Collaborative Research:GOALI: Sensitivity and Resolution Enhancement in Solid-State NMR Spectroscopy

Intellectual merit: Two of the biggest challenges in solid-state nuclear magnetic resonance (SSNMR) spectroscopy of crystalline and amorphous organic compounds such as pharmaceuticals are sensitivity and resolution. Sensitivity is an issue because the 13C and 15N SSNMR of many organic compounds have broad peaks and long proton relaxation times, leading to low signal to noise ratios even for several hours to days of signal acquisition. In addition, for many crystalline compounds, such as those containing aromatic groups, the peaks can be extremely broad due to an effect called anisotropic bulk magnetic susceptibility, resulting in linewidths of > 1 ppm for crystalline organic molecules having multiple aromatic species. Over the past 20 years, ongoing projects in the Munson lab focused on developing abilities to both increase sensitivity and resolution. For example, a multiple sample NMR probe was developed to run more than one sample at a time, resulting in time savings of a factor of four or more, and line widths could be decreased by 25%. In this proposal two approaches to increase sensitivity by an order of magnitude or more are proposed and will be evaluated for their ability to analyze these compounds, including detecting different crystalline forms, quantifying forms in complex mixtures such as drug formulations, and investigating phase miscibility in amorphous systems. The first approach, Dynamic Nuclear Polarization (DNP), has great promise, but has not been fully evaluated for its ability to investigate pharmaceuticals, greatly slowing its adoption in this area. The alternative, simply cooling the sample to liquid nitrogen temperatures, does not produce the same degree of enhancement of DNP, but still results in up to an order of magnitude increase in sensitivity compared to room temperature spectra. Finally, proton NMR spectroscopy is limited by poor resolution, where even in an ideal sample, obtaining line widths below 0.2 ppm is extremely challenging. Experiments using heteronuclear correlation spectroscopy have shown that a factor of three enhancement in resolution is possible, with even greater gains possible with improved decoupling methods. Overview: In this proposal, three methods to increase sensitivity and/or improve resolution in solid-state nuclear magnetic resonance (SSNMR) spectroscopy of crystalline and amorphous organic compounds will be pursued. The first method uses dynamic nuclear polarization (DNP) to increase the sensitivity of crystalline organic compounds by orders of magnitude. DNP is an emerging technology that has primarily been used to investigate frozen protein solutions, but high quality DMP spectra have been acquired for several pharmaceutical systems. Quantitation remains one of the biggest challenges facing DMP. A comprehensive study applying DMP to pure active pharmaceutical ingredients as well as drug formulations will be performed. Specific areas of research will be studied include the impact on particle size for DMP enhancement and quantitation; the investigation of phase separation in amorphous solid dispersions; and the identification of polymorphic forms in formulations. The second method is to develop a mechanical magic-angle spinning (MAS) NMR probe that operates at liquid nitrogen temperatures. Currently, MAS probes spin samples for DNP using compressed nitrogen gas that must be cooled to close to 100 K. A typical DNP NMR probe (3.2 mm spinning assembly) may use up to 600 L of liquid nitrogen for both spinning and cooling, which costs ~$200/day in cryogen costs alone. Moreover, an elegant but complicated setup is needed to ensure constant maintaining of the cooling gas. A simpler solution is to entirely eliminate the need for gas by using mechanical MAS. The advantages of mechanical MAS include: much lower costs to operate at liquid nitrogen/helium temperatures and the ability to control the environment in the MAS rotor, including water content and vacuum, without contamination from spinning gases. A mechanical MAS probe operating at close to liquid nitrogen temperatures should have about an order of magnitude increase in sensitivity compared to a standard NMR probe operating at 298 K as well as improved radiofrequency performance. A third area of research is to improve the resolution of pharmaceutical compounds through correlating heteronuclear proton-carbon two-dimensional NMR spectroscopy (HETCOR). Resolution enhancements of over a factor of three have been observed using this approach, and methods to obtain additional improvements are proposed. Broader impacts: The translation of scientific advancement to practical knowledge to improve the safety and efficacy of drugs requires mutual understanding of both the practical problems that exist in the pharmaceutical community as well as the fundamental knowledge of the most advanced techniques for being able to characterize these systems. The practical implementation of techniques such as DNP NMR into pharmaceutical research at companies requires that it shows demonstrable value beyond traditional approaches to characterize drug substance and drug product. The following four approaches will be used: 1) short courses on the basics of solidstate NMR and its applications to pharmaceutical and material science will be presented, either prior to or in conjunction with a conference such as the American Association of Pharmaceutical Scientists, the Rocky Mountain Conference on Solid-State NMR Spectroscopy, or the Small Molecules Are Still Hot (SMASH) NMR conference. The target audience is graduate students and postdocs who are probably not familiar with the challenges of using techniques such as solidstate NMR to study these systems; 2) visits to companies and universities where these short courses and seminars can be held; 3) establish a visiting scientist program for students, industrial representatives, and members of governmental agencies such as the FDA to visit the Munson lab (to learn pharmaceutical characterization) and Rossini lab (to learn DNP); 4) reach out to other disciplines, such as chemical engineering, food science, and agriculture to highlight how new developments can be used to solve their problems.