Solid State Mutarotation of Glucose - The Journal of Physical

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Solid State Mutarotation of Glucose N. Dujardin,† E. Dudognon,* J.-F. Willart, A. Hedoux, Y. Guinet, L. Paccou, and M. Descamps Universite Lille Nord de France, F-59000 Lille, France USTL, UMET (Unite Materiaux et Transformations), UMR CNRS 8207 F-59650 Villeneuve d’Ascq, France ABSTRACT: It has been recently shown that mechanical milling can amorphize without any mutarotation, giving rise to an anomerically pure amorphous sample. We have taken advantage of this exceptional possibility to study the kinetic of mutarotation in the amorphous solid state. The investigations have been performed in situ by time-resolved Raman spectroscopy. The results reveal an unexpected coupling between the mutarotation process and the structural relaxations involved in the glassy state. D-glucose

1. INTRODUCTION Mutarotation is a striking features of many sugars.1-3 It consists of the conversion of one anomeric form of a sugar to the other one. The process by which anomers can be interconverted is not yet clearly understood. Among the various suggested scenarios for the mutarotation process,4-6 the most commonly accepted requires the formation of a free aldehyde form separated from the R and β ring forms by two transition states according to the energy profile shown in Figure 1.4 For instance, glucose molecules can be found in two anomeric forms (R and β) which differ by the orientation of the hydroxyl group at the anomeric carbon (C1) (see Figure 1). Up to now, this phenomenon has been studied mainly in solution. Since the first observation of mutarotation phenomenon,7,8 several experimental studies have been performed using a rich variety of solvents: aqueous media,9-12 benzene,13-15 2-pyridone,14 benzoic acid,14 formamidine,16 benzamidine,16 sometimes a mixed solution of benzene/methanol,17 dimethylsulfoxide/water,18 and phenol/pyridine.14 However, very few investigations have been performed in pure liquid state,19 and even fewer in the crystalline20 and amorphous solid states. The lack of investigation in the solid state is due to the fact that anomerically pure amorphous samples cannot be formed by the usual amorphization routes, such as thermal quench of the liquid state, spray-drying, and lyophilization, which unavoidably provokes a strong mutarotation effect. The question thus arises whether mutarotation can also take place in the solid state. Very few investigations report such an observation and discuss this fundamental question.21,22 It has been recently shown that the amorphization of glucose by milling leads to the formation of anomerically pure amorphous compounds.23,24 We took advantage of this exceptional opportunity to study the mutarotation directly in the solid state. We report here direct experimental evidence of the mutarotation of glucose by means of Raman scattering measurements. In the Results section, we will examine the influence of the physical state r 2011 American Chemical Society

on the mutarotation propensity, and then, we will study the mutarotation of glucose by an original way in the amorphous equimassic alloy of β-glucose/trehalose. In the Discussion part, the determination of the kinetics of the mutarotation process and its evolution with temperature will enlighten the relation between the chemical and the physical stability.

2. MATERIALS AND METHODS Crystalline anhydrous R-D-glucose (Gr) (purity g99.5%) and crystalline anhydrous β-D-glucose (Gβ) (standard reagent grade) were purchased from Fluka and Promochem, respectively. Pure anomerically amorphous R-glucose and β-glucose were obtained by ball-milling the respective initial crystalline state for 14 h at -15 °C, as described in ref 24. Crystalline R-R anhydrous trehalose (purity g99%) was purchased from Promochem. Amorphous equimassic β-glucose/ trehalose mixtures are obtained by comilling the initial crystalline powders at -15 °C for 14 h. All the compounds were used without further purification. The Raman spectra of the different physical states of glucose covering the medium frequency region from 300 to 1500 cm-1 were recorded with an XY Dilor spectrometer equipped with a N2-cooled CCD system using a mixed argon-krypton coherent laser operating at 514.5 nm. Samples (milled powders) were introduced in a spherical Pyrex cell (Ø = 3 mm). To experiment between 5 and 160 °C, a regulated Oxford nitrogen flux device that keeps temperature fluctuations within 0.1 °C was used. The spectral deconvolution of the Raman spectra was achieved by means of Gaussian approximation of the actual Raman line shape, and integration of the best model refinements was realized by means of the PeakFit v4.12 software. The fraction of R-glucose anomer was identified to the ratio IR/(IR þ Iβ) of the integrated Received: September 30, 2010 Revised: December 22, 2010 Published: January 27, 2011 1698

dx.doi.org/10.1021/jp109382j | J. Phys. Chem. B 2011, 115, 1698–1705

The Journal of Physical Chemistry B

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Figure 1. Energy pathway for the interconversion between the R and β forms of glucose via an intermediate free aldehyde form (γ). EA represents the energy barrier separating the transient state from the R-form. Molecule drawings are adapted from ref 33.

intensities IR,β of the same modes of vibration. The absolute accuracy of this fraction is about 4%. The differential scanning calorimetry (DSC) experiments were performed with the DSC 2920 calorimeter of TA Instruments. All the experiments have been performed with a heating rate of 5 °C/min. For all the experiments, the sample was placed in an open aluminum pan (container with no cover) and was flushed with highly pure nitrogen gas. Temperature and enthalpy readings were calibrated using pure Indium at the same scan rates and with the same kind of pans used in the experiments. The glass transition temperature (Tg) was taken as the midpoint between the onset and end temperatures of the Cp-jump.

3. RESULTS (a). Raman Spectroscopy: A Well-Suited Tool for Qualitative and Quantitative Investigations of Mutarotation. Fig-

ure 2 shows the Raman spectra of crystalline β-glucose (Gβ) and crystalline R-glucose (GR) recorded at room temperature (RT) in the 300-1500 cm-1 wavenumber range. These two spectra are clearly different: (i) In the 730-850 cm-1 wavenumber range, the Raman spectrum of crystalline GR shows two characteristic bands appearing at approximately 769 and 838 cm-1, whereas crystalline Gβ yields almost no Raman optical activity (only one specific mode at 896 cm-1). Consequently, this region, called the anomeric region,25 easily allows the identification of both glucose anomers and makes the Raman diffusion a precious tool to discriminate readily the two anomers directly in the solid state. However the anomeric region is not convenient for a quantitative study because these vibrational modes are not yet well assigned. (ii) To quantify the mutarotation phenomenon, we will focus on vibrational modes that are rightly assigned. This is the case of the C2-C1-O1 bending mode (δC2-C1-O1) and the C-O stretching mode (νC-O). The bending mode is located at 522.7 cm-1 for β-glucose and 542.9 cm-1 for R-glucose.26 The stretching mode is located at 1021.6 and 1051.2 cm-1, respectively, for R- and β-glucose.26 These vibrational modes of glucose are well-identified and

Figure 2. Raman spectra of crystalline glucose anomers GR and Gβ recorded at room temperature. Some specific frequencies of each anomer are reported on the corresponding spectrum. Frequencies at 522.7 cm-1 for Gβ and 542.9 cm-1 for GR correspond to the C2C1-O1 bending mode (δC2-C1-O1), and frequencies at 1021.6 and 1051.2 cm-1 are assigned to the C-O stretching mode (νC-O) for R and β-glucose, respectively.

assigned to the same molecular motions so that the fraction of R-glucose anomer can be identified to the 1699

dx.doi.org/10.1021/jp109382j |J. Phys. Chem. B 2011, 115, 1698–1705

The Journal of Physical Chemistry B ratio of the integrated intensities IR,β of the same modes of vibration.26 We have validated this quantification method on the quench liquid for which the anomeric concentration is well-known (44% R and 56% β27). Raman scattering allows one to easily distinguish and quantify glucose anomers. Moreover, Raman spectroscopy is a relatively convenient, rapid, and nondestructive method. Thus, it is welladapted to study the evolution of mutarotation in glucose. (b). Study of Mutarotation upon Heating Anomerically Pure Glucose. We have recently shown that milling glucose leads to anomerically pure amorphous samples.24 These original amorphous states cannot be achieved by the usual amorphization techniques, such as thermal quench of the liquid, lyophilization, or spray-drying because a strong mutarotation cannot be avoided upon melting or during solubilization. The possibility to obtain anomerically pure glucose thus constitutes an exceptional opportunity to study the mutarotation in the amorphous solid state. Figure 3a shows the Raman spectra of amorphous Gβ recorded from 5 to 160 °C in the anomeric region (730-950 cm-1). (i) From 5 to 55 °C, the spectrum of pure amorphous β-glucose reveals only the vibrational modes of β-glucose around 900 cm-1. These modes are broader than those observed in the crystalline state (Figure 2), as expected for an amorphous sample. On the DSC scan of the sample recorded at 5 °C/min (Figure 3b), the amorphous character is confirmed by the Cp-jump characteristic of the glass transition around 38 °C. (ii) Upon heating to 65 °C, no change can be detected in the Raman spectrum, except a strong sharpening of all vibration modes. This indicates on one side the recrystallization of the compounds (confirmed by the exothermic peak on the DSC scan in Figure 3b) and on the other side the absence of a mutarotation process. This result differs from that observed in lactose,22 for which a strong mutarotation accompanies the recrystallization process, giving rise to a cocrystal made of the R and β anomers.28 The absence of mutarotation in glucose during the crystallization process of the metastable liquid shows that the mutarotation is not a phenomenon triggered by the crystallization. (iii) From 65 to 150 °C, the Raman spectrum of crystalline Gβ reveals only the previous characteristic modes of crystalline β-glucose, while no sign of the R-anomer is detected. However, these bands slightly broaden with increasing temperature as a result of the increasing thermal agitation. (iv) At 160 °C, the melting of crystalline Gβ occurs, as indicated by the endotherm in the DSC scan. Thus, the spectrum recorded at 160 °C corresponds to the melted glucose. This Raman spectrum still shows the characteristic bands of β-glucose, but it also reveals the specific modes of the R-anomer (around 770 and 840 cm-1), indicating that the mutarotation process occurred. These features persist in the spectrum recorded after return to ambient temperature. The chemical behavior of crystalline GR upon thermal perturbation is found to be similar to that of crystalline Gβ; that is, only the specific vibrational bands of the R-anomer are present as long as the sample is crystalline. Once the melting occurs, the corresponding spectrum reveals the characteristic bands of both glucose anomers. The occurrence of the mutarotation process upon melting is thus established.

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Figure 3. (a) Raman spectra of anomerically pure amorphous β-glucose recorded upon heating from 5 °C to the melting temperature. From 55 to 65 °C, the sample recrystallizes (thinner vibrational bands) without any trace of mutarotation (no R-glucose mode). After melt-quenching, broader vibrational bands at room temperature confirmed the amorphous character of the sample, and R-glucose modes (around 770 and 840 cm-1) show that mutarotation has occurred. (b) Corresponding DSC scan recorded upon heating at 5 °C/min. The insert shows a closeup view of the glass transition.

The above results indicate that upon heating β-glucose in the solid state, no mutarotation is observed whether in the amorphous state form up to the crystallization (Tcr = 65 °C) or in the crystalline form up to the melting point (Tm = 160 °C). However, a strong mutarotation readily occurs at 160 °C when glucose melts. This last result can be interpreted two ways: either the mutarotation is independent of the physical state and appears systematically at 160 °C or the mutarotation is dependent on the physical state change, and a strong mutarotation occurs as the sample melts. In this latter case, it would also suggest that the mutarotation strongly depends on the molecular mobility, since the mutarotation is not observed in the glassy amorphous state. c). Study of Mutarotation in β-Glucose/Trehalose Equimassic Alloy. Since the recrystallization of milled glucose occurs systematically a few degrees above Tg, the mutarotation in the metastable liquid state can be investigated in only a very small range of temperature. To extend this investigation range, we have comilled glucose with a high Tg excipient. Such a processing is expected to produce an amorphous molecular alloy whose glass transition temperature is located between those of the pure compounds. Here, we have used trehalose for several reasons: (i) It is a disacharride that is basically made up of two glucose units (ii) It has a high glass transition temperature (Tg = 120 °C29) (iii) It is known to amorphize upon milling26 1700



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Figure 4. DSC scans recorded upon heating at 5 °C/min: (a) milled β-glucose, (b) milled equimassic mixture of β-glucose/trehalose, and (c) milled trehalose. All compounds are fully amorphized by milling. The inset highlights the glass transition of the binary mixture of β-glucose/trehalose.

(iv) It does not show any mutarotation that could interfere with that of glucose Figure 4 shows the heating DSC scans of pure milled β-glucose (a), pure milled trehalose (c), and an equimassic mixture of comilled β-glucose and trehalose (b), all recorded upon heating at 5 °C/min after a 14 h milling process at -15 °C. As already reported, pure glucose and pure trehalose show a glass transition at 38 and 120 °C, respectively, similar to that of the quenched liquid and a strong recrystallization.24,29 This clearly confirms that upon milling, both compounds undergo a direct transformation from crystal to glass. The binary mixture glucose/trehalose also shows a glass transition (Tg = 62 °C) located between those of the pure compounds indicating that the comilled mixture is characterized by a single main relaxation process. This clearly proves that the mixing of the two chemical species has been performed at the molecular level, giving rise to a real amorphous molecular alloy. Moreover, this alloy does not show any sign of recrystallization upon heating at 5 °C/min. This behavior is strongly different from that of the pure milled compounds for which the recrystallization cannot be avoided for similar heating conditions. The absence of recrystallization in the alloy is essential, since it gives the exceptional opportunity to investigate the mutarotation in a wide range of temperature from below Tg up to the melting point. Figure 5 shows the Raman spectra of the amorphous equimassic alloy of β-glucose/trehalose recorded in the 475-560 cm-1 wavenumber range for increasing temperatures. Since the amorphization by milling does not induce any mutarotation,24,30 no R-glucose molecule results from the milling process. However, the Raman spectrum of comilled β-glucose/trehalose shows a contribution at 542 cm-1 due to the C2-C1-O1 bending mode of R-glucose molecules. This is simply due to the fact that trehalose molecules are composed of two units of Rglucose and, consequently, present some similar vibrational modes with those of R-glucose. As a result, the intensity ratio I542/(I542 þ I522), giving the concentration of R-glucose anomers

Figure 5. Temperature-dependent Raman spectra of amorphous equimassic alloy of β-glucose/trehalose in the 475-560 cm-1 wavenumber range. The circles are experimental results and the solid lines are fit of the data to simple Gaussian functions.

resulting from the mutarotation process, is fixed equal to zero at 23 °C. At this temperature, the intensity of the mode at 542 cm-1 is clearly less important than that of the β-anomer at 522 cm-1. Upon heating, the intensity at 522 cm-1 decreases while the one at 542 cm-1 increases. It reveals the progressive apparition of R-glucose at the expense of β-glucose. This behavior clearly reflects the conversion of β-glucose to R-glucose (i.e., the mutarotation process) right from the first increases of temperature. To our knowledge, this constitutes the first direct in situ observation of this phenomenon for glucose molecules. Thus, the mutarotation occurs below 160 °C in the amorphous state. Since there is no mutarotation in the crystalline form, this result 1701



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Figure 6. Evolution with temperature of the out-of-equilibrium R-glucose fraction in the β-glucose/trehalose amorphous mixture, as measured by Raman spectroscopy from bands ratio of the C2-C1-O1 bending mode (O) and the C-O stretching mode (b). After reaching 140 °C, samples are quenched down to room temperature. The solid line is a guide for the eye.

shows that the trigger of the mutarotation is dependent on the physical state. In the crystalline state, the hydrogen atoms of both glucose anomeric forms implied in the mutarotation process are involved in hydrogen bonds.31,32 Thus, the network cohesion ensured by hydrogen bonding is likely to prevent the mutarotation from occurring in the crystalline state. On the contrary, in the glassy state, hydrogen bonds are known to be weaker and could give the possibility of the mutarotation to occur. Figure 6 compiles the evolution of the R-glucose fraction of glucose molecules as the temperature increases in the amorphous equimassic alloy of β-glucose/trehalose. It has been deduced from the integrated intensities ratio (I542/(I542 þ I522), I1021/ (I1021 þ I1051)) of the two previously mentioned vibrational modes (the C2-C1-O1 bending mode (δC2-C1-O1) shown in Figure 5, and the C-O stretching mode (νC-O), not shown here). The increase in the R-glucose fraction upon heating (equivalent heating rate ∼ 1 °C/min) clearly shows that mutarotation occurs. The measured R-fraction is found to increase from RT to 110 °C and reaches an equilibrium value close to 50%. The anomeric concentration then remains unchanged upon further heating or upon cooling back to RT. Since the equilibrium concentration is close to 50%, this strongly suggests that the low anomeric concentrations measured at lower temperatures are out of equilibrium on the time scale of the Raman experiment. To assess this out-of-equilibrium character, the time evolution of the anomeric concentration was monitored during several isothermal annealings in the temperature range 40-110 °C. Seven annealing temperatures between 40 and 110 °C were investigated. As an example, Figure 7 shows a few spectra recorded during the annealing at 70 °C. To highlight the mutarotation phenomenon, the intensity, I, of each spectrum is normalized by the intensity, Iβ, of the δβ mode (around 520 cm-1). At the beginning of the isothermal annealing, the intensity of the δβ mode is higher than that of the δR mode. During the annealing, the δR mode develops while the δβ mode shrinks so that at the end of the annealing, the two modes have similar intensity. This evolution clearly reveals the isothermal conversion between the two anomeric forms; that is to say, the kinetic of mutarotation. Figure 8 shows the time evolution of the R-glucose fraction, X(t), of glucose molecules in the alloy for different annealing

Figure 7. Raman spectra of the equimassic amorphous mixture β-glucose/trehalose recorded at 70 °C. The development of δR during the isothermal experiment is a clear sign of the kinetic of the mutarotation of glucose.

Figure 8. Mutarotational kinetic curves of glucose in amorphous equimassic alloy of β-glucose/trehalose at seven temperatures from 40 to 110 °C. Results of fits (see text) are also represented. Insert shows that after normalization by the half-time of reaction, all the kinetic curves superimpose onto a single curve.

temperatures given by X(t) = I542(t)/(I542(t) þ I522(t)), where I542(t) and I522(t) are intensities deduced from Figure 7. These kinetic curves show that the higher the annealing temperature, the faster the mutarotation kinetic. For the high annealing temperatures (from 60 to 110 °C), the R-fraction reaches an equilibrium value closed to 50% in less than 10 h. On the contrary, for lower annealing temperatures (40 and 50 °C), this equilibrium value is not yet reached after 28 h of annealing, and the R-glucose fraction is still far from equilibrium on this time scale. 1702



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Table 1. Characteristic Times of the Mutarotational Kinetic Curves of Glucose in β-Glucose/Trehalose Equimassic Amorphous Alloy at Different Temperaturesa

a

temperature (°C)

characteristic time τ(s)

110

10 102 ( 2 102

90

22 102 ( 2 102

70

50 102 ( 2 102

65

62 102 ( 2 102

60 50

130 102 ( 2 102 914 102 ( 2 102

40

4606 102 ( 2 102

The values correspond to the best fits of eq 1 to the experimental kinetics of mutarotation shown in Figure 8.

4. ANALYSIS AND DISCUSSION The R-glucose fraction equilibrium value Xeq has been reported to slightly increase with temperature, at least in aqueous solutions.12 For our work, taking into account the experimental error range, it was not possible to detect such an evolution. Thus, in a first approximation, the hypothesis that the R-glucose fraction equilibrium value is 50% whatever the annealing temperature was made. The R-glucose fraction X(t) normalized by Xeq has been reported versus time rescaled by the half time reaction, for each annealing temperature (see insert, Figure 8). It appears that all these kinetic curves are superimposed onto a single curve (see insert, Figure 8). This result shows that the kinetic of mutarotation of glucose represented by the evolution versus time of the fraction of R-glucose molecule in the alloy, X(t), obeys a unique physical law, which appears to be exponential: ð1Þ XðtÞ ¼ Xeq 1 - eð - t=τ Þ where τ is a time that is characteristic of the kinetic of mutarotation. For each temperature, experimental kinetics have been fitted by this law. Results are added in Figure 8, and the corresponding fit parameters are reported in Table 1. It appears that the higher the temperature, the lower the characteristic time. This confirms the increase in the rate of mutarotation with temperature. It must be noted that these characteristic times, τ, remain relatively high, suggesting that the mutarotation phenomenon is a relatively slow process, slower than the R relaxation (τR ∼ 100 s at Tg) associated with the glass transition. To determine the behavior law of the characteristic time, τ, the values have been reported in Figure 9 in an Arrhenius plot, showing the evolution of ln(τ) versus 1000/T. A clear break in the evolution of ln(τ(T)) is observed around 65 °C. Below and above this temperature, ln(τ(T)) follows a linear evolution, indicating that the time that is characteristic of the kinetic obeys an Arrhenius law given by Ea ð2Þ lnðτðTÞÞ ¼ lnðτ0 Þ þ RT where τ0 is a pre-exponential factor, R is the universal gas constant, and EA is the activation energy of the mutarotation process (height of the energy barrier of the process; cf., Figure 1). The striking point is that the break in the evolution of the characteristic time occurs in the same temperature range as the glass transition. This reveals upon cooling an increase in the activation energy from 51 ( 4 kJ/mol to 156 ( 5 kJ/mol. This sudden change in the activation energy shows that the

Figure 9. Arrhenius diagram representing the temperature dependence of the characteristic time of mutarotation, τ, in a β-glucose/trehalose equimassic amorphous mixture. A sudden change in the activation energy reveals a coupling between the mutarotation process and the glass transition. (Open circle data is deduced from reference 27.)

mutarotation kinetic of glucose is strongly affected by passing through the glass transition domain. As the temperature decreases, the glass transition marks the catastrophic slowing down of the molecular mobility, leading to the out-of-equilibrium glassy state. Thus, the sudden change of the behavior law of the mutarotation phenomenon in this temperature range suggests a coupling between this process and the molecular dynamic. Such a coupling indicates that the mutarotation process is not a local chemical process. This is underlined by the magnitude of the characteristic times that is a lot more important than the classically admitted time of the main relaxation at Tg (∼100s) and, all the more, than the times of the secondary relaxations (assigned to more localized motions). Actually, even if it is not fully understood, the mechanism governing the mutarotation process implies a protonation of the oxygen O5 (see Figure 1) and the ring-opening of the glucose molecule through a free aldehyde form by which the interconversion takes place (see Figure 1,22,33), so to occur, the mutarotation requires a certain mobility of a glucose molecule and its closed neighboring molecules, in order their configurations allow the successive mechanisms (protonation, ring-opening) to occur. It should be noted that each step of the mutarotation process is quite localized, and its time scale is quite fast, but the characteristic time of the mutarotation process represents the global time of the process. Thus, the shorter the time the molecules take to be in a favorable configuration, the faster the frequency of the successful attempts to mutarotate (the shorter the characteristic time). Consequently, the strong slowing down of the molecular mobility in the vicinity of the glass transition could slow down the mutarotation process by slowing down the frequency of the successful attempts of the molecules to be in a favorable configuration. The mutarotation process would then follow the dynamic of the glass. Table 2 indicates some values of the mutarotation process activation energy that are reported in the literature. It can be seen that these values highly depend on the environment (presence and type of solvent), which underlines that the mutarotation process also depends on the type and strength of interactions with surrounding molecules. 1703



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Table 2. Values Reported in the Bibliography for Activation Energy of the Mutarotation Process of Glucose solvent calculated values

experimental values

activation energy (kJ/mol) ref

without

174

33

1 H2O

198 116

34 33

128

34

water

103

34

94

18

104

2

102

6

80

35

methanol acetic acid

93 85

2 6

2-methyl-2-butanol

45

36

This result does not permit us to determine the type of mechanism of the mutarotation process. Indeed, two types have been proposed: the protonation of the oxygen O5 could be intramolecular or intermolecular; that is, assisted by another molecule.33 However, the coupling between the mutarotation process and the molecular mobility suggests to us an intermolecular protonation, which would need freedom of movement on a larger scale than the one that would be needed by an intramolecular protonation. Neighboring glucose molecules could assist this protonation, as suggested by Broido et al.27 It should be noted that the role of trehalose molecules is not fully understood. As previously seen, trehalose molecules raise the glass transition of the alloy leading to a smaller mobility at a given temperature, but since they are composed of two glucose units, they could also assist the protonation process. However, the characteristic time of the kinetic of mutarotation determined from the results of Broido et al.27 at 151 °C on pure melted glucose (τ = 529 s, represented as an open circle in Figure 9) is very close to the characteristic time extrapolated for the alloy at the same temperature (τ = 316 s). This seems to indicate that the influence of trehalose on the glucose mutarotation process, if any, is quite small. It would be interesting to investigate this point by studying the mutarotation of glucose in a sample with different trehalose content portions.

5. CONCLUSION In this paper, we have taken advantage of the possibility to obtain anomerically pure amorphous glucose by mechanical milling of the crystalline forms to perform the first investigations of the mutarotation in the solid state. The investigations have been performed in situ by time-resolved Raman spectroscopy both in pure glucose and in the 1:1 glucose/trehalose mixture. The latter has the advantage over pure glucose to be reluctant to recrystallization upon heating, making it possible to follow the kinetics of mutarotation in the whole range of temperatures from below Tg up to the melting point. Moreover, contrary to the amorphous state, the pure crystalline forms of GR and Gβ did not show any sign of mutarotation upon heating. This clearly reveals the influence of the physical state of the material on its mutarotation propensity. Such a behavior has been attributed to the network of hydrogen bonds spreading in the crystalline state and which strongly stabilizes the OH groups involved in the mutarotation mechanism.

A striking result is the evolution of the characteristic time of the mutarotation process in the whole range of temperatures, which reveals a clear break in the glass transition temperature region (around 65 °C). This divides the evolution into two Arrhenian parts characterized by very different activation energies. As the temperature decreases, the activation energy becomes three times higher. This behavior strongly suggests that there is a coupling between the mutarotation process and the molecular mobility, in particular, the one assigned to the glass transition. This coupling and the surprisingly long characteristic times seem to indicate that the mutarotation process is not as localized as it would have been expected to be for a chemical process.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ33 3 20 43 68 15. E-mail: emeline.dudognon@ univ-lille1.fr. Present Addresses †

Current address: CERTES EA 3481, Universite Paris-Est, Creteil, France.

’ ACKNOWLEDGMENT This work was supported by the interreg IV “2 mers seas zee€en” cross-border cooperation programme 2007-2013. ’ REFERENCES (1) Isbell, H. S.; Pigman, W. W. A study of the R- and β-aldoses and their solutons by bromine oxidation and mutarotation measurements. J. Org. Chem. 1937, 1 (6), 505–539. (2) Isbell, H. S.; Pigman, W. Mutarotation of sugars in solution. Part II: Catalytic processes, isotope effects, reaction mechanisms, and biochemical aspects. Adv. Carbohydr. Chem. Biochem. 1969, 24, 13–65. (3) Pigman, W.; Isbell, H. S. Mutarotation of sugars in solution. Part I: History, basic kinetics, and composition of sugars solution. Adv. Carbohydr. Chem. Biochem. 1968, 23, 11–57. (4) Pedersen, K. J. The theory of protolytic reactions and prototropic isomerization. J. Phys. Chem. 1934, 38 (5), 581–600. (5) Christiansen, J. A. Hypotheses concerning the transition states in mutarotation of pyranoses. J. Colloid Interface Sci. 1966, 22 (1), 1. (6) Schmid, H. Der Reaktionsmechanismus der allgemeinen Basenkatalyse der Mutarotation der Glucose. Monatsh. Chem. 1963, 94 (6), 1206. (7) Dubrunfaut, M. Sur quelques phenomenes rotatoires et sur quelques proprietes des sucres. Ann. Chim. Phys. 1846, 3 (18), 99–107. (8) Dubrunfaut, M. Note sur quelques phenomenes rotatoires et sur quelques proprietes des sucres. C. R. Hebd. Seances Acad. Sci. 1846, 23, 38–44. (9) Lemieux, R. U.; Stevens, J. D. The proton magnetic resonance spectra and tautomeric equilibria of aldoses in deuterium oxide. Can. J. Chem. 1966, 44 (3), 249–262. (10) Pigman, I. Mutarotation sugars solution 1. History basic kinetics composition sugar solutions. Adv. Carbohydr. Chem. Biochem. 1968, 23, 11–57. (11) Swain, C. G. Concerted displacement reactions. V. The mechanism of acid-base catalysis in water solution. J. Am. Chem. Soc. 1950, 72 (10), 4576–4583. (12) Barc’h, N. L.; et al. Kinetic study of the mutarotation of D-glucose in concentrated aqueous solution by gas-liquid chromatography. Food Chem. 2001, 74, 119–224. (13) Swain, C. G.; Brown, J. F. Concerted displacement reactions. VIII. Polyfunctional catalysis. J. Am. Chem. Soc. 1952, 74 (10), 2538– 2543. 1704



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Solid State Mutarotation of Glucose - The Journal of Physical

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