Solid-State NMR ssNMR

In an early study, Mauritz et al. investigated anion—cation interactions within Nation sulfonate membranes versus degree of hydration using FTIR/ ATR and solid state NMR (SSNMR) spectroscopies. An understanding of the dynamic ionic—hydrate molecular structures within and between the sulfonate clusters is essential for a fundamental understanding of the action of these membranes in ion transport. This information can be directly related to the equilibrium water swelling that, in turn, influences molecular migration. 

To understand the function of membrane-active peptides, it is important to know the structure and orientation of the peptide in the membrane. As is evident from Figure 18.1, it is possible to distinguish between, for example, carpet and pore mechanisms of action by determining the peptide s orientation in the membrane. Various techniques, such as electron spin resonance (ESR) [35], infrared (IR) spectroscopy [36-38], circular dichroism (CD) [35, 39,40], and solid-state NMR (SSNMR) [4-7] are used to investigate membrane-active peptides in a quasi-native lipid bilayer environment. In the following sections, methods to determine peptide structure and orientation are presented. 

Solid-state NMR (ssNMR) is a useful and widely used tool to determine the structures of CMPs. Due to the amorphous nature and insolubility of most CMP networks, the precise structure of CMPs is difficult to determine. Solid-state NMR can give information about the bonding within the networks and can be useful to ascertain the incorporated functional groups. Due the large amount of carbon and hydrogen atoms in the networks, and the relatively quick collection times, cross-polarisation... 

Several methods are suitable to characterize the crystal structure of a solid, such as single-crystal X-ray diffraction, X-ray powder diffraction (XRPD), Raman, infrared (IR), near infrared (NIR), terahertz (THz), and solid-state NMR (ssNMR) spectroscopy [2-5]. Each method has its advantages and disadvantages. The most widely used ones will be described in some more detail. 

The following discussion focuses exclusively on what is termed small molecules, namely compounds with a molecular mass on the order of 2000 Da or less. The study of proteins, polymers, and other such macromolecules by NMR warrants an entirely different approach that is beyond the scope of this book. Similarly, we will restrict our discussion to liquid-state NMR spectroscopy, since the quantity of sample available for the characterization of a degradant or impurity is typically far below reasonable amounts needed for most solid-state NMR (SSNMR) techniques. The overall characterization strategy we will take for NMR is shown schematically in Figure 29. 

Once these techniques indicate that there may be multiple polymorphs or solvates present, more complex and involved techniques such as X-ray powder diffraction or solid-state NMR (SSNMR) can be employed to confirm the initial test and provide more information about the exact polymorphic species in the API. ... 

This method is often used for studying structures and conformations, as well as for analyzing molecular interactions and motions. Although most NMR experiments are performed on liquid samples, solid-state NMR (SSNMR) is also rapidly growing as a powerful method for studying organic solids. 

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