Probing chemical shifts of invisible states of proteins with relaxation dispersion NMR spectroscopy: how well can we do?

New labeling approach and NMR experiments are described along with those that has probed the exchange by using [super 15]N and [super 1]H N nuclei. The experimental data has showed that for many systems accurate excited-state chemical shifts can be measured, thus opening a possibility of obtaining quantitative structural information of invisible states of the sort that is recorded on ground states of proteins.