Exploring Molecular Dynamics of Adsorbed CO2 Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation

Previous studies on CO2 adsorbents have mainly addressed the identification and quantification of adsorbed CO2 species in amine-modified porous materials. Investigation of molecular motion of CO2 species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO2 species chemi- and physisorbed at amine-modified silica materials through the determination of the rotating frame spin-lattice relaxation times (T 1ρ) by solid-state NMR. Rotational correlation times (τC) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen-Purcell-Pound theories. As expected, the τC values for the two physisorbed CO2 species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO2 species (162, 62, and 123 μs). The differences in molecular dynamics between the different chemisorbed species correlate well with the structures previously proposed. In the case of the physisorbed CO2 species, the τC values of the CO2 species displaying faster molecular dynamics falls in the range of viscous liquids, whereas the species presenting slower dynamics exhibit T 1ρ and τC values compatible with a CO2 layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and 1H-13C heteronuclear dipolar couplings have also been estimated from T 1ρ measurements, for each adsorbed CO2 species. The CSA tensor parameters obtained from fitting the relaxation data agree with the experimentally measured CSA values, thus showing that the theories are well suited to study CO2 dynamics in silica surfaces.publishe

Contribution of non-ionic interactions on bile salt sequestration by chitooligosaccharides: potential hypocholesterolemic activity

Chitooligosaccharides have been suggested as cholesterol reducing ingredients mostly due to their ability to sequestrate bile salts. The nature of the chitooligosaccharides-bile salts binding is usually linked with the ionic interaction. However, at physiological intestinal pH range (6.4 to 7.4) and considering chitooligosaccharides pKa, they should be mostly uncharged. This highlights that other type of interaction might be of relevance. In this work, aqueous solutions of chitooligosaccharides with an average degree of polymerization of 10 and 90 % deacetylated, were characterized regarding their effect on bile salt sequestration and cholesterol accessibility. Chitooligosaccharides were shown to bind bile salts to a similar extent as the cationic resin colestipol, both decreasing cholesterol accessibility as measured by NMR at pH 7.4. A decrease in the ionic strength leads to an increase in the binding capacity of chitooligosaccharides, in agreement with the involvement of ionic interactions. However, when the pH is decreased to 6.4, the increase in charge of chitooligosaccharides is not followed by a significant increase in bile salt sequestration. This corroborates the involvement of non-ionic interactions, which was further supported by NMR chemical shift analysis and by the negative electrophoretic mobility attained for the bile salt-chitooligosaccharide aggregates at high bile salt concentrations. These results highlight that chitooligosaccharides non-ionic character is a relevant structural feature to aid in the development of hypocholesterolemic ingredients.publishe

Targeted DNP for biomolecular solid-state NMR

International audienceHigh-field dynamic nuclear polarization is revolutionizing the scope of solid-state NMR with newapplications in surface chemistry, materials science and structural biology. In this perspective article, wefocus on a specific DNP approach, called targeted DNP, in which the paramagnets introduced topolarize are not uniformly distributed in the sample but site-specifically located on the biomolecularsystem. After reviewing the various targeting strategies reported to date, including a bio-orthogonalchemistry-based approach, we discuss the potential of targeted DNP to improve the overall NMRsensitivity while avoiding the use of glass-forming DNP matrix. This is especially relevant to the study ofdiluted biomolecular systems such as, for instance, membrane proteins within their lipidic environment.We also discuss routes towards extracting structural information from paramagnetic relaxationenhancement (PRE) induced by targeted DNP at cryogenic temperature, and the possibility to recoversite-specific information in the vicinity of the paramagnetic moieties using high-resolution selective DNPspectra. Finally, we review the potential of targeted DNP for in-cell NMR studies and how it can be usedto extract a given protein NMR signal from a complex cellular background

“Hidden” CO2 in amine-modified porous silicas enables full quantitative NMR identification of physi- and chemisorbed CO2 species

Although spectroscopic investigation of surface chemisorbed CO2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO2 molecules in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chemical shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR analysis of CO2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times (T1). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO2 signals, which contributed to an empirical model of CO2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T1 values confirm the presence of CO2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed 13C-labeled CO2 as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO2 species, showing that 45% of chemisorbed CO2 versus 55% of physisorbed CO2 is formed from the overall confined CO2 in amine-modified hybrid silicas. A total of six distinct CO2 environments were identified from which three physisorbed CO2 were discriminated, coined here as “gas, liquid, and solid-like” CO2 species. The complex nature of physisorbed CO2 in the presence and absence of chemisorbed CO2 species is revealed, shedding light on what fractions of weakly interacting CO2 are affected upon pore functionalization. This work extends the current knowledge on CO2 sorption mechanisms providing new clues toward CO2 sorbent optimization.Fundação para a Ciência e a Tecnologia; European Research Council; Horizon2020; FEDER; PT2020; COMPETE 2020, POCI, PORL, PIDDAC.publishe

Selective high-resolution DNP-enhanced NMR of biomolecular binding sites

International audienceSel-DNP allows selective recovery of high-resolution information from protein binding sites by DNP-enhanced NMR

Truncated dipolar recoupling in solid-state nuclear magnetic resonance

We describe a solid-state NMR concept for the estimation of individual spin–spin couplings in strongly-coupled homonuclear spin systems. A radiofrequency pulse sequence, synchronised with the magic-angle sample rotation recouples zero-quantum dipolar interactions as well as a frequency-dispersing interaction such as the chemical shift anisotropy. The combination of these two recoupled interactions causes the spin system to behave in an approximately weakly-coupled fashion. Individual spin–spin couplings may then be disentangled by using frequency-selective radiofrequency pulses. Theoretical results and numerical simulations are compared with experimental data for the 13C nuclei in [2H7,13C3,15N]-l-alanine

Estimation of internuclear couplings in the solid-state NMR of multiple-spin systems. Selective spin echoes and off-magic-angle sample spinning

Internuclear couplings between selected homonuclear spin pairs in a multiply-labelled spin system are determined by NMR spin echo experiments in the solid-state. The spin echoes are induced by an amplitude-modulated shaped pulse. The time shift in the echo modulation curve is treated by average Hamiltonian theory and verified by numerical simulation. The J-couplings may be estimated by experiments on samples spinning at the magic-angle, while the direct dipole-dipole couplings may be estimated by off-magic-angle spinning. The concept is tested on a uniformly C-13-enriched sample of L-histidine hydrochloride monohydrate. (C) 2008 Elsevier B.V. All rights reserved