HU BiaoYU Di HUANG Kai YU Shu-Juan（College of Light Industry and Food Science， South China University of Technology， Guangzhou， Guangdong 510640， China）

Metal-ion Interactions with Sugars: Crystal Structure of Bis(4-dehydro-L-arabinose) Calcium Methanol Bishydrate①

HU Biao②YU Di HUANG Kai YU Shu-Juan
（College of Light Industry and Food Science， South China University of Technology， Guangzhou， Guangdong 510640， China）

The crystal structure of the metal-organic frame Ca（C5H9O5）2·CH3OH·2H2O （1） has been synthesized and characterized. Complex 1 belongs to a tetragonal P43212 space group. In complex 1， the sugar moiety shows a beta-L configuration of pyranose form. The calcium（II） is eight-coordinated， binding to four such sugar moieties， via O（1）， O（2） of two molecules and O（3），O（4） of the other two， with the 4-hydroxy group being deprotonated. The water and methanol molecules are not coordinated to the calcium ion.

metal-organic framework, Ca-L-arabinose complex, crystal structure;

1 INTRODUCTION

Although calcium-carbohydrate complexes are involved in several biological processes， such as calcium transport［1， 2］， calcification［3， 4］， and glycoprotein binding to the cell surface［5］， little is known about the factors that regulate calcium-carbohydrate interactions， or those that may affect the final structures of the resulting complexes. Therefore， the investigation of the complexes between calcium and simple sugars is important not only to understand the chemistry behind chiral compounds， but also those structural factors that play a major role in these biological processes. Several calcium-sugar complexes have been reported， including CaCl2·2fructose·3H2O［6， 7］， CaBr2·L-arabinose·4H2O［8］， CaCl2·D-ribose·3H2O［9］， CaBr2·galactose·4H2O［10］， CaBr2·lactose·4H2O［11］， CaBr2·tetrahalose·H2O［12］， Ca（D-glucose）2X2·4H2O， and Ca（D-glucose）X2· 4H2O［13］. In these complexes， Ca（II） is likely to be six- or eight-coordinated with hydroxyl groups and water molecules as well as with L-arabinose， an important pentose widely found in hemicellulose. L-arabinose， which exists in solution in an equilibrium mixture of mainly six isomers， coordinates with the calcium ions to form complexes with different configurations in aqueous solution［8， 14， 15］. However，the detailed three-dimensional structures of those complexes have not been solved yet. In order to synthesize the calcium chloride-L-arabinose complexes for subsequent structural analysis， we prepared a new colorless diamond-like hygroscopic substance.

The morphology， crystallographic structure， and unit cell of the obtained compound were determined in this paper. Our results showed that the structure of the compound strongly differs from the calcium halide sugar complexes previously reported. In particular， the free sugar of the complex is deprotonated， and the water molecule is not coordinated with the calcium ion. This new binding mode provides new insight into the formation of metal-carbohydrate complexes.

2 EXPERIMENTAL

2. 1 Synthesis and crystallization

Beta-L-arabinose （30.6 g， 0.2 mol） and an equivalent molar of anhydrous calcium chloride were dissolved in distilled water. After 4 months of slow evaporation at room temperature， colorless octahedral-shaped crystals 1 were obtained. The crystals were washed with cold absolute ethanol and then recrystallized from aqueous solution （small crystal seeds were added to the solution）. After a week， recrystallized crystals 1 were obtained by slow evaporation， and kept in methanol until further use. The compound is hygroscopic， and is easily dissolved in water and hot ethanol.

2. 2 Structure determination

X-ray diffraction （XRD） analysis was carried out with a Bruker D8 Advance X-ray diffractometer （BRUKER， Germany） using a conventional CuKα radiation （λ =1.54 Å） at voltage of 33 kV， and current of 45 mA. Data were collected from 2θ values of 5.0～ 60.0°， with a step width of 0.02° and speed of 12°·min-1. The X-ray powder diffraction patterns were compared to those of an equimolar mixture of L-arabinose and anhydrous calcium chloride by MDI Jade 6.0 （Materials Data，Inc， California）.

For single-crystal X-ray diffraction， a suitable single crystal with dimensions of 0.30mm × 0.22mm × 0.20mm （the same crystal shape as that used in SEM experiments） was carefully selected under an optical microscope and glued to a thin glass fiber with epoxy resin. The X-ray diffraction intensity data were collected with a Rigaku R-Axis diffractometer （Rigaku， Japan） with an ω-φ scan mode in the 2θ range of 3.19 ～ 27.45°. All measurements were performed at room temperature using graphitemonochromatized MoKa radiation （λ = 0.71073 Å）. A total of 17856 （2095 independent， Rint= 0.0273）reflections were collected. The structure was solved by direct methods and refined using a full-matrix least-squares procedure on F2with SHELXL-97［16］. Anisotropic displacement parameters for all nonhydrogen atoms and isotropic temperature factors for hydrogen atoms were introduced. The hydrogen atoms connected to carbon atoms were included in calculated positions from the geometry of molecules. The final cycle of refinement shows that complex 1 is in the tetragonal system， P43212 space group with a = 12.790（9）， b = 12.790（9）， c = 11.191（2） Å， V = 1830.7（5） Å3and Z = 4. The final full-matrix leastsquares refinement converged to R = 0.0664， wR = 0.2020， and S = 1.063. The largest difference peak and hole are 0.584 and -0.530 e·Å-3， respectively.

3 RESULTS AND DISCUSSION

3. 1 Powder diffraction pattern

The obtained crystals of 1 （bulk microcrystalline powder）， a mixture of equal molar of L-arabinose and anhydrous calcium chloride， were analyzed by powder diffraction pattern （Fig. 1）.

Fig. 1. X-ray diffraction (XRD) of complex 1 as well as a mixture of L-arabinose and calcium chloride

The spectrum of the complex is remarkably different from that of the mixture， suggesting that the compound is in fact not a mixture of L-arabinose and calcium chloride， but a new compound resulting by the combination of the two. The spectrum of complex 1 is very consistent with the theoretical spectrum simulated by diamond （version 3.1），indicating that the single crystals are actually the grown-up of the bulk microcrystalline powder.

3. 2 Description of the structure

According to single-crystal diffraction data， the studied compound has the following formula: Ca（C5H9O5）2·CH3OH·2H2O， with Mr= 406.41. The crystal belongs to a tetragonal， P43212 space group with a = 12.790（9）， b = 12.790（9） and c = 11.191（2）Å. Selected bond lengths and bond angles are collected in Table 1. The coordination environment of the Ca（II） ion of complex 1 is shown in Fig. 2，and the complete molecular structure of complex 1 is shown in Fig. 3.

Fig. 2. Coordination environment of the Ca2+ions in complex 1

Fig. 3. Molecular structure of complex 1

Table 1. Bond Lengths (Å) and Bond Angles (º) of Complex 1

The bond lengths of Ca-O（1）， Ca-O（2）， Ca-O（3）and Ca-O（4） are 2.451， 2.411， 2.488 and 2.427 Å，respectively （Table 1）. Unlike a typical sixmembered ring of formula CaCl2·D-ribose·3H2O［9］，characterized by the cis-cis and axial-equatorialaxial （ax-eq-ax） sequences， our compound shows only a cis-cis sequence in a five-membered ring between calcium， oxygen， and carbon atoms， namely Ca-O（1）-C（1）-C（2）-O（2）-Ca and Ca-O（3）-C（3）-C（4）-O（4）-Ca. This is because the calcium ion in the CaCl2-D-ribose·3H2O complex is coordinated with O（1）， O（2） and O（3） of one molecule as well as O（4）， O（5） of the other molecules. In contrast， in the title complex， the calcium atom is eight-coordinated with O（1）， O（2） of two sugar molecules as well as O（3）， O（4） of the other sugar molecules （Fig. 4）. The O（6） of methanol and O（7） of water molecules are not coordinated with calcium， and the hydrogen atom in the 4-hydroxy group being deprotonated （Fig. 3）. This differs from the previously published complexes， such as Zn（L-arabinose）2X2·4H2O，Cd（L-arabinose）2X2·4H2O， Sr（L-arabinose）2X2·4H2O，Ba（L-arabinose）2X2·4H2O， and UO2（L-arabinose）2X2·4H2O， in which the metal ions are sixcoordinated， binding to two arabinose moieties via O（3）， O（4） of the first and O（1）， O（5） of the second sugar molecule as well as two water molecules［17-19］. Since calcium（II） binds more easily to less sterically hindered O（2） atoms than to the ring oxygen atoms O（5）， it can be speculated that the calcium-L-arabinose complex may be more stable than the Zn（II）-， Cd（II）-， Ba（II）-， and UO2-L-arabinose complexes. The one described in this work is a new metal-sugar binding mode that may be of great importance for several biological processes.

The calcium-oxygen distances are in the range of 2.41～2.48 Å （Table 1）， typical of a Ca-OH （neutral）bond. The calcium ions were found to be eightcoordinated （square antiprismatic molecular geometry）. Unlike the previously published complexes，such as Ca（α-L-arabinose）X2·4H2O［8］， Zn（α-L-arabinose）2X2·4H2O， Cd（α-L- arabinose）2X2·4H2O［20］，and other metal-halide L-arabinose hydrates that have an alpha-furanose configuration， the sugar moiety of the title complex exists as a beta-anomer configuration of pyranose. This is because Ca（II），Zn（II）， and Cd（II） are coordinated with the ring oxygen atom O（5）； this type of binding， which also includes metal cation-halide anion coulombic attractive interactions， produces significant changes in the conformation of carbohydrate at the cation binding sites. In contrast， the calcium cation of Ca（C5H9O5）2·CH3OH·2H2O is only coordinated with the hydroxyl groups of the sugar moiety， which preserves the beta-conformation upon metallization. As expected， no intramolecular or intermolecular hydrogen bonds were found between molecules because all four hydroxyl groups of the sugar are coordinated with the calcium ion. In addition， the water molecules of the title complex are not coordinated with the calcium ion， in contrast to other metal-halide L-arabinose hydrates mentioned above. Although the preparation of the title complex followed the previously reported studies， the final compound obtained in this work is not a halide and is not characterized by a Ca（L-arabinose）X2·4H2O structure. The compound studied in this work exhibits a new metal-carbohydrate binding mode. In particular， calcium does not coordinate with the halide anion， the water and methanol molecules， but only binds with the hydroxyl groups of the sugar moiety. Such a conformation is persevered upon metallization. The new binding mode described in this work may be of great significance in several biological processes in which these complexes are involved.

3. 3 Thermal analysis

To determine the thermal stability of the crystal，simultaneous thermogravimetric analysis （TGA） and differential scanning calorimetry （DSC） were carried out between 30 and 500 ℃ in nitrogen atmosphere at a heating rate of 10 ℃·min-1using TG/ DSC （NETZSCH STA 449F3）. Based on the DSC analysis， the melting point of the complex was detected with a hot stage polarizing microscope （XPH-300Z， Shanghai Changfang， China）.

Thermal degradation patterns proposed for the metal complexes under N2are presented in Fig. 4.

Fig. 4. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of complex 1

The TG/DSC curves （Fig. 4） showed the complex loss of 4.97% of the total mass at low temperature （58.4～74.7 ℃）. This loss and the low endo peak area （15.54 J·g-1， the first endo peak） indicated that it is the hygroscopic moisture. According to Luo’s［21］research that thermal behavior of L-arabinose consists of three stages， the first stage is among 160～180 ℃， during which two endo peaks appear with no mass change on the TG curve. However， a high endo peak （the second endo peak with an areaof 253.3 J·g-1） together with a mass loss of 7.46% was observed in TG/DSC of the complex from 161.0 to 179.2 ℃. So， it can be concluded that the mass loss at this stage is caused by the removal of two coordinated water molecules， which amounts to a theoretical mass loss of 8.65%. This is also consistent with Wang’s researches that the departure of coordinated water molecule normally occurs at 120～180 ℃［22， 23］. According to Luo’s research［21］，L-arabinose began the first degradation step from 185 to 230 ℃， with a weight loss of 16.8%. The immediately followed degradation stage falls in the 230～330 ℃ region， with a weight loss of about 46.0%. The third endo peak of the title compound （with an area of 123.5 J·g-1and weight loss of 23.63%） may be caused by the loss of methanol and the thermal degradation of L-arabinose moiety （theoretical mass change of 25.43%）. The fourth endo peak is caused by the degradation of Ca-L-arabinose residues with a low peak height （0.2307 mW/mg） and mass loss of 15.75%. The mass loss of this stage is smaller than that of pure L-arabinose （46%）［21］， and the possible reason is that the Ca ion is bonded with the degraded L-arabinose residues and thus prevents further degradation.

In conclusion: the TG/DSC reveals that the thermal behavior of the complex consists of three steps: i） loss of two crystalline water molecules from 161.0 to 179.2 ℃， ii） removal of methanol molecules together with the degradation of L-arabinose moiety from 180.0 to 230 ℃， and iii）degradation of the degraded Ca-L-arabinose residues from 230 to 450 ℃ with a low peak height （0.2307 mW/mg） and mass loss of 15.75%. Based on the DSC analysis， the melting point of the complex detected with a hot stage polarizing microscope is 160.8～ 162.4 ℃.

4 CONCLUSION

Although calcium-carbohydrate complexes are known to play a crucial role in several biological processes， little is known about the factors that affect the formation of those complexes. During the synthesis of calcium chloride-L-arabinose complexes， a new metal-organic framework， namely，Ca（C5H9O5）2·CH3OH·2H2O， was obtained. The compound is not a halide， and the water and methanol molecules are not coordinated with the calcium ion. Calcium（II） is eight-coordinated，binding to four sugar moieties via O（1）， O（2） of two molecules and O（3）， O（4） of the other two， with O（4）being deprotonated.

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Received 9 September 2014； accepted 16 March 2015 （CCDC 999914）

① This project was supported by the Ministry of Science and Technology through the Agriculture Science and Technology Achievements Transformation Fund （No. 2013GB23600669）， and the Science and Technology Planning Project of Guangzhou Municipality， China （No. 2011Y2-00012）

② Corresponding author. Male. E-mail: hubiao2003@126.com

10.14102/j.cnki.0254-5861.2011-0517