Subtraction of DC Conductivity and Annealing: Approaches To Identify Johari–Goldstein Relaxation in Amorphous Trehalose

• Published in: Molecular pharmaceutics Vol. 8; no. 4; pp. 1416 - 1422
• Main Authors: Bhardwaj, Sunny P, Suryanarayanan, Raj
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01.08.2011
• Subjects: Dielectric Spectroscopy; Trehalose - chemistry; molecular mobility; Kramers−Kronig; trehalose; annealing; dielectric spectroscopy; Johari−Goldstein relaxation
• ISSN: 1543-8384; 1543-8392
• DOI: 10.1021/mp2000154

Amorphous trehalose finds extensive use as a stabilizer of biomolecules including proteins and phospholipids. Hypothesizing that molecular mobility is a determinant of its stability, dynamic dielectric spectroscopy (DDS) was used to map the different modes of mobility. Isothermal dielectric relaxation profiles of amorphous trehalose were obtained, over the frequency range of 10–1–107 Hz, and at temperatures ranging from 30–170 °C. At temperatures close to the glass transition (T g), the α-relaxation was not readily discernible due to interference from dc conductivity. We used Kramers–Kronig transformation that enabled not only the complete resolution of α-relaxation but also the identification of an excess wing, in the high frequency tail of α-relaxation. On annealing, this excess wing developed into a partially resolved and hitherto unidentified β-relaxation peak. This peak, due to its position in the dielectric spectrum, its annealing time dependence and the good agreement with the calculated independent relaxation time, was assigned to the Johari–Goldstein process. This work demonstrates the utility of conductivity subtraction coupled with sub-T g annealing to successfully study all the modes of mobility in amorphous trehalose. This approach can potentially be extended to situations wherein dc conductivity impedes the complete characterization of molecular mobility.