Myelography Iodinated Contrast Media. 2. Conformational Versatility of Iopamidol in the Solid State

The phenomenon of polymorphism is of great relevance in pharmaceutics, since different polymorphs have different physicochemical properties, e.g., solubility, hence, bioavailability. Coupling diffractometric and spectroscopic experiments with thermodynamic analysis and computational work opens to a methodological approach which provides information on both structure and dynamics in the solid as well as in solution. The present work reports on the conformational changes in crystalline iopamidol, which is characterized by atropisomerism, a phenomenon that influences both the solution properties and the distinct crystal phases. The conformation of iopamidol is discussed for three different crystal phases. In the anhydrous and monohydrate crystal forms, iopamidol molecules display a syn conformation of the long branches stemming out from the triiodobenzene ring, while in the pentahydrate phase the anti conformation is found. IR and Raman spectroscopic studies carried out on the three crystal forms, jointly with quantum chemical computations, revealed that the markedly different spectral features can be specifically attributed to the different molecular conformations. Our results on the conformational versatility of iopamidol in different crystalline phases, linking structural and spectroscopic evidence for the solution state and the solid forms, provide a definite protocol for grasping the signals that can be taken as conformational markers. This is the first step for understanding the crystallization mechanism occurring in supersaturated solution of iopamidol molecules.