of conventional DSC and fast scanning calorimeters was used to study denaturation of lysozyme dissolved in glycerol. Glycerol was chosen as a solvent to prevent evaporation from the micro-sized samples of the fast scanning calorimeter.

RESULTS

The lysozyme denaturation temperatures in the range of scanning rates from 5 K/min to ca. 500,000 K/min follow the Arrhenius law. The experimental results for FSC and conventional DSC fall into two distinct clusters in a Kissinger plot, which are well approximated by two parallel straight lines.

CONCLUSIONS

The transition temperatures for the unfolding process measured on fast scanning calorimetry sensor are significantly lower than what could be expected from the results of conventional DSC using extrapolation to high scanning rates. Evidence for the influence of the relative surface area on the unfolding temperature was found.

GENERAL SIGNIFICANCE

For the first time, fast scanning calorimetry was employed to study protein denaturation with a range of temperature scanning rates of 5 orders of magnitude. Decreased thermal stability of the micro-sized samples on the fast scanning calorimeter raise caution over using bulk solution thermal stability data of proteins for applications where micro-sized dispersed protein solutions are used, e.g., spray drying.

Unexpected behavior of ultra-thin films of blends of polystyrene/poly(vinyl methyl ether) studied by specific heat spectroscopy

Specific heat spectroscopy (SHS) employing AC nanochip calorimetry was used to investigate the glassy dynamics of ultra-thin films (thicknesses: 10 nm-340 nm) of a polymer blend, which is miscible in the bulk. In detail, a Poly(vinyl methyl ether) (PVME)/Polystyrene (PS) blend with the composition of 25/75 wt. % was studied. The film thickness was controlled by ellipsometry while the film topography was checked by atomic force microscopy. The results are discussed in the framework of the balance between an adsorbed and a free surface layer on the glassy dynamics. By a self-assembling process, a layer with a reduced mobility is irreversibly adsorbed at the polymer/substrate interface. This layer is discussed employing two different scenarios. In the first approach, it is assumed that a PS-rich layer is adsorbed at the substrate. Whereas in the second approach, a PVME-rich layer is suggested to be formed at the SiO₂ substrate. Further, due to the lower surface tension of PVME, with respect to air, a nanometer thick PVME-rich surface layer, with higher molecular mobility, is formed at the polymer/air interface. By measuring the glassy dynamics of the thin films of PVME/PS in dependence on the film thickness, it was shown that down to 30 nm thicknesses, the dynamic T_g of the whole film was strongly influenced by the adsorbed layer yielding a systematic increase in the dynamic T_g with decreasing the film thickness. However, at a thickness of ca. 30 nm, the influence of the mobile surface layer becomes more pronounced. This results in a systematic decrease in T_g with the further decrease of the film thickness, below 30 nm. These results were discussed with respect to thin films of PVME/PS blend with a composition of 50/50 wt. % as well as literature results.

Dynamics of supercooled liquid and plastic crystalline ethanol: Dielectric relaxation and AC nanocalorimetry distinguish structural α- and Debye relaxation processes

Physical vapor deposition has been used to prepare glasses of ethanol. Upon heating, the glasses transformed into the supercooled liquid phase and then crystallized into the plastic crystal phase. The dynamic glass transition of the supercooled liquid is successfully measured by AC nanocalorimetry, and preliminary results for the plastic crystal are obtained. The frequency dependences of these dynamic glass transitions observed by AC nanocalorimetry are in disagreement with conclusions from previously published dielectric spectra of ethanol. Existing dielectric loss spectra have been carefully re-evaluated considering a Debye peak, which is a typical feature in the dielectric loss spectra of monohydroxy alcohols. The re-evaluated dielectric fits reveal a prominent dielectric Debye peak, a smaller and asymmetrically broadened peak, which is identified as the signature of the structural α-relaxation and a Johari-Goldstein secondary relaxation process. This new assignment of the dielectric processes is supported by the observation that the AC nanocalorimetry dynamic glass transition temperature, T_α, coincides with the dielectric structural α-relaxation process rather than the Debye process. The combined results from dielectric spectroscopy and AC nanocalorimetry on the plastic crystal of ethanol suggest the occurrence of a Debye process also in the plastic crystal phase.

First Clear-Cut Experimental Evidence of a Glass Transition in a Polymer with Intrinsic Microporosity: PIM-1

Polymers with intrinsic microporosity (PIMs) represent a novel, innovative class of materials with great potential in various applications from high-performance gas-separation membranes to electronic devices. Here, for the first time, for PIM-1, as the archetypal PIM, fast scanning calorimetry provides definitive evidence of a glass transition ( T_g = 715 K, heating rate 3 × 10⁴ K/s) by decoupling the time scales responsible for glass transition and decomposition. Because the rigid molecular structure of PIM-1 prevents any conformational changes, small-scale bend and flex fluctuations must be considered the origin of its glass transition. This result has strong implications for the fundamental understanding of the glass transition and for the physical aging of PIMs and other complex polymers, both topical problems of materials science.

Vapor pressure of ionic liquids at low temperatures from AC-chip-calorimetry

The very low vapor pressure of ionic liquids is challenging to measure. At elevated temperatures the liquids might start to decompose, and at relatively low temperatures the vapor pressure becomes too low to be measured by conventional methods. In this work we developed a highly sensitive method for mass loss determination at temperatures starting from 350 K. This technique is based on an alternating current calorimeter equipped with a chip sensor that consists of a free-standing SiNx-membrane (thickness <1 μm) and a measuring area with lateral dimensions of the order of 1 mm. A small droplet (diameter ca. 600 μm) of an ionic liquid is vaporized isothermally from the chip sensor in a vacuum-chamber. The surface-to-volume-ratio of such a droplet is large and the relative mass loss due to evaporation is therefore easy to monitor by the changing heat capacity (J K(-1)) of the remaining liquid. The vapor pressure is determined from the measured mass loss rates using the Langmuir equation. The method was successfully tested for the determination of the vapor pressure and the vaporization enthalpy of an archetypical ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf2]). The data set created in this way in an extremely broad temperature range from 358 K to 780 K has allowed the estimation of the boiling temperature of [EMIm][NTf2]. The value (1120 ± 50) K should be considered as the first reliable boiling point of the archetypical ionic liquid obtained from experimental vapor pressures measured in the most possible close proximity to the normal boiling temperature.

Determination of electrophysical and structural properties of human cancellous bone and synthetic bone substitute material using impedance spectroscopy and X-ray powder diffraction

Electrophysical stimulation is used to support fracture healing and bone regeneration. For design optimization of electrostimulative implants, in combination with applied human donor bone or synthetic bone scaffolds, the knowledge of electrophysical properties is fundamental. Hence further investigations of the structural properties of native and synthetic bone is of high interest to improve biofunctionality of bone scaffolds and subsequent healing of the bone defect. The investigation of these properties was taken as an objective of this study. Therefore, surgically extracted fresh cylindrical and consecutively ashed cancellous bone samples from human osteoarthritic femoral heads were characterized and compared to synthetic bone substitute material. Thereby, impedance spectroscopy is used to determine the electrophysical properties and X-ray powder diffraction (XRD) for analysis of structural information of the bone samples. Conductivity and permittivity of fresh and ashed cancellous bone amounted to 1.710-2 S/m and 7.5106 and 210-5 S/m and 7.2103, respectively. Electrical conductivity and dielectric permittivity of bone scaffold resulted in 1.710-7 S/m and 49. Analysis of the structural properties showed that the synthetic bone scaffolds made of Brushite exhibited some reflections which correspond to the native bone samples. The information in present study of the bone material (synthetic and autologous) could be used for later patient individual application of electrostimulative implants.

New experimental melting properties as access for predicting amino-acid solubility

New experimental melting properties combined with PC-SAFT allow quantitative solubility predictions of amino acids in water.

Beating Homogeneous Nucleation and Tuning Atomic Ordering in Glass-Forming Metals by Nanocalorimetry

In this paper, the amorphous Ce₆₈Al₁₀Cu₂₀Co₂ (atom %) alloy was in situ prepared by nanocalorimetry. The high cooling and heating rates accessible with this technique facilitate the suppression of crystallization on cooling and the identification of homogeneous nucleation. Different from the generally accepted notion that metallic glasses form just by avoiding crystallization, the role of nucleation and growth in the crystallization behavior of amorphous alloys is specified, allowing an access to the ideal metallic glass free of nuclei. Local atomic configurations are fundamentally significant to unravel the glass forming ability (GFA) and phase transitions in metallic glasses. For this reason, isothermal annealing near T_g from 0.001 s to 25,000 s following quenching becomes the strategy to tune local atomic configurations and facilitate an amorphous alloy, a mixed glassy-nanocrystalline state, and a crystalline sample successively. On the basis of the evolution of crystallization enthalpy and overall latent heat on reheating, we quantify the underlying mechanism for the isothermal nucleation and crystallization of amorphous alloys. With Johnson-Mehl-Avrami method, it is demonstrated that the coexistence of homogeneous and heterogeneous nucleation contributes to the isothermal crystallization of glass. Heterogeneous rather than homogeneous nucleation dominates the isothermal crystallization of the undercooled liquid. For the mixed glassy-nanocrystalline structure, an extraordinary kinetic stability of the residual glass is validated, which is ascribed to the denser packed interface between amorphous phase and ordered nanocrystals. Tailoring the amorphous structure by nanocalorimetry permits new insights into unraveling GFA and the mechanism that correlates local atomic configurations and phase transitions in metallic glasses.

Homogeneous crystal nucleation in polymers

The pathway of crystal nucleation significantly influences the structure and properties of semi-crystalline polymers. Crystal nucleation is normally heterogeneous at low supercooling, and homogeneous at high supercooling, of the polymer melt. Homogeneous nucleation in bulk polymers has been, so far, hardly accessible experimentally, and was even doubted to occur at all. This topical review summarizes experimental findings on homogeneous crystal nucleation in polymers. Recently developed fast scanning calorimetry, with cooling and heating rates up to 10⁶ K s^-1, allows for detailed investigations of nucleation near and even below the glass transition temperature, including analysis of nuclei stability. As for other materials, the maximum homogeneous nucleation rate for polymers is located close to the glass transition temperature. In the experiments discussed here, it is shown that polymer nucleation is homogeneous at such temperatures. Homogeneous nucleation in polymers is discussed in the framework of the classical nucleation theory. The majority of our observations are consistent with the theory. The discrepancies may guide further research, particularly experiments to progress theoretical development. Progress in the understanding of homogeneous nucleation is much needed, since most of the modelling approaches dealing with polymer crystallization exclusively consider homogeneous nucleation. This is also the basis for advancing theoretical approaches to the much more complex phenomena governing heterogeneous nucleation.

Silk I and Silk II studied by fast scanning calorimetry

Using fast scanning calorimetry (FSC), we investigated the glass transition and crystal melting of samples of B. mori silk fibroin containing Silk I and/or Silk II crystals. Due to the very short residence times at high temperatures during such measurements, thermal decomposition of silk protein can be significantly suppressed. FSC was performed at 2000K/s using the Mettler Flash DSC1 on fibroin films with masses around 130-270ng. Films were prepared with different crystalline fractions (ranging from 0.26 to 0.50) and with different crystal structures (Silk I, Silk II, or mixed) by varying the processing conditions. These included water annealing at different temperatures, exposure to 50%MeOH in water, or autoclaving. The resulting crystal structure was examined using wide angle X-ray scattering. Degree of crystallinity was evaluated from Fourier transform infrared (FTIR) spectroscopy and from analysis of the heat capacity increment at the glass transition temperature. Silk fibroin films prepared by water annealing at 25°C were the least crystalline and had Silk I structure. FTIR and FSC studies showed that films prepared by autoclaving or 50%MeOH exposure were the most crystalline and had Silk II structure. Intermediate crystalline fraction and mixed Silk I/Silk II structures were found in films prepared by water annealing at 37°C. FSC results indicate that Silk II crystals exhibit endotherms of narrower width and have higher mean melting temperature T_m(II)=351±2.6°C, compared to Silk I crystals which melt at T_m(I)=292±3.8°C. Films containing mixed Silk I/Silk II structure showed two clearly separated endothermic peaks. Evidence suggests that the two types of crystals melt separately and do not thermally interconvert on the extremely short time scale (0.065s between onset and end of melting) of the FSC experiment.

STATEMENT OF SIGNIFICANCE

Silkworm silk is a naturally occurring biomaterial. The fibroin component of silk forms two types of crystals. Silk properties depend upon the amount and type of crystals, and their stability. One measure of stability is crystal melting temperature. Crystals which are more stable have a higher melting temperature. Until now, it has been challenging to study thermal behavior of silk crystals because they degrade at high temperature. To avoid degradation, and study the melting properties of silk biomaterial, we heated silk at a very fast rate of 2000K/s using a special calorimeter. We have shown that the two crystal types have very different melting temperatures, indicating that one crystal type is much more stable than the other.

Characterization of human cancellous and subchondral bone with respect to electro physical properties and bone mineral density by means of impedance spectroscopy

Computational simulation of electrical bone stimulation of the electrical and dielectric parameters of osteoarthritic bone tissue is useful for an exact patient-individual adaptation of the bone models. Therefore, we investigated electrical and dielectric parameters at a frequency of 20Hz of cancellous and subchondral human femoral head bone samples. Furthermore, the mechanical properties and the bone mineral density (BMD) were determined. Finally, these data were compared with the electrical and dielectric parameters. The bone samples were taken from patients with hip osteoarthritis. Electrical conductivity and dielectric permittivity of cancellous bone amounted to 0.043S/m and 8.1⋅10⁶. BMD of the bone samples determined by dual-x-ray-absorptiometry (DXA) and ashing resulted in 193 ± 70mg/cm² and 286 ± 59mg/cm³ respectively. Structural modulus (E_S) and ultimate compression strength (σ_max) were measured with 227 ± 94N/mm² and 6.5 ± 3.4N/mm². No linear correlation of the electrical and dielectric parameters compared with BMD and mechanical properties of cancellous bone samples was found. Electrical conductivity and dielectric permittivity of subchondral bone resulted in 0.029S/m and 8.97×10⁶.

Temperature fluctuations and the thermodynamic determination of the cooperativity length in glass forming liquids

The aim of this paper is to decide which of the two possible thermodynamic expressions for the cooperativity length in glass forming liquids is the correct one. In the derivation of these two expressions, the occurrence of temperature fluctuations in the considered nanoscale subsystems is either included or neglected. Consequently, our analysis gives also an answer to the widely discussed problem whether temperature fluctuations have to be generally accounted for in thermodynamics or not. To this end, the characteristic length-scales at equal times and temperatures for propylene glycol were determined independently from AC calorimetry in both the above specified ways and from quasielastic neutron scattering (QENS), and compared. The result shows that the cooperative length determined from QENS coincides most consistently with the cooperativity length determined from AC calorimetry measurements for the case that the effect of temperature fluctuations is incorporated in the description. This conclusion indicates that-accounting for temperature fluctuations-the characteristic length can be derived by thermodynamic considerations from the specific parameters of the liquid at glass transition and that temperature does fluctuate in small systems.

Experimental Test of Tammann's Nuclei Development Approach in Crystallization of Macromolecules

Prediction of the supermolecular structure and with that of properties of crystallizable polymers requires in-depth knowledge about the crystallization behavior, in particular the temperature-dependence of the nucleation kinetics. Typically, at low supercooling of the melt the nucleation rate/nuclei density often is assessed by optical microscopy, through an analysis of the evolution of the spherulitic superstructure. This approach fails if the nuclei density is too high, or if nuclei formation is not followed by growth due to chain-mobility constraints. In such cases, Tammann's two-stage crystal nuclei development method can be applied. It includes the formation of crystal nuclei at high supercooling of the melt, and their detection at higher temperature, after their growth to crystals. Though initially developed for analysis of low molecular mass compounds, this approach has recently also successfully been employed for analysis of the nucleation behavior of polymers, which is demonstrated here on the examples of poly (L-lactic acid) (PLLA), and poly (∊-caprolactone) (PCL). While in case of PLLA the ability to gain information about isothermal and non-isothermal nucleation is explained, in case of PCL new information about the thermal stability of nuclei is presented. The importance of such analyses in the context of understanding structure formation of polymers at processing-relevant cooling conditions is discussed.

Lactoferrin-Immobilized Surfaces onto Functionalized PLA Assisted by the Gamma-Rays and Nitrogen Plasma to Create Materials with Multifunctional Properties

Both cold nitrogen radiofrequency plasma and gamma irradiation have been applied to activate and functionalize the polylactic acid (PLA) surface and the subsequent lactoferrin immobilization. Modified films were comparatively characterized with respect to the procedure of activation and also with unmodified sample by water contact angle measurements, mass loss, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), atomic force microscopy (AFM), and chemiluminescence measurements. All modified samples exhibit enhanced surface properties mainly those concerning biocompatibility, antimicrobial, and antioxidant properties, and furthermore, they are biodegradable and environmentally friendly. Lactoferrin deposited layer by covalent coupling using carbodiimide chemistry showed a good stability. It was found that the lactoferrin-modified PLA materials present significantly increased oxidative stability. Gamma-irradiated samples and lactoferrin-functionalized samples show higher antioxidant, antimicrobial, and cell proliferation activity than plasma-activated and lactoferrin-functionalized ones. The multifunctional materials thus obtained could find application as biomaterials or as bioactive packaging films.

Glass transition and stable glass formation of tetrachloromethane

Physical vapor deposition (PVD) has been used to prepare organic glasses with very high kinetic stability and it has been suggested that molecular anisotropy is a prerequisite for stable glass formation. Here we use PVD to prepare glasses of tetrachloromethane, a simple organic molecule with a nearly isotropic molecular structure. In situ AC nanocalorimetry was used to characterize the vapor-deposited glasses. Glasses of high kinetic stability were produced by deposition near 0.8 Tg. The isothermal transformation of the vapor-deposited glasses into the supercooled liquid state gave further evidence that tetrachloromethane forms glasses with high kinetic stability, with the transformation time exceeding the structural relaxation time of the supercooled liquid by a factor of 10(3). The glass transition temperature of liquid-cooled tetrachloromethane is determined as Tg ≈ 78 K, which is different from previously reported values. The frequency dependence of the glass transition was also determined and the fragility was estimated as m ≈ 118. The successful formation of PVD glasses of tetrachloromethane which have high kinetic stability argues that molecular asymmetry is not a prerequisite for stable glass formation.

Crystallization of Polyethylene at Large Undercooling

Extremely fast crystallization of high-density polyethylene and random copolymers of ethylene with up to 16 mol % 1-octene was observed for the first time by ultrafast scanning calorimetry. In order to account for the inherently high crystallization rate of polyethylenes, in nonisothermal and isothermal crystallization experiments cooling rates up to 1 000 000 K/s and crystallization times as short as 10 μs, respectively, were employed. It was possible to supercool the melt of high-density polyethylene down to 57 °C and the melt of a random ethylene/1-octene copolymer with 16 mol % 1-octene down to -33 °C, without prior crystallization. At these temperatures, the characteristic time of the primary crystallization process is of the order of magnitude of 100 μs. Complete vitrification of the liquid would require cooling even faster than 1 000 000 K/s. Compared to the homopolymer, the cooling-rate dependence of the crystallization temperatures and the temperature dependence of the characteristic time of primary crystallization of random ethylene/1-octene copolymers both are nearly parallel shifted to lower temperatures. Fast crystallization under conditions of reduced linear crystal growth rate is possibly caused by boosting homogeneous nuclei density up to 10²⁷ m^-3 and urgently requires further investigation.

Stability studies of ionic liquid [EMIm][NTf2] under short-term thermal exposure

Mass spectrometer coupled TG analysis was used to identify decomposition products of thermally highly stressed ionic liquids.

Vapor-deposited glasses of methyl-m-toluate: How uniform is stable glass transformation?

AC chip nanocalorimetry is used to characterize vapor-deposited glasses of methyl-m-toluate (MMT). Physical vapor deposition can prepare MMT glasses that have lower heat capacity and significantly higher kinetic stability compared to liquid-cooled glasses. When heated, highly stable MMT glasses transform into the supercooled liquid via propagating fronts. We present the first quantitative analysis of the temporal and spatial uniformities of these transformation fronts. The front velocity varies by less than 4% over the duration of the transformation. For films 280 nm thick, the transformation rates at different spatial positions in the film differ by about 25%; this quantity may be related to spatially heterogeneous dynamics in the stable glass. Our characterization of the kinetic stability of MMT stable glasses extends previous dielectric experiments and is in excellent agreement with these results.