Determination of 3J(HNi,C'i) coupling constants in proteins with the C′-FIDS method

We introduce the C'-FIDS-1H,15N-HSQC experiment, a new method for the determination of 3J(HinfisupN,Cinfisup') coupling constants in proteins, yielding information about the torsional angle ?. It relies on the 1H,15N-HSQC or HNCO experiment, two of the the most sensitive heteronuclear correlation experiments for isotopically labeled proteins. A set of three 1H,15N-HSQC or HNCO spectra are recorded: a reference experiment in which the carbonyl spins are decoupled during t1 and t2, a second experiment in which they are decoupled exclusively during t1 and a third one in which they are coupled in t1 as well as t2. The last experiment yields an E.COSY-type pattern from which the 2J(HinfisupN,Cinfi-1sup') and 1J(Ni,Cinfi-1sup') coupling constants can be extracted. By comparison of the coupled multiplet (obtained from the second experiment) with the decoupled multiplet (obtained from the first experiment) convoluted with the 2J(HinfisupN,Cinfi-1sup') coupling, the 3J(HinfisupN,Cinfisup') coupling can be found in a one-parameter fitting procedure. The method is demonstrated for the protein rhodniin, containing 103 amino acids. Systematic errors due to differential relaxation are small for nJ(HN,C') couplings in biomacromolecules of the size currently under NMR spectroscopic investigation.