Mechanical Behavior of Polymer Gels for RDCs and RCSAs Collection: NMR Imaging Study of Buckling Phenomena

An Acrylonitrile-Based Copolymer Gel as an NMR Alignment Medium for Extraction of Residual Dipolar Couplings of Small Molecules in Aqueous Solution

An NMR weakly-aligning polymer gel has been prepared by copolymerization of acrylonitrile and 2-acrylamide-2-methyl-1-propanesulfonic acid in the presence of 1,4-butanediol diacrylate as a cross-linker. The polymer readily swells in water in a large range of temperatures, although the swelling ratio is decreased in saline solutions. The swollen gel can be mechanically compressed, in a reversible way, generating anisotropy, as easily shown in ² H NMR experiments, and allowing measurement of ¹ D_CH residual dipolar couplings (RDCs) through F1-coupled HSQC experiments. The performance of this gel as a NMR alignment medium was evaluated in several water-soluble organic molecules and, while it provided RDCs of proper size for sucrose and even such as small molecule as 5-norbornen-2-ol, in the case of azidothymidine and cefuroxime sodium salt the strong interaction of these molecules with the gel prevented successful extraction of the RDCs.

Connection of Isolated Stereoclusters by Combining 13C-RCSA, RDC, and J-Based Configurational Analyses and Structural Revision of a Tetraprenyltoluquinol Chromane Meroterpenoid from Sargassum muticum

The seaweed Sargassum muticum, collected on the southern coast of Galicia, yielded a tetraprenyltoluquinol chromane meroditerpene compound known as 1b, whose structure is revised. The relative configuration of 1b was determined by J-based configurational methodology combined with an iJ/DP4 statistical analysis and further confirmed by measuring two anisotropic properties: carbon residual chemical shift anisotropies (13C-RCSAs) and one-bond 1H-13C residual dipolar couplings (1DCH-RDCs). The absolute configuration of 1b was deduced by ECD/OR/TD-DFT methods and established as 3R,7S,11R.

Single experiment measurement of residual dipolar couplings in aqueous solution using a biphasic bisperylene imide chromonic liquid crystal

The use of the biphasic isotropic/nematic region in a bisperylene imide-based lyotropic liquid crystal system allows the extraction of proton-carbon ¹ D_CH residual dipolar couplings in aqueous solution from a single F1-coupled HSQC experiment. The method was successfully applied to the RDC-based conformational analysis of sucrose.

"One-Shot" Measurement of Residual Chemical Shift Anisotropy Using Poly-γ-benzyl-l-glutamate as an Alignment Medium

A method for the measurement of residual chemical shift anisotropy in one experiment using a biphasic isotropic/anisotropic lyotropic liquid crystalline medium based on poly-γ-benzyl-l-glutamate as the alignment medium is presented. This approach is demonstrated on the model compound strychnine and neotricone, a depsidone natural product with a questionable structural assignment based on comparison with the closely related excelsione and in-depth density functional theory calculations.

Chemically cross-linked polyacrylonitrile. A DMSO compatible NMR alignment medium for measurement of residual dipolar couplings and residual chemical shift anisotropies

Chemically cross-linked polyacrylontrile polymer gels, have been prepared as an alignment medium compatible with DMSO-d₆. These gels allow measurement of residual dipolar couplings (RDCs) and residual chemical shift anisotropies (RCSAs) with good accuracy as tested with brucine and α-santonin natural compounds. The gels can be reversibly compressed allowing easy measurement of RCSAs. They also present good physical homogeneity, clean HSQC spectra with little background ¹H signals, and allow unambiguous referencing of ¹³C spectra for RCSA extraction.

Cyclic Peptide Design Guided by Residual Dipolar Couplings, J-Couplings, and Intramolecular Hydrogen Bond Analysis

Cyclic peptides have long tantalized drug designers with their potential ability to combine the best attributes of antibodies and small molecules. An ideal cyclic peptide drug candidate would be able to recognize a protein surface like an antibody while achieving the oral bioavailability of a small molecule. It has been hypothesized that such cyclic peptides balance permeability and solubility using their solvent-dependent conformational flexibility. Herein we report a conformational deconvolution NMR methodology that combines residual dipolar couplings, J-couplings, and intramolecular hydrogen bond analysis along with conformational analysis using molecular dynamics simulations and density functional theory calculations for studying cyclic peptide conformations in both low-dielectric solvent (chloroform) and high-dielectric solvent (DMSO) to experimentally study the solvent-dependent conformational change hypothesis. Taken together, the combined experimental and computational approaches can illuminate conformational ensembles of cyclic peptides in solution and help identify design opportunities for better permeability.

Measurement of residual chemical shift anisotropies in compressed polymethylmethacrylate gels. Automatic compensation of gel isotropic shift contribution

Mechanical compression of polymer gels provides a simple way for the measurement of residual chemical shift anisotropies, which then can be employed, on its own, or in combination with residual dipolar couplings, for structural elucidation purposes. Residual chemical shift anisotropies measured using compression devices needed a posteriori correction to account for the increase of the polymer to solvent ratio inside the swollen gel. This correction has been cast before in terms of a single-free parameter which, as shown here, can be simultaneously optimized along with the components of the alignment tensor while still retaining discriminating power of the different relative configurations as illustrated in the stereochemical analysis of α-santonin and 10-epi-8-deoxycumambrin B.

Current developments in homonuclear and heteronuclear J-resolved NMR experiments

Two-dimensional J-resolved (Jres) NMR experiments offer a simple, user-friendly spectral representation where the information of coupling constants and chemical shifts are separated into two orthogonal frequency axis. Since its initial proposal 40 years ago, Jres has been the focus of considerable interest both in improving the basic pulse sequence and in its successful application to a wide range of studies. Here, the latest developments in the design of novel Jres pulse schemes are reviewed, mainly focusing on obtaining pure absorption lineshapes, minimizing strong coupling artifacts, and also optimizing sensitivity and experimental measurements. A discussion of several Jres versions for the accurate measurement of a different number of homonuclear (J_HH ) and heteronuclear (J_CH ) coupling constants is presented, accompanied by some illustrative examples.

Computer-Assisted 3D Structure Elucidation (CASE-3D) of Natural Products Combining Isotropic and Anisotropic NMR Parameters

A computer-assisted structural elucidation (CASE-3D) strategy based on the use of isotropic and/or anisotropic NMR data is proposed to elucidate relative configuration and preferred conformation in complex natural products. The methodology involves the selection of conformational models through the use of the Akaike Information Criterion and scoring of the different configurations. As illustrative examples, the methodology furnished the correct configuration of the already known compounds artemisinin (1) and homodimericin A (2). Revised structures (5 and 6), including their absolute configuration, for the recently reported curcusones I (3) and J (4) are proposed.

2JHH-resolved HSQC: Exclusive determination of geminal proton-proton coupling constants

The measurement of two-bond proton-proton coupling constants (²J_HH) in prochiral CH₂ groups from the F2 dimension of 2D spectra is not easy due to the usual presence of complex multiplet J patterns, line broadening effects and strong coupling artifacts. These drawbacks are particularly pronounced and frequent in AB spin systems, as those normally exhibited by the pair of diastereotopic CH₂ protons. Here, a novel ²J_HH-resolved HSQC experiment for the exclusive and accurate determination of the magnitude of ²J_HH from the doublet displayed along the highly-resolved indirect F1 dimension is described. A pragmatic ²J_HH NMR profile affords a fast overview of the full range of existing ²J_HH values. In addition, a ²J_HH/δ(¹³C)-scaled version proves to be an efficient solution when severe signal overlapping complicate a rigorous analysis. The performance of the method is compared with other current techniques and illustrated by the determination of challenging residual dipolar ²D_HH coupling constants of small molecules dissolved in weakly orienting media.

Perfect 1JCH-resolved HSQC: Efficient measurement of one-bond proton-carbon coupling constants along the indirect dimension

A versatile ¹J_CH-resolved HSQC pulse scheme for the speedy, accurate and automated determination of one-bond proton-carbon coupling constants is reported. The implementation of a perfectBIRD element allows a straightforward measurement from the clean doublets obtained along the highly resolved F1 dimension, even for each individual ¹J_CHa and ¹J_CHb in diastereotopic H_aCH_b methylene groups. Real-time homodecoupling during acquisition and other alternatives to minimize accidental signal overlapping in overcrowded spectra are also discussed.