Implications of Global and Local Mobility in Amorphous Sucrose and Trehalose as Determined by Differential Scanning Calorimetry |
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Authors: | Ion Dranca Sisir Bhattacharya Sergey Vyazovkin Raj Suryanarayanan |
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Institution: | (1) Department of Pharmaceutics, College of Pharmacy, University of Minnesota, 9-177 Weaver Densford Hall, 308 Harvard Street SE, Minneapolis, Minnesota 55455, USA;(2) Department of Chemistry, University of Alabama at Birmingham, 901 South 14th Street, Birmingham, Alabama 35294, USA;(3) Present address: Center of Physical Chemistry and Nanocomposites, Institute of Chemistry, Academy of Sciences of Moldova, Chisinau, MD 2028, Moldova;(4) Forest Research Institute, Forest Laboratories, Inc., 49 Mall Drive, Commack, NY 11725, USA; |
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Abstract: | Purpose To investigate the local and global mobility in amorphous sucrose and trehalose and their potential implications on physical
stability.
Methods Amorphous sucrose was prepared by lyophilization while amorphous trehalose was prepared by dehydration of trehalose dihydrate.
The variation in the effective activation energy of α-relaxation through glass transition has been determined by applying
an isoconversional method. β-Relaxations were detected as shallow peaks, at temperatures below the glass transition temperature,
caused by annealing glassy samples at different temperatures and subsequently heating at different rates in a differential
scanning calorimeter. The effect of heating rate on the β-relaxation peak temperature formed the basis for the calculation
of the activation energy.
Results α-Relaxations in glassy trehalose were characterized by larger activation energy barrier compared to sucrose, attributable
to a more compact molecular structure of trehalose. The effect of temperature on viscous flow was greater in trehalose which
can have implications on lyophile collapse. The size of the cooperatively rearranging regions was about the same for sucrose
and trehalose suggesting similar dynamic heterogeneity at their respective glass transition temperatures. The activation energy
of β-relaxations increased with annealing temperature due to increasing cooperative motions and the increase was larger in
sucrose. The temperature at which β-relaxation was detected for a given annealing time was much less in sucrose implying that
progression of local motions to cooperative motions occurred at lower temperatures in sucrose.
Conclusions Trehalose, having a lower free volume in the glassy state due to a more tightly packed molecular structure, is characterized
by larger activation energies of α-relaxation and experiences a greater effect of temperature on the reduction in the activation
energy barrier for viscous flow. The pronounced increase in cooperative motions in sucrose upon annealing at temperatures
below (T
g −50) suggest that even a small excursion in temperature could result in a significant increase in mobility. |
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Keywords: | activation energy β -relaxation DSC lyoprotectant molecular mobility sucrose trehalose |
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