Solid state carbon-13 NMR studies on pharmaceuticals and chemical modifications of nucleic acids

Nina Sipahimalani Cauchon, Purdue University

Abstract

I. In order to investigate the relationship between molecular mobility and solid-state reactivity, several pharmaceutically active compounds were studied by solid-state $\sp{13}$C NMR spectroscopy and X-ray crystallography. The activation energies of the methyl carbons in the two 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors lovastatin and simvastatin were calculated from their variable-temperature spin-lattice relaxation times. The side-chain terminal methyls, which had the largest thermal ellipsoids from the X-ray crystallographic studies, also had the lowest activation energies. In the low-temperature solid-state spectra, two distinct rotamers of the more highly disordered simvastatin side chain were observed. The ibuprofen methyl-group activation energies were correlated with thermal parameters (U values) from variable-temperature X-ray crystallography. The methyl group with the higher activation energy had the smaller change in its U value with temperature. No significant difference was found in the methyl-group activation energies of crystalline and amorphous indomethacin, but the T$\sb1\sp{\rm c}$ values of the side-chain carbons were found to be significantly lower in the amorphous form. Differences in the T$\sb{1\rho}\sp{\rm c}$ values of the side-chain carbons were also seen. These results suggest more conformational mobility for the amorphous form. Several polymorphs of indomethacin and mefenamic acid have been shown to be readily distinguishable by solid-state $\sp{13}$C NMR in their solid dosage forms. This technique can thus give valuable information about molecular mobility, which complements that obtained by single-crystal X-ray diffraction, and holds particular promise for the study of polymorphism and dynamics of drugs in solid dosage forms. II. The chemical modification of polyguanylic acid with a simple methylating agent, methyl methanesulfonate, was also studied by solid-state $\sp{13}$C NMR spectroscopy. The major product was found to be the result of methylation at the N-7 position of guanine. This was confirmed by enzymatic degradation to the bases, using ribonuclease T$\sb1$, spleen phosphodiesterase, 3$\sp\prime$-nucleotidase, and purine nucleoside phosphorylase, followed by reverse-phase HPLC analysis. Solid-state $\sp{13}$C NMR is therefore a direct and non-destructive method for the analysis of methylated polyguanylic acid. The solid-state $\sp{13}$C NMR spectra of several other nucleosides were also obtained, and were found to provide valuable conformational information.

Degree

Ph.D.

Advisors

Byrn, Purdue University.

Subject Area

Pharmacology

COinS