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

Abstract

Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or Δν ), appear in solution-state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p-HEMA) is an easy, user-friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro-imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross-linker and the length of the gel.

New Stretching Method for Aligning Gels: Its Application to the Measurement Residual Chemical Shift Anisotropies (RCSAs) without the Need for Isotropic Shift Correction

“…In this method, when the gel is compressed, the 1D 2 H spectrum shows a doublet corresponding to the quadrupolar splitting (Δ ν Q ) of the solvent inside the gel and a singlet from the solvent present in isotropic pockets outside the gel. This singlet is observed even at full compression, as shown in Figure A and as described previously . With this method, RDCs can be straightforwardly applied to the structural analysis of small molecules.…”

“…In other words, the ratio of gel to solution increases inducing isotropic chemical shift variations. These variations change linearly with the degree of compression . In order to avoid these complications, as well as to avoid the use of open‐ended tubes, we have developed a new method that involves the modification of an existing device.…”

“…of 4 mm; in the original device it was of 5 or 6 mm, depending on the desired conditions. With the larger diameter the gels tend to buckle when swollen due to their elasticity and smaller diameter (≈3.8 mm) . This buckling effect randomly breaks some gels; but with the new device this buckling effect is restricted.…”

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

“…The difference in chemical shift between signals at maximum and minimum compression depends on the RCSA and the isotropic contribution. However, in the minimum compression state, a layer of isotropic solution surrounds the gel, leading to the observation of analyte carbon signals from inside (anisotropic: ) and outside (isotropic: ) of the polymer gel. The Δ in and refers to the calibration of the chemical shifts with respect to a reference signal that can be freely chosen .…”

Current developments in homonuclear and heteronuclear J‐resolved NMR experiments

“…The outstanding features of J ‐resolved HSQC experiments to perform accurate measurements along F 1 have been demonstrated in the direct and challenging determination of both the magnitude and the sign of 1 D CH and 2 D HH RDCs of 1 weakly aligned in a CDCl 3 –compatible poly(methyl methacrylate) (PMMA) gel . The use of mechanically compressed PMMA gels as orienting media is supported by several reasons: (a) simple and fast sample preparation, (b) the sample can be ready for NMR measurement in less than 24 hr, (c) the gel can be cleaned and recovered for later use, (d) reversible compression allows collect complementary NMR experiments at different alignment degrees with the same NMR tube, (e) isotropic and anisotropic environments can be visualized and measured simultaneously, (f) optimum lineshapes are achievable for the anisotropic components even at maximum compressions, and (g) measurable 1 D CH and 2 D HH of similar magnitudes are available for alignments around Δν Q ( 2 H) = 30–40 Hz. As an example, Figure shows the perfect 1 J CH ‐resolved HSQC spectrum of 3 dissolved in a CDCl 3 ‐soluble PMMA gel media.…”