New Application of Proton Nuclear Spin Relaxation Unraveling the Intermolecular Structural Features of Low-Molecular-Weight Organogel Fibers

Abstract

Proton nuclear spin relaxation has been for the first time extensively used for a structural and dynamical study of low-molecular-weight organogels. The gelator in the present study is a modified phenylalanine amino acid bearing a naphthalimide moiety. From T(1) (spin-lattice relaxation time in the laboratory frame) and T(1ρ) (spin-lattice relaxation time in the rotating frame) measurements, it is shown that the visible gelator NMR spectrum below the liquid-gel transition temperature corresponds to a so-called isotropic compartment, where gelator molecules behave as in a liquid phase but exchange rapidly with the molecules constituting the gel structure. This feature allows one to derive, from accessible parameters, information about the gel itself. Nuclear Overhauser effect spectroscopy (NOESY) experiments have been exploited in view of determining not only cross-relaxation rates but also specific longitudinal rates. The whole set of relaxation parameters (at 25 °C) leads to a correlation time of 5 ns for gelator molecules within the gel structure and 150 ps for gelator molecules in the isotropic phase. This confirms, on one hand, the flexibility of the organogel fibers and, on the other hand, the likely presence of clusters in the isotropic phase. Concerning cross-relaxation rates, a thorough theoretical investigation in multispin systems of direct and relayed correlations in a NOESY spectrum allows one to make conclusions about contacts (around 2-3 Å) not only between naphtalimide moieties of different gelator molecules but also between the phenyl ring and the naphtalimide moiety again of different gelator molecules. As a result, not only is the head-to-tail structure of amino acid columns confirmed but also the entangling of nearby columns by the naphthalimide moieties is demonstrated.

Supramolecular Amino Acid Based Hydrogels: Probing the Contribution of Additive Molecules using NMR Spectroscopy

“…Negative nOe enhancements were detected in 1 H‐ 1 H 2D NOESY NMR spectra for Phe protons in Phe hydrogels (Figure S9 in Supporting Information). This is characteristic of large molecules which transfer magnetisation efficiently through dipolar interactions . Since these Phe‐based hydrogel systems are composed exclusively of low molecular weight species, these findings indicated that molecules in solution contain information from the fibrous network due to their fast dynamics of exchange in the NMR frequency time scale.…”

“…Since these Phe‐based hydrogel systems are composed exclusively of low molecular weight species, these findings indicated that molecules in solution contain information from the fibrous network due to their fast dynamics of exchange in the NMR frequency time scale. The phenomenon of solution‐state NMR spectra containing information from the hydrogel fibres due to fast molecular exchange between solution and gel states has been described previously and further confirmed by the determination of a single diffusion regime (see above). Strong negative cross‐peaks were also recorded between Phe and Trp or Tyr, supporting the hypothesis that Trp and Tyr were in close proximity with Phe due to their incorporation in the Phe/Trp and Phe/Tyr hydrogel fibres.…”

“…Bouguet‐Bonnet et al. reported a similar observation for single component organogels of a derivative of phenylalanine and naphthalimide . During stage II, a heterogeneous distribution of T 1 times typical of Phe solutions was observed, reflecting the disassembly of the network and the presence of more Phe molecules in the solution state.…”

Biphenyl Bis(amino alcohol) Oxalamide Gelators: Complex Gelation Involving Coupled Equilibria, Central‐to‐Axial Chirality Transfer, Dia­stereoisomer Interconversion, and Self‐Sorting

“…[16,17] In the 1 H NMR spectrum of the [D 8 ]toluene gel sample (c = 1.48 × 10 -2 M) recorded at room temperature, signals of the dissolved major and minor diastereomer could be observed ( Figure S5, Supporting Information). When the temperature was increased, the intensity of all the gelator signals increased due to disassembly of NMR-silent gel network into smaller NMR-observable aggregates.…”

“…When the temperature was increased, the intensity of all the gelator signals increased due to disassembly of NMR-silent gel network into smaller NMR-observable aggregates. [16,17] In the 1 H NMR spectrum of the [D 8 ]toluene gel sample (c = 1.48 × 10 -2 M) recorded at room temperature, signals of the dissolved major and minor diastereomer could be observed ( Figure S5, Supporting Information). The variations of the rela-tive concentrations of the dissolved major (R,aR,R)-3 diss and minor (R,aS,R) diss , diastereomers, as well as that of the assembled diastereomer, that occur with an increase in temperature were calculated based on the intensity changes of the biphenyl NH, C*-NH and biphenyl C-3-H signals relative to the intensity of the internal standard (Figure 7, a).…”