Mapping Molecular Perturbations by a New Form of Two-Dimensional Spectroscopy

We propose a new general form of two-dimensional spectroscopy where the indirect “evolution” dimension is derived using the Radon transform. This idea is applicable to several types of spectroscopy but is illustrated here for the case of NMR spectroscopy. This “projection spectroscopy” displays characteristic correlation peaks that highlight perturbations of chemical shifts caused by temperature, pressure, solvent, molecular binding, chemical exchange, hydrogen bonding, pH variations, conformational changes, or paramagnetic agents. The results are displayed in a convenient format that allows the chemist to see all of the chemical shift perturbations at a glance and assess their rates of change and directions. As a proof of principle, we present two simple, practical examples that display two-dimensional representations of the effects of temperature and solvent on NMR spectra

Relaxation-Assisted Magnetization Transfer Phenomena for a Sensitivity-Enhanced 2D NMR

2D NOESY and TOCSY play central roles in contemporary NMR. We have recently discussed how solvent-driven exchanges can significantly enhance the sensitivity of such methods when attempting correlations between labile and nonlabile protons. This study explores two scenarios where similar sensitivity enhancements can be achieved in the absence of solvent exchange: the first one involves biomolecular paramagnetic systems, while the other involves small organic molecules in natural abundance. It is shown that, in both cases, the effects introduced by either differential paramagnetic shift and relaxation or by polarization sharing among networks of protons can provide a similar sensitivity boost, as previously discussed for solvent exchange. The origin and potential of the resulting enhancements are analyzed, and experiments that demonstrate them in protein and natural products are exemplified. Limitations and future improvements of these approaches are also briefly discussed