HNCO-based measurement of one-bond amide 15N-1H couplings with optimized precision.

A pair of 3D HNCO-based experiments have been developed with the aim of optimizing the precision of measurement of (1)J(NH) couplings. Both pulse sequences record (1)J(NH) coupling evolution during the entire constant time interval that (15)N magnetization is dephasing or rephasing with respect to the directly bonded (13)C' nucleus, with (15)N(13)C' multiple quantum coherence maintained during the (13)C' evolution period. The first experiment, designed for smaller proteins, produces an apparent doubling of the (1)J(NH) coupling without any accompanying increases in line width. The second experiment is a J-scaled TROSY-HNCO experiment in which the (1)J(NH) coupling is measured by frequency difference between resonances offset symmetrically about the position of the downfield component of the (15)N doublet (i.e. the TROSY resonance). This experiment delivers significant gains in precision of (1)J(NH) coupling measurement compared to existing J-scaled TROSY-HNCO experiments. With the proper choice of acquisition parameters and sufficient sensitivity to acquire a 3D TROSY-HNCO experiment, it is shown that (1)J(NH) couplings can be measured with a precision which approaches or exceeds the precision of measurement with which the frequency of the TROSY resonance itself can be determined.