The "Hidden" Exchange Partner in NMR Spectroscopy. Kinetic Properties and Implications in NOE Analysis

Abstract

The presence of hidden exchange partners is a possible but not generally recognized source of negative effects in the 1H {1H} NOE difference spectra of molecules that fall in the extreme narrowing region. The true origin of these negative signals is not easily identified and this can lead to erroneous molecular structural conclusions. By introducing into the modified Bloch equations an extra term for transition probabilities induced by continuous selective RF irradiation, it is shown that in a two-site problem the "saturability" of the minor signal can be much lower than that of the major signal; this accentuates the possibility of spectral misinterpretations in NOE difference experiments. The occurrence of such a hidden partner in 1H spectroscopy is limited to a relatively narrow range of characteristic dynamical parameters (exchange rates, population ratios, relaxation times, and chemical-shift differences). Diagrams are provided for the assessment of population ratios using peak-height ratios determined from saturation-transfer difference experiments.