Juan A. Aguilar

Reversible interactions with para-hydrogen enhance NMR sensitivity by polarization transfer.

The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.

Pure Shift 1H NMR: A Resolution of the Resolution Problem?

Suppressing multiplet structure in 1H??NMR spectra offers a large improvement in spectral resolution (see picture), equivalent to the use of a spectrometer in the GHz range. Such ldquopure shiftrdquo techniques are readily extended to multidimensional methods, for example DOSY.

Simultaneously Enhancing Spectral Resolution and Sensitivity in Heteronuclear Correlation NMR Spectroscopy

BIRDs eye view: Adding periodic BIRD J-refocusing (BIRD=bilinear rotation decoupling) to data acquisition in an HSQC experiment causes broadband homonuclear decoupling, giving a single signal for each proton chemical shift. This pure shift method improves both resolution and signal-to-noise ratio, without the need for special data processing.

True Chemical Shift Correlation Maps: A TOCSY Experiment with Pure Shifts in Both Dimensions

Signal resolution in (1)H NMR is limited primarily by multiplet structure. Recent advances in pure shift NMR, in which the effects of homonuclear couplings are suppressed, have allowed this limitation to be circumvented in 1D NMR, gaining almost an order of magnitude in spectral resolution. Here for the first time an experiment is demonstrated that suppresses multiplet structure in both domains of a homonuclear two-dimensional spectrum. The principle is demonstrated for the TOCSY experiment, generating a chemical shift correlation map in which a single peak is seen for each coupled relationship, but the principle is general and readily extensible to other homonuclear correlation experiments. Such spectra greatly simplify manual spectral analysis and should be well-suited to automated methods for structure elucidation.

Decoupling Two‐Dimensional NMR Spectroscopy in Both Dimensions: Pure Shift NOESY and COSY

An increase in the resolving power in 2D NMR spectra is obtained by collapsing 2D signals with multiplet structure into 2D singlets. This resolution gain is achieved by combining 2D experiments with pure shift techniques and covariance processing (see picture). The method should be of value in both manual and automated structure determination.

Copper complexes of polyaza[n]cyclophanes and their interaction with DNA and RNA ☆

Abstract The complexation properties of Cu 2+ ions with the cyclophane receptors 2,6,9,13-tetraaza[14]metacyclophane ( L1 ), 2,6,10,13,17,21-hexaaza[22]metacyclophane ( L2 ) and 2,6,10,13,17,21-hexaaza[22]paracyclophane ( L3 ) are presented. Formation of mononuclear complexes in the case of L1 and of mono- and binuclear complexes in the case of the hexaaazcyclophane ligands L2 and L3 is observed. The coordination numbers around each Cu 2+ in the binuclear complexes involve at most three nitrogen donors for each metal ion. Cyclophanes L1 – L3 and several acyclic polyamine ligands are tested for their affinity towards double-stranded nucleic acid models of RNA and DNA. The binding affinity of the acyclic and macrocyclic polyamines towards DNA and RNA models, measured by changes in their melting temperature Δ T M , increases progressively with the average number of charges present on the amine, with higher stabilisation for RNA. Unusually large differences of up to Δ(Δ T M )=30°C were observed with tripropylenetetraamine ( L15 ) and with the macrocyclic amines ( L2 and L3 ). Introduction of copper in the ligands leads to considerable affinity variations. One macrocyclic copper complex (with L3 ) shows a strong discrimination between the RNA and DNA polymers with a record value of ΔΔ T M =41°C, and a small destabilisation of the DNA. The copper complexes exhibit nuclease activity; with plasmid DNA nicking was increased by one complex by a factor of k / k un =10 7 . Experiments with hydroxyl radical quenchers indicate a predominant redox cleavage mechanism.

Only para-hydrogen spectroscopy (OPSY), a technique for the selective observation of para-hydrogen enhanced NMR signals.

An NMR method is reported for the efficient removal of signals derived from nuclei with thermally equilibrated spin state populations whilst leaving, intact, signals derived from para-hydrogen induced polarisation (PHIP) through gradient assisted coherence selection.

Multifunctional molecular recognition of ATP, ADP and AMP nucleotides by the novel receptor 2,6,10,13,17,21-hexaaza[22]metacyclophane

The novel cyclophane receptor 2,6,10,13,17,21-hexaaza[22]metacyclophane L presents a molecular architecture which enables recognition in aqueous solution of ATP, ADP and AMP through electrostatic, hydrogen bonding and π-stacking interactions; electrostatic interactions occur between the polyammonium sites of L and the phosphate chain of the nucleosides, and π-stacking interactions occur between the m-phenylene subunit incorporated in the receptor as a non-pendant integral part of the macrocyclic framework and the adenine ring of the nucleotides.

J-modulation effects in DOSY experiments and their suppression: The Oneshot45 experiment

Diffusion-ordered spectroscopy (DOSY) is a powerful NMR method for identifying compounds in mixtures. DOSY experiments are very demanding of spectral quality; even small deviations from expected behaviour in NMR signals can cause significant distortions in the diffusion domain. This is a particular problem when signals overlap, so it is very important to be able to acquire clean data with as little overlap as possible. DOSY experiments all suffer to a greater or lesser extent from multiplet phase distortions caused by J-modulation, requiring a trade-off between such distortions and gradient pulse width. Multiplet distortions increase spectral overlap and may cause unexpected and misleading apparent diffusion coefficients in DOSY spectra. These effects are described here and a simple and effective remedy, the addition of a 45° purging pulse immediately before the onset of acquisition to remove the unwanted anti-phase terms, is demonstrated. As well as affording significantly cleaner results, the new method allows much longer diffusion-encoding pulses to be used without problems from J-modulation, and hence greatly increases the range of molecular sizes that can be studied for coupled spin systems. The sensitivity loss is negligible and the added phase cycling is modest. The new method is illustrated for a widely-used general purpose DOSY pulse sequence, Oneshot.

Simultaneous enhancement of chemical shift dispersion and diffusion resolution in mixture analysis by diffusion-ordered NMR spectroscopy

Mixture analysis by high resolution diffusion-ordered NMR spectroscopy (HR-DOSY) requires differences in both chemical shift and diffusion coefficient; resolution can be greatly enhanced by exploiting the chemical specificity of lanthanide shift reagent binding to increase chemical shift and diffusion dispersion simultaneously.