Application of NMR SHAPES screening to an RNA target

Several NMR screening techniques have been developed in recent years to aid in the identification of lead drug compounds. These NMR methods have traditionally been used for protein targets, and here we examine their applicability for an RNA target. We used the SHAPES compound library to test three different NMR screening methodologies: the saturation transfer difference (STD), the 2D trNOESY, and the WaterLOGSY experiments. We found that the WaterLOGSY experiment was the most sensitive method for our RNA target, the P4P6 domain of the Tetrahymena thermophila Group I intron. Using the WaterLOGSY experiment, we found that 23 of the 112 SHAPES compounds interact with P4P6. To identify which of these 23 hits bind through nonspecific interactions, we counterscreened with a linear duplex RNA control and identified one of the SHAPES compounds as interacting with P4P6 specifically. We thus demonstrated that the WaterLOGSY experiment in combination with the SHAPES compound library can be used to efficiently find RNA binding lead compounds.     

A note on maximal antichains in ordered sets

We examine the question of when two consecutive levels in a product of -chains form an ordered set such that for any antichain, there is a maximal antichain disjoint from it. We characterize the pairs of consecutive levels in the product of t2 -chains that have this property. We also show that there is no upper bound on the heights of ordered sets having this property.The graph of an ordered set is the graph whose points are the elements of the ordered set, and whose edges are the ordered set's 2-element maximal antichains. We construct a class of ordered sets of all widths at least three such that the graph of each ordered set is a path, and we construct an ordered set of infinite width having a connected graph.Research supported by NSERC undergraduate student summer research fellowship, and by NSERC operating grant 69-3378Research supported by ONR contract N00014-85-K-0769     

NMR Reveals That a Highly Reactive Nonheme FeIV=O Complex Retains Its Six-Coordinate Geometry and S=1 State in Solution

The [Fe^IV(O)(Me₃NTB)]²⁺ (Me₃NTB=tris[(1-methyl-benzimidazol-2-yl)methyl]amine) complex 1 has been shown by Mössbauer spectroscopy to have an S=1 ground state at 4 K, but is proposed to become an S=2 trigonal-bipyramidal species at higher temperatures based on a DFT model to rationalize its very high C−H bond-cleavage reactivity. In this work, ¹H NMR spectroscopy was used to determine that 1 does not have C₃-symmetry in solution and is not an S=2 species. Our results show that 1 is unique among nonheme Fe^IV=O complexes in retaining its S=1 spin state and high reactivity at 193 K, providing evidence that S=1 Fe^IV=O complexes can be as reactive as their S=2 counterparts. This result emphasizes the need to identify factors besides the ground spin state of the Fe^IV=O center to rationalize nonheme oxoiron(IV) reactivity.     

19F-detected dual-optimized inverted 1JCC 1,n-ADEQUATE

Modification of the recently reported ¹⁹F-detected 1,1-ADEQUATE experiment that incorporates dual-optimization to selectively invert a wide range of ¹J_CC correlations in a 1,n-ADEQUATE experiment is reported. Parameters for the dual-optimization segment of the pulse sequence were modified to accommodate the increased size of ¹J_CC homonuclear coupling constants of poly- and perfluorinated molecules relative to protonated molecules to allow broadband inversion of the ¹J_CC correlations. The observation and utility of isotope shifts are reported for the first time for 1,1- and 1,n-ADEQUATE correlations.     

Cellulose Derivatives Synthesized via Isocyanate and Activated Ester Pathways in Homogeneous Solutions of Lithium Chloride/N,N-Dimethylacetamide

Abstract

Cellulose carbamate and ester derivatives were synthesized in homogeneous solutions of lithium chloride (LiCl)/N,N-dimemyl-acetamide (DMAc) by the reaction of cellulose with ethyl 4-isocyanatobenzoate and the activated esters of N,N-dimethyl-aminobenzoic acids. Comparative reactions were performed with phenyl isocyanate and the activated ester of benzoic acid. All reactions were followed spectroscopically by FTIR, ¹H NMR, and ¹³C NMR. Degrees of substitution were calculated utilizing UV spectroscopy. The isocyanate reactions are facile allowing controllable degrees of substitution and high yields. By contrast, the activated ester pathway inherently results in lower degrees of substitution and lower yields due in part to undesirable side reactions.     

Using LR-HSQMBC to observe long-range H–N correlations

The recently reported LR-HSQMBC experiment has been optimized for ¹H–¹⁵N long-range heteronuclear couplings. Several previously unreported four-bond correlations, consistent with the predicted by DFT calculations (0.2–0.3 Hz ⁴J_NH couplings), have been observed for strychnine using 2 Hz optimization of the LR-HSQMBC experiment. This experiment offers an advantage over accordion-optimized experiments such as IMPEACH and CIGAR for the observation of long-range ¹H–¹⁵N correlations in that the experiment is refocused and employs a CLIP pulse sequence element to bring the long-range correlations into phase, allowing broadband X-decoupling to be employed during acquisition.     

Mixed‐Metal MIL‐100(Sc,M) (M=Al,Cr, Fe) for Lewis Acid Catalysis and Tandem CC Bond Formation and Alcohol Oxidation

The trivalent metal cations Al³⁺, Cr³⁺, and Fe³⁺ were each introduced, together with Sc³⁺, into MIL‐100(Sc,M) solid solutions (M=Al, Cr, Fe) by direct synthesis. The substitution has been confirmed by powder X‐ray diffraction (PXRD) and solid‐state NMR, UV/Vis, and X‐ray absorption (XAS) spectroscopy. Mixed Sc/Fe MIL‐100 samples were prepared in which part of the Fe is present as α‐Fe₂O₃ nanoparticles within the mesoporous cages of the MOF, as shown by XAS, TGA, and PXRD. The catalytic activity of the mixed‐metal catalysts in Lewis acid catalysed Friedel–Crafts additions increases with the amount of Sc present, with the attenuating effect of the second metal decreasing in the order Al>Fe>Cr. Mixed‐metal Sc,Fe materials give acceptable activity: 40 % Fe incorporation only results in a 20 % decrease in activity over the same reaction time and pure product can still be obtained and filtered off after extended reaction times. Supported α‐Fe₂O₃ nanoparticles were also active Lewis acid species, although less active than Sc³⁺ in trimer sites. The incorporation of Fe³⁺ into MIL‐100(Sc) imparts activity for oxidation catalysis and tandem catalytic processes (Lewis acid+oxidation) that make use of both catalytically active framework Sc³⁺ and Fe³⁺. A procedure for using these mixed‐metal heterogeneous catalysts has been developed for making ketones from (hetero)aromatics and a hemiacetal.     

HMBC‐1,1‐ADEQUATE via generalized indirect covariance: a high sensitivity alternative to n,1‐ADEQUATE

1,1‐ADEQUATE and the related long‐range 1,n‐ and n,1‐ADEQUATE variants were developed to provide an unequivocal means of establishing ²J_CH and the equivalent of ⁿJ_CH correlations where n = 3,4. Whereas the 1,1‐ and 1,n‐ADEQUATE experiments have two simultaneous evolution periods that refocus the chemical shift and afford net single quantum evolution for the carbon spins, the n,1‐variant has a single evolution period that leaves the carbon spin to be observed at the double quantum frequency. The n,1‐ADEQUATE experiment begins with an HMBC‐type ⁿJ_CH magnetization transfer, which leads to inherently lower sensitivity than the 1,1‐ and 1,n‐ADEQUATE experiments that begin with a ¹J_CH transfer. These attributes, in tandem, serve to render the n,1‐ADEQUATE experiment less generally applicable and more difficult to interpret than the 1,n‐ADEQUATE experiment, which can in principle afford the same structural information. Unsymmetrical and generalized indirect covariance processing methods can complement and enhance the structural information encoded in combinations of experiments e.g. HSQC‐1,1‐ or ?1,n‐ADEQUATE. Another benefit is that covariance processing methods offer the possibility of mathematically combining a higher sensitivity 2D NMR spectrum with for example 1,1‐ or 1,n‐ADEQUATE to improve access to the information content of lower sensitivity congeners. The covariance spectrum also provides a significant enhancement in the F₁ digital resolution. The combination of HMBC and 1,1‐ADEQUATE spectra is shown here using strychnine as a model compound to derive structural information inherent to an n,1‐ADEQUATE spectrum with higher sensitivity and in a more convenient to interpret single quantum presentation. Copyright © 2012 John Wiley & Sons, Ltd.