The Interaction between Water and Cephalexin in the Crystalline and Noncrystalline States

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Solid cephalexin (CEX) has been obtained as crystals, the noncrystalline solid (NC) produced by freeze-drying and the noncrystalline solid (GNC) produced by grinding in a centrifugal mill for 4 h. Each product formed an anhydride, monohydrate and dihydrate. The interaction between water and CEX in the crystalline and noncrystalline states was studied by the infrared (IR) spectral method and the thermal kinetic analytical method (Criado method) using differential scanning calorimetry. In the IR spectrum of CEX, the hydroxyl v_<OH> band was observed at 3400 cm^<-1>, the amido v_<NH> band at 3100 cm^<-1>, the amine v_<NH_3^+> band at 2600 cm^<-1>, the β-lactam v_<C=O> band at 1760 cm^<-1>, the amido v_<C=O> band at 1680 cm^<-1>, and the carboxylate v_<CO_2^-> band at 1600 cm^<-1>. The IR spectral intensity ratios were calculated from the peak heights of the bands with respect to the band at 1280 cm^<-1> as an internal standard peak. The changes of the IR spectral intensity ratios with increasing water content led us to the following conclusions. In the crystalline monohydrate (phase IV), the first water molecule interacted with the amine and carboxylate groups of different molecules in the crystal of CEX. In the crystalline dihydrate (phase II), the second water molecule interacted with NH and C=O of the amido group of CEX. In NC-H_2O, the water molecule is adsorbed at the carboxylate group of CEX, while in NC-2H_2O, the second water molecule is adsorbed at the amine group. The dehydrations of phase IV, GNC-H_2O, GNC-2H_2O and NC-2H_2O followed first-order kinetics and the activation energies (E) were 15.67,14.83,14.58 and 12.50 kcal/mol, respectively, while that of NC-H_2O followed three-dimensional diffusion kinetics and its E value was 15.81 kcal/mol. The E of phase II (second water molecule) was 11.74 kcal/mol as determined by the Kissinger method. These results suggest that the binding energies between the various forms of CEX and water reflect the characteristics of the groups of the CEX molecule to which water is actually bound in each case.
社団法人日本薬学会の論文
1984-11-25

The Interaction between Water and Cephalexin in the Crystalline and Noncrystalline States

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