Add time:09/24/2019         Source:sciencedirect.com

Lipids regularly exhibit complicated thermotropic and lyotropic phase behavior. In this study, the utility of 13C solid‐state nuclear magnetic resonance spectroscopy (SSNMR) in characterizing the phase properties of pharmaceutical lipids was investigated. Variable temperature 13C SSNMR spectra and spin‐lattice relaxation times (T1C) were obtained for high‐purity oleic acid (OA) and propranolol oleate (POA). Spectral changes took place following OA γ‐to‐α phase transition that indicated increased nuclear inequivalence of aliphatic chain carbons in the α phase. T1C data for the α phase demonstrated considerable conformational changes throughout the aliphatic chain, not solely in the methyl side chain as previously reported. These data support α‐OA classification as a conformationally disordered crystalline phase. The prevalence of low T1C values in both POA I and II suggested the absence of a rigid crystalline molecular lattice, so both phases were described as conformationally disordered crystalline phases. A two‐phase mixture of POA I and II was also identified, emphasizing the sensitivity of this technique. 13C SSNMR provided valuable information regarding the nuclear environment of specific functional groups in lipid crystalline and mesomorphic structures. Understanding phase behavior at the molecular level can aid selection of appropriate formulation strategies for lipids by allowing prediction of processing properties, and physical and chemical stability. 13C SSNMR is a powerful technique for pharmaceutical lipid characterization. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89:1286–1295, 2000

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