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 phi angles that are smaller than -120 degrees from those larger than -120 degrees.