The kinetics of mutarotation in L-fucose as monitored by dielectric and infrared spectroscopy

Studying the Intermolecular Interactions, Structural Dynamics, and Non-Equilibrium Kinetics of Cilnidipine Infiltrated into Alumina and Silica Pores

In the present study, the behavior of the calcium channel blocker cilnidipine (CLN) infiltrated into silica (SiO₂) and anodic aluminum oxide (AAO) porous membranes characterized by a similar pore size (d = 8 nm and d = 10 nm, respectively) as well as the bulk sample has been investigated using differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and Fourier-transform infrared spectroscopy (FTIR) techniques. The obtained data suggested the existence of two sets of CLN molecules in both confined systems (core and interfacial). They also revealed the lack of substantial differences in inter- and intramolecular dynamics of nanospatially restricted samples independently of the applied porous membranes. Moreover, the annealing experiments (isothermal time-dependent measurements) performed on the confined CLN clearly indicated that the whole equilibration process under confinement is governed by structural relaxation. It was also found that the β_anneal parameters obtained from BDS and FTIR data upon equilibration of both confined samples are comparable (within 10%) to each other, while the equilibration constants are significantly different. This finding strongly emphasizes that there is a close connection between the inter- and intramolecular dynamics under nanospatial restriction.

Impact of Amorphization Methods on the Physicochemical Properties of Amorphous Lactulose

The influence of the amorphization technique on the physicochemical properties of amorphous lactulose was investigated. Four different amorphization techniques were used: quenching of the melt, milling, spray-drying, and freeze-drying, and amorphous samples were analyzed by differential scanning calorimetry, NMR spectroscopy, and powder X-ray diffraction analysis. Special attention was paid to the tautomeric composition and to the glass transition of amorphized materials. It was found that the tautomeric composition of the starting physical state (crystal, liquid, or solution) is preserved during the amorphization process and has a strong repercussion on the glass transition of the material. The correlation between these two properties as well as the plasticizing effect of the different tautomers was clarified by molecular dynamics simulations.

Dramatic slowing down of the conformational equilibrium in the silyl derivative of glucose in the vicinity of the glass transition temperature

The vitrification process is usually preceded by a significant change (around 6-8 decades) in the viscosity, structural relaxation times, or diffusion that occurs in a relatively small range of temperatures in fragile liquids. Along with this phenomenon, conformations of the molecules vary as well. In fact, this process is studied in bulk polymers and high molecular weight materials deposited in the form of thin films. On the other hand, spatial rearrangement of small glass formers in the supercooled liquid state has not been intensively investigated, so far. Herein, data obtained from measurements carried out using various experimental techniques on supercooled 1,2,3,4,6-penta-O-(trimethylsilyl)-d-glucopyranose (S-GLU) have revealed that rotations of silyl moieties along with the deformation in the saccharide ring are significantly slowed down in the vicinity of the glass transition temperature (Tg). These intramolecular reorganizations affect the structural relaxation time, atomic pair distribution function, integrated intensity, as well as a number of bands and signals observed, respectively, in the Raman and NMR spectra. Data reported herein offer a better understanding of the conformational variation and time scale of this process in the complex and flexible molecules around the Tg.

Studies on dynamics and isomerism in supercooled photochromic compound Aberchrome 670 with the use of different experimental techniques

Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and Broadband Dielectric (BD) spectroscopies were applied to investigate the thermal, structural, photochemical and dynamical properties of a fulgide-type photochromic compound, Aberchrome 670 (Ab670). In the original crystals, characterized by a pale yellow color, molecules take the E conformation. However, upon UV irradiation of either the crystalline or glassy compound, it isomerizes to the closed (C) form, characterized by the intense red tone. Although, we have found that such conversion is not complete (far below 100%). It was shown that due to UV irradiation as well as heating of the studied fulgide to high temperature (above the melting point), the Z isomer is formed. Further FTIR measurements performed on the UV irradiated and molten compound indicated that upon annealing of the sample in the vicinity of the glass transition temperature the Z isomer reverts back to the original E form. The final confirmation of this supposition has come from BDS studies, where the strong shift of the structural relaxation process during time-dependent isothermal measurements was noticed. One can add that a similar pattern of behavior has been observed previously by some of us in the case of tautomerism or mutarotation [Z. Wojnarowska et al., J. Chem. Phys., 2010, 133, 094507; W. Kossack et al., J. Chem. Phys., 2014, 140, 215101; P. Wlodarczyk et al., J. Phys. Chem. B, 2009, 113, 4379-4383; P. Wlodarczyk et al., J. Non-Cryst. Solids, 2010, 356, 738-742]. From the analysis of the time variation of the structural relaxation times, the activation barrier, E_A = 18 kJ mol^-1, for Z to E isomerization in Ab670 was calculated. Interestingly, it agrees well with the one determined for a similar kind of transformation in stilbenes. Therefore, we found that dielectric spectroscopy can be a very useful technique to track Z to E interconversion in the highly viscous supercooled state. Consequently, a unique opportunity to follow this kind of isomerism at high pressures, high electric fields and under nanometric spatial confinement in pure supercooled compounds appeared.

Unexpected Crossover in the kinetics of mutarotation in the supercooled region: the role of H-bonds

Intra- and intermolecular studies on the molten L-sorbose have been carried out at variable temperature conditions to determine the crosover temperature (T_c). In addition, isothermal time-dependent FTIR and Raman measurements were performed to probe the pace of mutarotation and activation energy of this reaction in the studied saccharide, which varied from 53–62 kJ/mol up to 177–192 kJ/mol below and above T_c, respectively. To explain the change in activation barrier for the mutarotation a complementary analysis using difference FTIR spectra collected around T_c = 365 K in the hydroxyl region has been done. It was found that the alteration of kinetic parameters and molecular dynamics around T_c are strictly related to the variation in the strength of H-bonds which above T_c are significantly weaken, increasing the freedom of rotation of functional groups and movement of individual molecules. That phenomenon most likely affects the proton transfer, underlying molecular mechanism of mutarotation, which may lead to the significant increase in activation barrier. The new insight into a molecular aspect of the mutarotation around T_c has created an opportunity to better understanding the relationship between physics of condensed matter and the potential role of H-bonds dynamics on the progress of the chemical reaction in highly viscous systems.

A study on the progress of mutarotation above and below the Tg and the relationship between constant rates and structural relaxation times

Comprehensive FTIR studies on the progress of mutarotation in d-fructose mixed with maltitol have been carried out over a wide range of temperatures, both above and below the glass transition temperature T_g. In addition to the analysis of single bands, we have developed a completely new approach considering the full spectral range to follow the overall progress of the reaction. We have found that at the calorimetric T_g, there is a clear change in the temperature dependence of constant rates. The activation barrier for mutarotation changes from around 59 kJ mol^-1 (the supercooled state) to around 249 kJ mol^-1 (the glassy state). This dramatic variation in the activation barrier is consistent with the change in the mechanism of this specific chemical conversion, as theoretically considered by Wlodarczyk et al. [Phys. Chem. Chem. Phys., 2014, 16, 4694-4698]. Alternatively, it can also be connected to the change in the viscosity of the sample. Additionally, we investigated the relationship between constant rates (k) of mutarotation, structural relaxation times (τ_α), and dc conductivity (σ_dc) above and below the glass transition temperature. It was found that there was a linear correlation between all these quantities; they scale with various exponents changing at T_g. Our results also indicate that a single activation barrier might not be sufficient to describe the mutarotation process.

Anomerization reaction of bare and microhydrated d-erythrose via explicitly correlated coupled cluster approach. Two water molecules are optimal

We present a comprehensive benchmark computational study which has explored a complete path of the anomerization reaction of bare d-erythrose involving a pair of the low-energy α- and β-furanose anomers, the former of which was observed spectroscopically (Cabezas et al., Chem. Commun. 2013, 49, 10826). We find that the ring opening of the α-anomer yields the most stable open-chain tautomer which step is followed by the rotational interconversion of the open-chain rotamers and final ring closing to form the β-anomer. Our results indicate the flatness of the reaction's potential energy surface (PES) corresponding to the rotational interconversion path and its sensitivity to the computational level. By using the explicitly correlated coupled cluster CCSD(T)-F12/cc-pVTZ-F12 energies, we determine the free energy barrier for the α-furanose ring-opening (rate-determining) step as 170.3 kJ/mol. The question of the number of water molecules (n) needed for optimal stabilization of the erythrose anomerization reaction rate-determining transition state is addressed by a systematic exploration of the PES of the ring opening in the α-anomer-(H₂ O)_n and various β-anomer-(H₂ O)_n (n = 1-3) clusters using density functional and CCSD(T)-F12 computations. These computations suggest the lowest free energy barrier of the ring opening for doubly hydrated α-anomer, achieved by a mechanism that involves water-mediated multiple proton transfer coupled with the furanose CO bond breakage. Among the methods used, the G4 performed best against the CCSD(T)-F12 reference at estimating the ring-opening barrier heights for both the hydrated and bare erythrose conformers. Our results for the hydrated species are most relevant to an experimental study of the anomerization reaction of d-erythrose to be carried out in microsolvation environment. © 2016 Wiley Periodicals, Inc.

Compressed-tube pressure cell for optical studies at ocean pressures: Application to glucose mutarotation kinetics

A self-contained compressed-tube pressure cell is tested to 25 MPa. The cell is very simple to construct and offers stable pressure control with optical access to fluid samples. The physical path length of light through the cell is large enough to measure optical activity. The entire system is relatively small and portable, and it is vibration-free, since a compressor is not used. Operation of the cell is demonstrated by measuring the mutarotation rate of aqueous glucose solutions at 25 °C. A logarithmic plot of the rate constant vs. pressure yields an activation volume for mutarotation of -22 cm³/mol, approximately twice the value measured previously at higher pressures.

Experimental (FTIR, BDS) and theoretical analysis of mutarotation kinetics of d-fructose mixed with different alcohols in the supercooled region

The mutarotation kinetics of pure molten d-fructose and its binary mixture with alcohols (i.e., sorbitol and maltitol) have been reported using Fourier transform infrared, broadband dielectric spectroscopy and density functional theory calculations.

Evidence of pressure induced intermolecular proton transfer via mutarotation: the case of supercooled d-fructose

This paper describes a systematic investigation on the role of pressure in mutarotation kinetics of supercooled d-fructose using dielectric spectroscopy. The structural relaxation time acts as a suitable dynamical observable to monitor the mutarotation process that enables the construction of the kinetic curves. The reaction kinetic shapes have been analyzed using the Avrami model. At low temperature, sigmoidal kinetic curves are noted, which correspond to the high concentration of furanosidic forms. The magnitude of activation energy of the process significantly decreases with increasing pressure and is comparable to the solvated systems at 100 MPa. A potential connection between cooperative motion and the origin of intermolecular proton transfer via mutarotation at elevated pressure is also discussed. These experimental observations have fundamental significance on theoretical explanation of the mechanism involving mutarotation in sugars.