Tuning the HNN experiment: generation of serine-threonine check points.

We describe here the tunability of the HNN experiment to obtain certain residue specific peak patterns in the spectra of ((15)N, (13)C) labeled proteins. This is achieved by tuning a band-selective 180 degrees pulse on the carbon channel in the pulse sequence, whereby one can tamper with the C(alpha)-C(beta) coupling evolutions for the different residues. Specifically, we generate distinctive peak patterns for serine and threonine and their neighbors in the different planes of the three dimensional spectrum. These provide useful anchor points during sequential assignment of backbone resonances. The performance of this experiment, referred to as HNN-ST here, is demonstrated using two proteins, one properly folded and the other completely denatured. With the availability of high field spectrometers, techniques such as TROSY, and ever increasing sensitivities in the probes, this experiment with its large number of check points has a great potential for rapid and unambiguous backbone resonance assignment in large proteins.