Isotope enrichment of NMR active atoms with low natural abundance, in particular 13C and 15N, has been a means to use NMR active probes that are selectively enhanced over background signals by a factor given by their isotope enrichment. NMR spectroscopy is understood from first principles and the interaction between magnetic moments can be used to enhance otherwise weak signals in a controlled manner by transfer of polarization from spins with high magnetic moments (usually protons and electrons) to nuclear spins with lower magnetic moments (e.g., 13C and 15N). During the last decade, a new generation of nuclear magnetic resonance probes has become popular that affords signal improvements relative to spectral noise and biological backgrounds of at least 3�C4 orders of magnitude.
This review consecutively covers nuclear spin hyperpolarization, assay designs for hyperpolarized NMR probing, emerging strategies and applications using designed and natural probes, current technological developments and future hopes for NMR assays based on hyperpolarized probes and labels. Several excellent reviews have recently described the development of hyperpolarized contrast agents for functional magnetic resonance imaging [6�C9], an application area that is therefore not discussed herein.2.?Hyperpolarization of Molecular ProbesHigh-resolution nuclear magnetic resonance (NMR) spectroscopy has established itself as a principal detection modality in a remarkable variety of disciplines [10�C12].
In the life sciences, many of these applications rely on the use of NMR for retrieving molecular information Brefeldin_A in close to natural environments and intact biofluids, often in order to probe molecular recognition events and biocatalysis. A principal shortcoming of NMR spectroscopy has remained its moderate sensitivity owing to the low equilibrium polarization of nuclear spins as defined for spin-1/2 nuclei by:Peq=n??n+n?+n+?tanh��?B02kbT(1)where n? and n+ are the numbers of nuclear spins in the lower and higher energy Zeeman eigenstates, ��B0 is the energy gap between the Zeeman eigenstates and kbT is the thermal energy [13]. The equilibrium nuclear spin determines the fraction of nuclear spins contributing to the detected signal. This fraction remains well below 0.1% for all nuclear spins at currently available NMR spectrometer fields (Figure 1).Figure 1.(A) Spin polarizations of electrons (��e��), 1H, 13C and 15N nuclei in a 3.35 Tesla DNP polarizer near liquid helium temperature, compared to spin polarizations of 1H, 13C and 15N in a 14.1 Tesla (600 MHz) spectrometer at 273�C373 …