Investigation of Polymorphism in Aspartame and Neotame Using Solid-State NMR Spectroscopy

Two-dimensional proton-detected 35Cl/1H correlation solid-state NMR experiment under fast magic angle sample spinning: application to pharmaceutical compounds

In this study, we have measured ³⁵Cl/¹H correlations in hydrochloride salts of active pharmaceutical ingredients (HCl APIs) using the D-HMQC pulse sequence at fast MAS.

35Cl solid-state NMR of HCl salts of active pharmaceutical ingredients: structural prediction, spectral fingerprinting and polymorph recognition

A series of HCl salts of active pharmaceutical ingredients (APIs) have been characterized via³⁵Cl solid-state NMR (SSNMR) spectroscopy and first-principles plane-wave DFT calculations of ³⁵Cl NMR interaction tensors.

Evolution of solid-state NMR in pharmaceutical analysis

The technique of solid-state NMR, as applied to pharmaceutical analysis, has evolved from a sparingly used technique into an integrated, information-rich technique that uses a variety of sophisticated 1D and 2D experiments. In this article, several key developments in the field are reviewed, highlighting the increasing sophistication of the analyses being utilized and the detailed results obtained, in addition to future directions and developments.

Pursuing structure in microcrystalline solids with independent molecules in the unit cell using 1H-13C correlation data

The (1)H-(13)C solid-state NMR heteronuclear correlation (HETCOR) experiment is demonstrated to provide shift assignments in certain powders that have two or more structurally independent molecules in the unit cell (i.e. multiple molecules per asymmetric unit). Although this class of solids is often difficult to characterize using other methods, HETCOR provides both the conventional assignment of shifts to molecular positions and associates many resonances with specific molecules in the asymmetric unit. Such assignments facilitate conformational characterization of the individual molecules of the asymmetric unit and the first such characterization solely from solid-state NMR data is described. HETCOR offers advantages in sensitivity over prior methods that assign resonances in the asymmetric unit by (13)C-(13)C correlations and therefore allows shorter average analysis times in natural abundance materials. The (1)H-(13)C analysis is demonstrated first on materials with known shift assignments from INADEQUATE data (santonin and Ca(OAc)(2) phase I) to verify the technique and subsequently is extended to a pair of unknown solids: (+)-catechin and Ca(OAc)(2) phase II. Sufficient sensitivity and resolution is achieved in the spectra to provide assignments to one of the specific molecules of the asymmetric unit at over 54% of the sites.

Structural characterization of an anhydrous polymorph of paclitaxel by solid-state NMR

The three-dimensional structure of a unique polymorph of the anticancer drug paclitaxel (Taxol) is established using solid state NMR (SSNMR) tensor ((13)C & (15)N) and heteronuclear correlation ((1)H-(13)C) data. The polymorph has two molecules per asymmetric unit (Z' = 2) and is thus the first conformational characterization with Z' > 1 established solely by SSNMR. Experimental data are correlated with structure through a series of computational models that extensively sample all conformations. For each computational model, corresponding tensor values are computed to supply comparisons with experimental information which, in turn, establishes paclitaxel's structure. Heteronuclear correlation data at thirteen key positions provide shift assignments to the asymmetric unit for each comparison. The two distinct molecules of the asymmetric unit possess nearly identical baccatin III moieties with matching conformations of the C10 acetyl moiety and, specifically, the torsion angle formed by C30-O-C10-C9. Additionally, both are found to exhibit an extended conformation of the phenylisoserine sidechain at C13 with notable differences in the dihedral angles centered around the rotation axes of O-C13, C2'-C1' and C3'-C2'.

NMR studies of organic polymorphs and solvates

This review article describes the applications of NMR to the study of polymorphs and related forms (solvates) of organic (especially pharmaceutical) compounds, for which it is of increasing academic and practical importance. The nature of the systems covered is briefly introduced, as are the techniques constituting solid-state NMR. The methodologies involved are then reviewed under a number of different headings, ranging from spectral editing through relaxation times to shielding tensors and NMR crystallography. In each case the relevant applications are described. Whilst most studies concentrate on structural matters, motional effects are not neglected. A special section discusses studies of solvates (especially hydrates), and another reviews quantitative analysis.

Quantitation of crystalline and amorphous forms of anhydrous neotame using 13C CPMAS NMR spectroscopy

Although most drugs are formulated in the crystalline state, amorphous or other crystalline forms are often generated during the formulation process. The presence of other forms can dramatically affect the physical and chemical stability of the drug. The identification and quantitation of different forms of a drug is a significant analytical challenge, especially in a formulated product. The ability of solid-state 13C NMR spectroscopy with cross polarization (CP) and magic-angle spinning (MAS) to quantify the amounts of three of the multiple crystalline and amorphous forms of the artificial sweetener neotame is described. It was possible to quantify, in a mixture of two anhydrous polymorphic forms of neotame, the amount of each polymorph within 1-2%. In mixtures of amorphous and crystalline forms of neotame, the amorphous content could be determined within 5%. It was found that the crystalline standards that were used to prepare the mixtures were not pure crystalline forms, but rather a mixture of crystalline and amorphous forms. The effect of amorphous content in the crystalline standards on the overall quantitation of the two crystalline polymorphic forms is discussed. The importance of differences in relaxation parameters and CP efficiencies on quantifying mixtures of different forms using solid-state NMR spectroscopy is also addressed.

Probing Structure in the Polymorphic Domain of the l-Enantiomer of N-Benzoyl-Phenylalanine by Means of 2D Solid-State NMR Spectroscopy and DFT Calculations

A study of polymorphism using a range of solid-state NMR techniques is presented. We demonstrate the existence of at least six polymorphs in a sample of N-benzoyl-L-phenylalanine. We also present methodology for the characterization of the protonation state, hydrogen bonding, and molecular conformation for the polymorphs, together with results of such a characterization for one of the polymorphs present in our sample. DFT modeling is used to investigate the separate effects hydrogen bonding and molecular conformation have on the chemical shift tensor.

Water molecules in hydroxy/acid networks as a competition between dynamics and bonding. Synthesis of a wet hydrophobic pore

In a model formed by hydroxy acids with a general structure (+/-)-1, we found that solid-state structures depend on steric interactions. Thus, with the exception of molecules 1b and 1e, compounds (+/-)-1a-(+/-)-1m, which possess bulky and conformationally rigid substituents, aggregate by forming tapes and sheets by alternating (+) and (-) subunits held together via carboxylic acid to alcohol hydrogen bonds. Homologue (+/-)-1n with conformationally flexible substituents, which allow conformational deformation gives, by way of the incorporation of water molecules, an efficient hexagonal assembly, which extends to the third-dimension to form tubular H-bonding networks. Each puckered channel can be described as being interconnected by closely packed hexagons in chair-like conformations. The ethyl groups presented in (+/-)-1n provided the volume required to lock the inner hexagonal wall into a rigid structure.

13C cross-polarization MAS NMR study of some steroidal sapogenins

Cross-polarization (CP) magic angle spinning (MAS) solid-state 13C NMR spectra of five steroidal sapogenins: tigogenin ((25R)-5alpha-spirostan-3beta-ol), hecogenin (3beta-hydroxy-(25R)-5alpha-spirostan-12-one), diosgenin ((delta5-(25R)-5alpha-spirosten-3beta-ol), sarsasapogenin ((25S)-55beta-spirostan-3beta-ol), and smilagenin ((25R)-5beta-spirostan-3beta-ol) were recorded. The solid-state chemical shifts are almost the same as for solution, which indicate that confirmations of sapogenins are similar in both phases. The doubling of some resonances in the spectra of solid diosgenin shows that there are two molecules in the crystallographic asymmetric unit. The cross-polarization time constants T(CP) and relaxation times in the rotating frame T(1rho)H were obtained from the variable-contact cross-polarization experiments for tigogenin and diosgenin. The values of T(CH) for methyl carbons indicate fast rotation of methyl groups and are close (0.30-0.35 ms), suggesting that the interaction with their intramolecular neighbors is similar. The values of T(1rho)H for carbons of tigogenin are longer than of diosgenin. Very efficient cross-polarization dynamics results in short time required for obtaining a spectrum of sapogenin of remarkably good quality.