A three-dimensional NMR experiment with improved sensitivity for carbonyl-carbonyl J correlation in proteins

Recently, a quantitative J correlation technique has been presented that permits measurement of 3JC'C' in proteins isotopically enriched with 13C [Hu, J.-S. and Bax, A.(1996) J. Am. Chem. Soc., 118, 8170-8171]. Here, we describe an analogous experiment that is less sensitive to transverse 13C' relaxation, which is the principal limiting factor in all 13C-13C long-range correlation experiments on macromolecules. The new scheme utilizes homonuclear Hartmann-Hahn cross polarization (TOCSY) instead of a COSY-type transfer to accomplish magnetization transfer; a description of the relevant relaxation terms is presented. The experiment is demonstrated for ubiquitin and HIV-1 Nef.The results show excellent agreement between 3JC'C' values measured for ubiquitin with the new scheme and those reported previously. The experiment is particularly useful for distinguishing backbone φ angles that are smaller than -120 deg. from those larger than -120 deg