Elevating Oxygen

J. Am. Chem. Soc. View Enlarged Image

¹⁷O Spectrum Quadrupole central transition NMR delivers separate ¹⁷O NMR peaks for the four oxygens (O1 to O4) in this protein oxalate ligand—a level of resolution never before obtained for oxygen in proteins. Dashed lines are hydrogen bonds, Arg = arginine, His = histidine, Thr = threonine, Tyr = tyrosine, Gly = glycine, and Ala = alanine.

Oxygen is the ugly duckling of biomolecular nuclear magnetic resonance. Although it is a key atom in proteins and nucleic acids, its siblings carbon, hydrogen, nitrogen, and phosphorus are analyzed far more often because ¹⁷O-based NMR spectra are poorly resolved. Now, in an effort to promote oxygen to its rightful place, researchers have developed a method that makes it more practical to obtain solution NMR spectra of ¹⁷O in biomolecules with much better resolution than was possible before.

Postdoc Jianfeng Zhu and NMR spectroscopist Gang Wu at Queen’s University, in Kingston, Ontario, developed the method (J. Am. Chem. Soc., DOI: 10.1021/ja1079207). Wu and coworkers earlier reported the use of ultrahigh magnetic fields to analyze large protein-ligand complexes for the first time by solid-state ¹⁷O NMR spectroscopy (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201002041). In the new work, they developed a technique called quadrupole central transition NMR that enabled them to extend high-resolution ¹⁷O NMR to the analysis of biomolecules in aqueous solution.

Both studies achieved unprecedented levels of resolution for ¹⁷O NMR, and the solution method increases the size limit of ¹⁷O-containing biomolecules accessible to NMR analysis by nearly three orders of magnitude over previous efforts. The solution technique is the more important, Wu says, “because it permits one to study biological molecules such as proteins in the native state” instead of in a rigid crystalline form.

The work “clearly shows that highly resolved ¹⁷O spectra of even quite large proteins can be obtained at very high magnetic fields,” says Eric Oldfield of the University of Illinois, Urbana-Champaign, who did pioneering work on biomolecular ¹⁷O NMR in the 1980s. “The challenge now is to label proteins and more-complex ligands” because it is difficult and expensive to make ¹⁷O-labeled proteins and complex ligands, he notes.

¹⁷O NMR is difficult because ¹⁷O is a quadrupolar nucleus, a type that generates broad, difficult-to-interpret NMR spectral lines. Zhu and Wu addressed this challenge by applying unusually high magnetic fields and focusing on just a single type of NMR energy change, called the central transition. These strategies—and the tendency of large biomolecules in solution to move slowly, which curbs broadening—enabled the researchers to reduce ¹⁷O NMR line widths and thus improve resolution considerably.

Quadrupole central transition NMR could be used to study bonding effects on oxygen chemical shifts, oxygen-containing reaction intermediate formation, and oxygen-containing ligand binding in biomolecules—such as the binding of oxalate to ovotransferrin, a subject of the new study.

Wu and coworkers believe the new technique could be applicable to protein complexes of up to 500 kDa—and perhaps also make the ¹⁷O ugly duckling a lot less lonely.

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