Sensitivity-enhanced detection of non-labile proton and carbon NMR spectra on water resonances

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.

Indirectly detected satellite-transition quadrupolar NMR via progressive saturation of the proton reservoir

Static satellite-transitions (ST) NMR line shapes from half-integer quadrupolar nuclei could be very informative: they can deliver insight about local motions over a wide range of timescales, and can report on small changes in the local electronic environments as reflected by variations in the quadrupolar parameters. Satellite transitions, however, are typically "invisible" for half-integer quadrupolar nuclei due to their sheer breadth, leading to low signal-to-noise ratio -especially for unreceptive low-gamma or dilute quadrupolar nuclei. Very recently we have introduced a method for enhancing the NMR sensitivity of unreceptive X nuclei in static solids dubbed PROgressive Saturation of the Proton Reservoir (PROSPR), which opens the possibility of magnifying the signals from such spins by repeatedly imprinting frequency-selective X-driven depolarizations on the much more sensitive ¹H NMR signal. Here, we show that PROSPR's efficacy is high enough for enabling the detection of static ST NMR for challenging species like ³⁵Cl, ³³S and even ¹⁷O -all at natural-abundance. The ensuing ST-PROSPR NMR experiment thus opens new approaches to probe ultra-wideline (6-8 MHz wide) spectra. These highly pronounced anisotropies can in turn deliver new vistas about dynamic changes in solids, as here illustrated by tracking ST line shapes as a function of temperature during thermally-driven events.

Sensitivity Enhancement by Progressive Saturation of the Proton Reservoir: A Solid-State NMR Analogue of Chemical Exchange Saturation Transfer

Chemical exchange saturation transfer (CEST) enhances solution-state NMR signals of labile and otherwise invisible chemical sites, by indirectly detecting their signatures as a highly magnified saturation of an abundant resonance─for instance, the 1H resonance of water. Stimulated by this sensitivity magnification, this study presents PROgressive Saturation of the Proton Reservoir (PROSPR), a method for enhancing the NMR sensitivity of dilute heteronuclei in static solids. PROSPR aims at using these heteronuclei to progressively deplete the abundant 1H polarization found in most organic and several inorganic solids, and implements this 1H signal depletion in a manner that reflects the spectral intensities of the heteronuclei as a function of their chemical shifts or quadrupolar offsets. To achieve this, PROSPR uses a looped cross-polarization scheme that repeatedly depletes 1H-1H local dipolar order and then relays this saturation throughout the full 1H reservoir via spin-diffusion processes that act as analogues of chemical exchanges in the CEST experiment. Repeating this cross-polarization/spin-diffusion procedure multiple times results in an effective magnification of each heteronucleus's response that, when repeated in a frequency-stepped fashion, indirectly maps their NMR spectrum as sizable attenuations of the abundant 1H NMR signal. Experimental PROSPR examples demonstrate that, in this fashion, faithful wideline NMR spectra can be obtained. These 1H-detected heteronuclear NMR spectra can have their sensitivity enhanced by orders of magnitude in comparison to optimized direct-detect experiments targeting unreceptive nuclei at low natural abundance, using modest hardware requirements and conventional NMR equipment at room temperature.

The Incorporation of Labile Protons into Multidimensional NMR Analyses: Glycan Structures Revisited

Glycan structures are often stabilized by a repertoire of hydrogen-bonded donor/acceptor groups, revealing longer-lived structures that could represent biologically relevant conformations. NMR provides unique data on these hydrogen-bonded networks from multidimensional experiments detecting cross-peaks resulting from through-bond (TOCSY) or through-space (NOESY) interactions. However, fast OH/H₂O exchange, and the spectral proximity among these NMR resonances, hamper the use of glycans’ labile protons in such analyses; consequently, studies are often restricted to aprotic solvents or supercooled aqueous solutions. These nonphysiological conditions may lead to unrepresentative structures or to probing a small subset of accessible conformations that may miss “active” glycan conformations. Looped, projected spectroscopy (L-PROSY) has been recently shown to substantially enhance protein NOESY and TOCSY cross-peaks, for ¹Hs that undergo fast exchange with water. This study shows that even larger enhancements can be obtained for rapidly exchanging OHs in saccharides, leading to the retrieval of previously undetectable 2D TOCSY/NOESY cross-peaks with nonlabile protons. After demonstrating ≥300% signal enhancements on model monosaccharides, these experiments were applied at 1 GHz to elucidate the structural network adopted by a sialic acid homotetramer, used as a model for α,2–8 linked polysaccharides. High-field L-PROSY NMR enabled these studies at higher temperatures and provided insight previously unavailable from lower-field NMR investigations on supercooled samples, involving mostly nonlabile nuclei. Using L-PROSY’s NOEs and other restraints, a revised structural model for the homotetramer was obtained combining rigid motifs and flexible segments, that is well represented by conformations derived from 40 μs molecular dynamics simulations.

Sensitivity enhancement of homonuclear multidimensional NMR correlations for labile sites in proteins, polysaccharides, and nucleic acids

Multidimensional TOCSY and NOESY are central experiments in chemical and biophysical NMR. Limited efficiencies are an intrinsic downside of these methods, particularly when targeting labile sites. This study demonstrates that the decoherence imparted on these protons through solvent exchanges can, when suitably manipulated, lead to dramatic sensitivity gains per unit time in the acquisition of these experiments. To achieve this, a priori selected frequencies are encoded according to Hadamard recipes, while concurrently subject to looped selective inversion or selective saturation procedures. Suitable processing then leads to protein, oligosaccharide and nucleic acid cross-peak enhancements of ≈200–1000% per scan, in measurements that are ≈10-fold faster than conventional counterparts. The extent of these gains will depend on the solvent exchange and relaxation rates of the targeted sites; these gains also benefit considerably from the spectral resolution provided by ultrahigh fields, as corroborated by NMR experiments at 600 MHz and 1 GHz. The mechanisms underlying these experiments’ enhanced efficiencies are analyzed on the basis of three-way polarization transfer interplays between the water, labile and non-labile protons, and the experimental results are rationalized using both analytical and numerical derivations. Limitations as well as further extensions of the proposed methods, are also discussed.

Here, the authors present an approach that enhances the sensitivity of basic 2D biomolecular NMR experiments like NOESY and TOCSY, when carried out in polysaccharides, proteins and nucleic acids. This method combines principles associated to quantum Anti-Zeno Effects and advanced data acquisition methods based on Hadamard multiplexing.

Looped-PROjected SpectroscopY (L-PROSY): A simple approach to enhance backbone/sidechain cross-peaks in 1H NMR

Cross-relaxation and isotropic mixing phenomena leading to the Nuclear Overhauser Effect (NOE) and to the TOCSY experiment, lie at the center of structural determinations by NMR. 2D TOCSY and NOESY exploit these polarization transfer effects to determine inter-site connectivities and molecular geometries under physiologically-relevant conditions. Among these sequences' drawback, particularly for the case of NOEs, are a lack of sensitivity arising from small structurally-relevant cross peaks. The present study explores the application of multiple Zeno-like projective measurements, to enhance the cross-peaks between spectrally distinct groups in proteins -in particular between amide and aliphatic protons. The enhancement is based on repeating the projection done by Ramsey or TOCSY blocks multiple times, in what we refer to as Looped, PROjected Spectroscopy (L-PROSY). This leads to a reset of the amide/aliphatic transfer processes; the initial slopes of the NOE- or J-transfer effects thus define the cross-peak growth, and a faster cross-peak buildup is achieved upon looping these transfers over the allotted time T₁. These projections also help to better preserve the magnetization originating in the amides, resulting in an overall improvement in sensitivity. L-PROSY's usefulness is demonstrated by incorporating it into two widely used protein NMR experiments: 2D ¹⁵N-¹H HMQC-NOESY and ¹⁵N-filtered 2D NOESY. Different parameters dictating the overall SNR improvement, particularly the protein correlation times and the amide-water chemical exchange rates, were examined, and L-PROSY's enhancements resulted for all tested proteins. The largest cross-peak enhancements were observed for unstructured proteins, where chemical exchanges with the solvent of the kind that tend to average out NOE cross-peaks in conventional NMR, boost L-PROSY's cross-peaks by replenishing the amide's magnetizations within each loop. Enhanced cross-peaks were also found in extensions involving TOCSY-based experiments when applied to proteins with unfolded segments.