Research

Professor Schaefer's specialty is high-resolution solid-state NMR. He is the co-inventor of cross-polarization magic-angle spinning (1976) and rotational-echo double resonance (1989). Both techniques have become standard methods and are currently in use worldwide. The first results in high-resolution rare-spin NMR spectra of solids, and the second allows the measurement of distance-dependent dipolar coupling between pairs of spins. His program at Washington University has applications focusing on the determination of structure and dynamics of protein binding sites, channels, and interfaces, and of chain packing in synthetic polymer systems. All of these applications are on materials not suited to diffraction or solution-state NMR measurements.

In the last 20 years, the combination of cross polarization for sensitivity and magic-angle spinning (with dipolar decoupling) for resolution has made ¹³C NMR a standard tool for the characterization of solid polymers. Local structure is revealed by ¹³C chemical shifts and local dynamics from a variety of relaxation measurements in both the laboratory and rotating reference frames. Some success has been achieved in relating these microscopic parameters to macroscopic properties of the polymers. Nevertheless, this important connection is hampered by the fact that conventional ¹³C NMR spectra must be interpreted primarily in terms of a single chain with the effects of neighboring chains inferred indirectly. However, an accurate, direct characterization of the interchain packing in solid, glassy polymers is crucial to the understanding of the mechanical properties of the polymers. Today, the distances between packed chains can be determined by the same types of rotational-echo double resonance (REDOR) NMR experiments developed for proteins and protein complexes. REDOR NMR for an I-S spin pair involves the dephasing of transverse S-spin magnetization by rotor-synchronized I-spin pulses. Coupling to abundant protons is removed using a third radio frequency channel. The results of REDOR experiments lead directly to the strength of the heteronuclear I-S dipolar coupling and hence I-S internuclear distances. In applications on glassy polymers, polycarbonate chains with ¹³C labels have been mixed with chains having ²H labels. The resulting distances determined by ¹³C-²H REDOR NMR define local packing geometry on a system for which diffraction experiments are impossible. Interfaces of blends of heterogeneous polymers like poly(p-fluorostyrene) and ¹³C-labeled polycarbonate can also be characterized quantitatively and accurately by REDOR NMR as shown in the figure below. The residual protons in perdeuterated polymers behave as a rare spin, and REDOR between isolated ¹H-¹³C pairs in perdeuterated poly-carbonate reveals the average location of the ubiquitous 0.3% (by weight) water in glassy polycarbonate.

Selected Publications

• D. R. Studelska, L. M. McDowell, M. Adler, R. D. O'Connor, A. K. Mehta, W. J. Guilford, J. L. Dallas, D. Arnaiz, D. R. Light, and J. Schaefer, "Conformation of a bound inhibitor of blood coagulant factor Xa," Biochemistry, 42, 7942 (2003).
• R. D. O'Connor, B. Poliks, D. H. Bolton, J. M. Goetz, J. A. Byers, K. L. Wooley, and J. Schaefer, "Chain packing in linear phenol-polycarbonate by 13C{2H} REDOR," Macromolecules, 35, 2608 (2002).
• S. J. Kim, L. Cegelski, D. R. Studelska, R. D. O'Connor, A. K. Mehta, and J. Schaefer, "Rotational-echo double resonance characterization of vancomycin binding sites in Staphylococcus aureus," Biochemistry, 41, 6967 (2002).